Bringing back python memory

Lately I've done work on the memory management of Evennia. Analyzing the memory footprint of a python program is a rather educational thing in general.

Python keeps tracks of all objects (from variables to classes and everything in between) via a memory reference. When other objects reference that object it tracks this too.
Once nothing references an object, it does not need to be in memory any more - in a more low-level languages this might lead to a memory leak. Python's garbage collector handles this for us though - it goes through all abandoned objects and frees the memory for usage by other things. The garbage collector will however not do its thing as long as some other object (which will not be garbage-collected) still holds a reference to the object. This is what you want - you don't want existing objects to stop working because an object they rely on is suddenly not there.

Normally in Django, whenever you retrieve an database model instance, that exists only in memory then and there. If you later retrieve the same object from the database, the model instance you have to work with is most likely a new one. This is okay for most usage, but Evennia's typeclass system (described in an earlier blog entry) as well our wish to store temporary properties on models (existing until next server restart) does not work if the model instance we get is always different. It would also help if we didn't have to load models from the database more than necessary.

For this reason, Evennia uses something called the idmapper. This is a cache mechanism (heavily modified for Evennia) that allows objects to be loaded from the database only once and then be reused when later accessed. The speedup achieved from this is important, but as said it also makes critical systems work properly.

The tradeoff of speed and utility is memory usage. Since the idmapper never drops those references it means that objects will never be garbage collected. The result was that the memory usage of Evennia could rise rapidly with an increasing number of objects. Whereas some objects (like those with temporary attributes) should indeed not be garbage collected, in a working game there is likely to be objects without such volatile data. An example might be objects that are not used some of the time - simply because players or the game don't need them for the moment. For such objects it may be okay to re-load them on demand rather than keep them in memory indefinitely.

When looking into this I found that simply force-flushing the idmapper did not clean up all objects from memory. The reason for this has to do with how Evennia references objects via a range of other means. The reference count never went to zero and so the garbage collector never got around to it. 

With the excellent objgraph library it is actually pretty easy to track just what is referencing what, and to figure out what to remove. Using this I went through a rather prolonged spree of cleanups where I gradually (and carefully) cleaned up Evennia's object referencing to a point where the only external reference to most objects were the idmapper cache reference. So removing that (like when deliberately flushing the cache) will now make the object possible to garbage-collect.

This is how the reference map used to look for one type of Evennia object (ObjectDB) before the cleanup. Note the several references into the ObjectDB and the cyclic references for all handlers (the cyclic reference is in itself not a problem for reference-counting but they are slow and unnecessary; I now made all handlers use lazy-loading with weak referencing instead).

This is how the reference map looks for the same object now. The __instance__ cache is the idmapper reference. There are also no more cyclic references for handlers (the display don't even pick up on them for this depth of recursion). Just removing that single link will now garbage-collect ObjectDB and its typeclass (ignore the g reference, that is just the variable holding the object in ipython).
We also see that the dbobj.typeclass <-> typeclass.dbobj references keep each other alive and when one goes the other one goes too - just as expected.

An curious aspect of Python memory handling is that (C-)Python does not actually release the memory back to operating system when flushing the idmapper cache. Rather Python makes it internally available so that it does not need to request any more. The result is that if you look at Evennia with the top command, its memory requirement (for example while continuously creating new objects) will not actually drop on a idmapper flush, it will just stop rising.  This is discussed at length in this blog, it was good to learn it was not something I did at least.


Apart from the memory stuff, there is work ongoing with fixing the latest batch of user issue reports. Another dev is working on cleaning up the web-related code, it should make it a lot cleaner to overload web functionality with custom code. One of those days I'll also try to sit down and finally convert our web client from long-polling to use web sockets now that Evennia suppports web sockets natively. Time, time ...

Imaginary realities volume 6, issue 1

I'm a bit late with writing about it, but the latest issue of Imaginary Realities has been out for a month or so now. You can find it here.

Here is a brief summary of the articles in the latest issue. 

• In A Journey Through Paradice, Part II, Matthew Chaplan continues his description of the C++ codebase Paradice9, notably focusing on its input handling, which uses character-mode telnet to produce plenty of interesting effects in a custom terminal client. There are plenty of interesting features (or potential features) discussed. An example is the client knowing to store the receiver of a "reply" command the moment the command is entered rather than waiting for the player the press return (at which point someone else might have written to you and the reply-to target would have changed in a traditional setup). There is no denying the power of having a custom client for your game. And whereas I think some more secure protocol than telnet would maybe be better to use when you control both server and client anyway, it's really interesting to see the power and features you can achieve with it. 
• Building a Giant Mech in Evennia - this is my entry for this issue; a short little tutorial on designing a machine of mirth and mayhem in Evennia. 
• Richard “KaVir” Woolcock's Describing a Virtual World covers the different uses of dynamically created descriptions in text games. He summarizes the types, from the most common, fully static room description up unto the character-dependent procedurally generated descriptions in his own GodWarsII mud. It's a fascinating read since it not only goes into rooms but also how to build piecemeal and situation-aware character and object descriptions as well as procedural help and quest info. The techniques are not for every type of game of course. But a good and informative read for anyone interested in game design.
• Dynamic room descriptions, by Jana, are also covering room descriptions although takes a more conservative conclusion than the full procedural contruction of KaVir's article. This covers the hybrid hand-written description - that is hand-written text that uses a markup language for optional or situation-dependent text. It makes for a good text on implementing such a system (an Evennia equivalent is extended_room.py found in our contrib folder. 
• Saddle Up - A Personal Story about Riding Your Demon to Success is a summary and inspirational story by Michael "Drakkos" Heron. It ties back to his work with Epitath and how it has affected and improved (and continues to improve) his personal and professional life. I like that he manages to include his game development work into his teaching and reasearch, a cool way to make use of your hobby. He has a point on the usability of a coding hobby like this: I myself have had lots of use and even landed project work based on my work with Evennia. One of our users landed his current job based on knowledge he learned working with Evennia. So there is definitely an advantage to mud-development outside the hobby realm.
• The Successful Quest Builder by John "TheDude" Robinette and Joanna "Lorana" Liberty covers the construction of a Quest from the designer's perspective. Rather than focusing on gameplay considerations the authors here focus on the technical aspects; learning the codebase's tools and things to think about debugging and developing something that is enjoyable for the players.
• The article Your MUD Should Have an Account System finally, is Matthew “Chaos” Sheahan's argument as to why a game should use a single login account system rather than the old way of creating a new account per player. Much of the argument is around converting an old-school code base into this configuration and how it's not as hard as one may think. I fully agree on his assessment (although I wonder just how "easy" it is to patch on such a system on an old-running mud). He even mentions Evennia as an example of a modern codebase having this functionality out of the box (yay!).

Deadline for the next issue is announced to be May 31 2014 so don't be shy to contribute your own article. Richard Tew hints at in his introduction, finding people to write articles is the tricky part still.

Moving from Google Code to GitHub

A few weeks back, the Evennia project made the leap from Google Code to GitHub (here). Things have been calming down so it's time to give a summary of how the process went.

Firstly I want to say that I liked Google Code. It did everything expected of it with little hassle. It had a very good Issue system (better than GitHub in my opinion) and it allowed us to use Mercurial instead of Git for version control (I just happen to like Mercurial better than Git, so sue me). Now, GitHub is getting to be something of a standard these days. But whereas our users have occationaly inquired about us making the move, I've been reluctant to do so. 

The problem I did have with Google Code was that I got the increasing feeling that Google didn't care all that much about it. It worked decently, but it was not really going anywhere either. What finally made me change my mind though was an event just after summer last year. There was a bug in Google Code that made the links to online clones disappear. It was worse than that - creating new online clones of the main repo didn't work - people wanting to contribute using a clone just couldn't. 

This is extremely critical functionality for a code-sharing website to have! I made a bug report and many other projects chimed in seeing the same issues. Eventually the links returned and everything worked the way it had. But it took several months before this critical bug was fixed. Even then Google didn't even bother to close my issue. This suggested quite strongly to me that Google Code is not really a priority even for its parent company. It was time to consider a move.

I was never personally a fan of Git. It is undoubtedly powerful, but I always felt its syntax way too archaic and the number of ways to shoot yourself in the foot way too many. But I do like GitHub better than BitBucket (I've used both in other projects), so that's where we nevertheless were heading.

Already last year I created an Evennia "organization" on GitHub and one of our  users first helped to set up a Git Mirror of our Mercurial repo. The idea was a good one - have a mirror on GitHub, allowing the transition to be more gradual. In the end this didn't work out though - there were some issue with the hg-git conversion and the mirror never didn't actually update. When I checked back and it was three months behind we just removed that first ill-fated version. 

In the end I decided to not fiddle about with it, but to move everything over in one go.

 Converting the repository

 

I set aside a new folder on my hard drive and cloned the original mercurial repo into a new sub folder. A good idea is to set up a quick Python virtual environment for easily getting updated dependencies of build scripts. 

I initialized an empty Git repository and used a program called hg-fast-export to convert. As it turned out there were some finer details to consider when doing that:

• The most obvious one was that the conversion initially failed, complaining about the Mercurial original containing "unnamed branches". These came from a contributor who did something to spawn off all sorts of weird branches with little purpose. I should not have merged those into main in the first place, but in those days I didn't know mercurial well enough to be  concerned. In the end I simply used mercurial's MQ extension to remove the unnamed (and unused) branches so the conversion could complete.
• The second issue was that Mercurial is less stringent about its author strings than Git is. Git's author string is "name <email>". Over the years we have gotten contributions from people with all sorts of combinations of names, with or without an email address. So for this we had to supply a mapping file to the converter. It's basically a list of old_author_string = new_author_string and allows for grouping the various used names as needed (some of them were the same person using slightly different author strings). 

Once this was in place, the repo conversion worked fine. It was just a matter of changing the .hgignore file to a .gitignore file and change some code that made use of mercurial to get and display the current revision id.

 Converting the Wiki, part one

Evennia's wiki consitutes our documentation, it's some 80+ pages or so by now. Definitely not something we want to loose. Google Code use a dialect of MediaWiki whereas GitHub's wiki supports a few other formats, like markdown or reST. I needed to convert between them.

Digging around a bit I found googlecode2github. This download contains python scripts for converting the wiki as well as Issues. I didn't really get the issues-converter to work, so I had to find another solution for that (see next section).

All in all, the initial wiki conversion worked decently - all the pages were converted over and were readable. I was even to the point of declaring success when finding the damn thing messed up the links. Googe Code writes links like this: [MyLink Text to see on page]. The script converted this to [[MyLink|Text to see on page]]. Which may look fine except it isn't. GitHub actually wants the syntax in the inverse order: [[Text to see on page|MyLink]].

Furthermore, in Google Code's wiki, code blocks were marked with

{{{ 
  <verbatim code> 
}}}

In markdown, code blocks are created just by indenting the block by four spaces. The converter dutifully did this - but it didn't add empty lines above and below the block, which is another thing markdown requires. The result was that all code ended up mixed into the running text output.

I could have gone back and fixed the converter script, but I suspected there would be enough small things to fix anyway. So in the end I went through 80+ pages of fixing link syntax and adding empty lines by hand. After that I could finally push the first converted wiki version up to the GitHub wiki repository.

Some time later I also found that there is a way to let GitHub wiki pages use syntax highlighting for the language of your choice. The way to do this is to enclose your code blocks like this:


```python

<verbatim code>

```

This is apparently "GitHub-flavoured" markdown. So another stint into all the pages followed, to update everything for prettiness.

 Converting Google Code Issues

I didn't want to loose our Issues from Google Code. I looked around a bit and tested some conversions for this (it helps to be able to create and delete repos on GitHub with abandon when things fail). I eventually settled on google-code-issues-migrator.

This is a Python script that gathers all the Issues from a given Google Code project. It then uses GitHub's API to re-post the issues. It retains the issue numbers and re-maps the Google Code Issue tags to GitHub's equivalent. It didn't retain most other formatting and whereas I ended up as the creator of all issues, the converter included the name of the original author as well as a link back to the original Google Code one. I found that to be quite sufficient for our needs. 

 Converting the IRC announcer

A lot of development discussion goes on in our IRC channel #evennia on Freenode. There is an announcer bot in there that I've written, that collates information from various sources and reports it in the IRC channel:

• Repository updates
• Wiki updates
• Issue creation and updates
• Mailing list/forum posts
• Dev-blog updates (this blog)

Say what you will about Google, but they are great at offering RSS feeds to all their stuff. So my IRC bot was basically a glorified threaded RSS reader that echoed changes to the channel as they came in. This had been working nicely for years.

GitHub does offer RSS feeds to -some- of their offerings, but it's a lot more patchy. I eventually had to do quite a bit of hacking to get everything reporting the way we were used to.

• GitHub has its own IRC announcer bot that reports to IRC. The problem is that this will connect, send message and then disconnect. This causes a lot of spam in the channel. We neither can nor want to set +n on our channel to allow external messages either. The way I solved this was to expand my own custom IRC bot to sit in two irc channels. The GitHub announcer connects to only one of them (so this gets all the spammy connect messages). My IRC bot picks up the announcement and echoes it cleanly to our main #evennia channel.  It works really well.
• Issues are handled by the GitHub announcer in the same way. 
• GitHub has no automatic way to report wiki updates. It doesn't even have a proper RSS feed. However, a user clued me in on using the pipes website to relay an RSS feed from github. I then configured my IRC bot to check that RSS and report it (I also changed the IRC colours to match the GitHub-announcer ones).
• Mailing list and blog haven't changed, so those are still handled via RSS as before.

All this done, the modified IRC announcement works well.

 Closing the book on Google Code

At this point all the critical things were moved over. So after some heads-up warnings on the mailing list (and users helping to rewrite our documentation to use Git instead of mercurial) we eventually made the official move.

One thing I really dislike is when a project switches hosts and don't let users know about it in their revision history. So I made a mercurial-only last commit announcing that the repo is closed and giving the link to the new one.

The Google Code page doesn't go anywhere, but I changed the front page to point to GitHub instead. I even made an issue in the Issue tracker with a title telling people not to use that tracker anymore. Finally I re-pointed all the links on http://www.evennia.com to GitHub and made a mailing list posting. Move was officially complete.

 Converting the Wiki, part 2

At this point were were officially moved over and I started to look into getting fancy with our documentation. We have for the longest time made automated translations of our wiki for compiling by ReadTheDocs.

Getting Google Code's special wikimedia syntax into reST (that ReadTheDocs uses) used to mean jumping through a few hoops. My hackish solution worked in two steps. First a custom python script (whose originating url I can no longer find, sorry) converted the Google Code wiki to HTML. Once this was done, pandoc converted the files from HTML to reST. The result was ... acceptable. There were some minor issues here and there but mostly the result was readable.

I figured that converting from the more standard Markdown of the GitHub wiki to reST should be a breeze by comparison. Not so.

The first hurdle was that the version of pandoc coming with my Linux distribution was too old to support Github-flavoured markdown syntax. I knew from before that Pandoc worked so I didn't want to start with something else. I had to download the several hundred MBs needed by the Haskell build environment and their package manager in order to get and compile all the dependencies and finally the latest version of pandoc. To their credit it was all a very streamlined experience, it just took quite some time.

The second hurdle came when finally looping pandoc to convert all wiki files. It turns out to be that the [[Text on page|address]] syntax I had manually corrected earlier is a special syntax offered by Gollum, the engine powering GitHub's wiki behind the scenes. None of the markdown-to-reSt converters I looked at (pandoc or otherwise) even recognized this syntax as a link at all. As it turns out, normal markdown actually expects its links in the format [Text on page](address).

I was not going to go through and edit all those pages again. So my next step was to write a script to scan and replace all the [[...|...]] syntax in our wiki and replace it with the standard markdown one. After this the markdown files converted to reST quite nicely -- formatting-wise they look much better than the old wiki to HTML to reST chain I had to use from Google Code.

Problem was that when compiling these reST pages into HTML with Sphinx, no links worked.

Each individual page looked okay, just that the links were not pointing to anything reasonable. In retrospect this was not so strange. Pandoc knows nothing about the relationships between files, and clearly the simple naming scheme used for addresses is something the wiki softwares knows and Sphinx does not.

Some thinking lead to a custom Python script for renaming the link targets in the converted pages to their html page name. This needed to handle the fact that wiki links also allows whitespace. So the [Start](Getting Started) link would be converted to [Start](GettingStarted.html), which seems to be the format with which Sphinx will generate its pages.

One also needs to have a "toc" (Table of Contents) to tie all those pages together for the benefit of Sphinx. I just used a "hidden" toc, letting my converter script add this to the bottom of my normal index file. As long as it's included somewhere, Sphinx will be happy.

Originally I put the reST files in a subfolder of the GitHub wiki repo, I thought I could just point ReadTheDocs to that repo later. The GitHub wiki has a strange "feature" though. It seems to pick its wiki pages from wherever they are in the repo, no matter if they are in the root or in subfolders. Suddenly I was starting to see reST-style pages appear in the online wiki, and sometimes I would get the markdown version (the two would go out of sync). Very strange and confusing.

Since the files clearly "polluted" our wiki, I had to move the converted reST files to a separate branch of the wiki repository. This has the advantage of keeping all the support scripts and converter mechanisms separate from the normal wiki content. ReadTheDocs can luckily be set to read its information from another branch than master, so finally the latest converted wiki can again be read there!

That concludes what I think was the last main conversion effort. Phew!

 Impressions so far

GitHub is nice. The merge requests and easy way to comment on them are really good. Since people are more familiar with using GitHub overall, it does seem to be a shorter step for people to make a fork and contribute small things. Doing the same in Google Code was probably not harder per se, just something less people were previously familiar with.

Due to the modular way Evennia is structured, people are recommended to make a fresh clone of the new Git repo and simply copy their plugin source files and database back into it. So far this seems to have gone smoothly.

The GitHub issue tracker is worse than the Google Code one. It has no good way to order Issues or list them in a more compact form (nor in a matrix). Not having good issue templates is really limiting; having to reply to issues only to ask for basic info they should include in their issue is an unnecessary time sink.

I also find that there is no clear way to announce an issue change (like "Needing more information"). Tags work partly for this, but setting them is not announced anywhere as far as I can tell - they are just there.

Most things also takes so much spaaace. Overall GitHub seems designed for people with big monitors. I have those, but most of the time I prefer working on my laptop. I'm sure it's a matter of habit, but Google Code is very compact by comparison. It gave a lot better overview of things. On GitHub I have to scroll everywhere and this is true both in the repo view, wiki and issues.

These small quips nonwithstanding, I think this move will serve us well. There is a good wibe of development and continuing improvement going on at GitHub. There's plenty of help and tutorials all over. Since so many people are using GitHub, problems are more likely to have been answered before. And of course we hope this will in effect help more people find Evennia and join the fun.

Looking forwards and backwards

We are almost a month into the new year, time to look forward.

But first a look backwards. The year of 2013 was a year of big development projects and lots of quiet in the main repository in between. Two major updates were released during the year.

The first update, the "many sessions per player" update, originated in a feature request that I thought would be easy to implement but which led to far-ranging changes (and honestly, improvements) to how Players and Sessions interconnect. It was a lot more more work than I anticipated.

The second update was about moving Evennia's web server from the Portal level into the Server-level. The actual moving of the server was actually considerably easier than I thought it would be. But it turned out that a truckload of other things came along with it. Not only did the cache system have to change in order to accommodate the new webs erver, I had to also finalize the Out-of-band structure, since this made use of the cache system. And while I were at it, other fixes were done and ... the update grew and grew. When it finally merged late last year it closed plenty of issues, but it would probably have been better to structure it into more, small updates instead.

Anyway, 2014 promises continued (and hopefully more continuous and gradual) development of Evennia. The closest upcoming upheaval is our move from Google Code to GitHub in a few days (I'll probably do a blog about that once it's done). Apart from that we are currently in a fixing state, cleaning up and fixing remnant issues from the big mergers.

Another Issue of the MUD e-zine Imaginary Realities is coming too. I just contributed with an Evennia-related article. If anyone reading this blog has anything MUD-related to write about, do consider contributing before January 31, they need more articles! I don't think you need to be too advanced, anything from a mud-development anecdote to retells of good MUD gaming memories might be interesting I would think.

Imaginary Realities is back

The Imaginariy Realities webzine was the place to go to for MUD game design articles in the late 90's. Last released in 2001, its articles are still worth the read for any game designers today. 

But guess what - this venerable ezine has now returned! You can find the new issue here.

I think this is a good community initiative worthy of support. I contibuted two articles myself (one of which is about Evennia) and would like to thank the staff/editors who took their work very seriously and did sterling work on getting everything in shape.

Thanks also to the other authors who penned some very interesting articles. Great job guys!

My impressions: 

• KaVir elaborates in A modern interface for a modern MUD on the advantages of not sticking with an outdated UI just for the sake of it. Adding a more accessible presentation is not that hard and won't ruin your game but rather help it. Whereas I have read his argument about this before, this is a good summary to take to heart. Evennia's javascript web client is currently mainly a telnet clone; there's a lot of things we could offer to make it easier for users to offer a more graphical presentation.
• Molly O’Hara, in her A well built zone is a work of art, outlines a list of useful things to keep in mind when designing a zone. While some of these vary with game type, others do not. I like the suggestion that scripting bugs need not be the most important aspect - syntactic errors can be handled by automated means as long as the design aspect of the zone is solid.
• A journey through Paradice [sic] is Matthew Chaplain's entry on designing a dice-roller using the telnet protocol. Some interesting things here, including making creative use of the telnet character-mode and VT100 control sequences. This ties a bit into KaVir's article, in that the interface used for "modern" MUDs are often voefully missing out on a lot of possibilities.
• Blind accessibility: challenges and opportunities by Matthew “Chaos” Sheahan, is based on interviews with a blind mudder and a game admin having implemented lots of support for seeing-impaired players. This was a really interesting article since I've been pondering myself what could be done from Evennia's core side to help players support players using screen readers. Most seem to be down to configuration options though, and avoiding making colour or ascii art the only sources of information. These are all things Evennia devs implement depend on their game. We may offer some good contribs to build from though.
• Evennia: an introduction - this is mine. It not-so-briefly sums up stuff about Evennia and the more important systems it relies on.
• Getting a roleplaying scene going - another article of mine. This is a light-hearted list of tropes for getting a RP scene going on an RP-mud. It's based on things I've tried or seen myself in play. 
• Darcie “Natilena” Laur laments on the often opaque newbie guides in Introducing new players and redesigning MUD School. It describes how she tested (and improved) her own MUD's starter area while testing it on her kids. It made me think more on having Evennia offering easier ways to dump text logs in all sorts of situations. And we find out that kids have the attention span of zombie squirrels - something new learned every day!
• Finally, The Hunger Game, or how I learned to break the ship from the bottle is Michael “Drakkos” Heron's epic about his journey developing and releasing his zombie-survival MUD Epitaph. Drakkos is a frequent blogger on the MUD-planet feed, so I knew some of this already, but it's a good read and contains some useful warnings and things-to-think-of for those thinking of starting their own MUD project. We already give some of the same advice (albeit with fewer words) in our wiki but I'm seriously considering linking to Drakkos post from there as well - it gives a more comprehensive treatment and offers a real-world example of the long road to a released game.

The issue also contains a call to help recover old MUDing resources that has gone offline since their heyday. Having browsed through the classic MUD-dev mailing list archives I can agree they are not quite as easily searchable as could be hoped. It would also be great to see the old comment posts tied to the articles of the old Imaginary Realities articles, if those could be found somehow.

This is actually one thing which I do miss with this first Imaginary Realities issue - a way for readers to comment on the articles. This would likely mean a much higher level of complexity and work though, so I can certainly see why it's not there - using the existing MUD forums is probably enough for now.

Anyway, I'm happy to see this thing getting off on a good start. I'm already looking forward to the next issue! 

Out-of-band mergings

As of today the development repository of Evennia, which has been brewing for a few months now, merged into the main repository. This update grew from one experimental feature to a relatively big update in the end. Together with the "many-character-per-player" feature released earlier, this update covers all the stuff I talked about in my Behind the Scenes blog post.

•  Evennia's webserver was moved from Portal to Server. This moves all database-modifying operations into the same process and neatly avoids race conditions when modifying a game world from various interfaces. 
• The OOB (Out Of Band) handler was implemented. This goes together with a protocol for telnet sub-negotiations according to the MSDP specification. The handler allows on-demand reporting whenever database fields update. It also offers regular polling of properties if needed. A user can customize which oob commands are available to the client and write whatever handlers are needed for their particular game. In the future we'll also add support for GMCP, but the lack of a central, official specification is off-putting (if there is a central document besides accounts of how individual games chose to implement GMCP, please let me know). For our own included web client, we'll likely just use JSON straight off.
• Our channel system is now typeclassed. If you are not familiar with Evennia this won't mean much to you - In short it means developers will be able to customize their channel system much easier than in the past since a channel can be treated pretty much like any Python class (thanks go to user Kelketek who actually did the implementation).
• We added the concept of Tagging, as a more generalized version of our old Alias system. Tagging is just what it sounds like - it allows you to tag all your objects in order to group them and easily (and efficiently) find them later. Tagging offers a powerful way to create what other code bases refer to as "zones". There are many other possible uses though, such as having effects appear only in certainly tagged rooms, indicate which Characters have joined a particular guild and so on. 
• Behind the scenes there were a lot of cleanups, along with minor API changes mentioned on the mailing list. A slew of older Issues were also fixed with this merge.

Now, in the immediate future we'll be working on updating the documentation. Evennia prides itself with having a large body of documentation and this update comes with small changes to various existing systems. There are also a few bigger new features (such as OOB). No doubt the near future will also see other issues popping up as more people try things out.

 

Imaginary Realities update

Apparently the reboot of Imaginary Realities (to which I contribute two articles) has been pushed forward a week or two. Reason being, apparently, to finalize the actual presentation of the content. I already signed off on the last editorial fixes way before deadline, so I guess it's just to wait and see what comes of it!

A list of Evennia topics

Some Evennia updates.

Development
Lots of work has been happening in the dev-clone of Evennia over the last few months.
As alluded to in the last blog, the main work has been to move Evennia's webserver component into the Server-half of Evennia for various reasons, the most obvious one to make sure that all database writes happen in the same process, avoiding race conditions. But this move lead to a rework of the cache system, which in turn lead to me having to finalize the plans for how Out-of-Band protocols should be implemented server-side. And once that was finalized, OOB was pretty much implemented anyway. As part of making sure OOB trackers were updated correctly at all times meant reworking some of the ways data is stored ... So one thing led to another making this a bigger update than originally planned.

I plan to make a more detailed post to the mailing list soon with more technical details of the (relatively minor) API changes existing users should expect. The merging of the clone into the main repo is still a little way off, but adventurous users have already started testing things.

Google Code
I like Google Code. It's easy to manage and maintain, it has a good wiki and Issue system, not to mention that it allows the use of Mercurial. But in the beginning of September, suddenly all links to our user's clone repositories were gone from the front of the project page. Not only that, creating new clones just didn't work anymore.
Now any site can have bugs, and we made an issue for it (other projects were similarly affected). But nothing happened for the longest time - at least two months given that we didn't report it right away. Just recently the functionality came back but there is no confirmation or comments from Google (our issue is not even closed).
That such a fundamental feature can go unheeded for so long is disturbing to me, driving home the fact that Google is certainly not putting much priority in their code hosting.

Community
Some furious activity in the IRC chat lately, with new people dropping in to chat and ask about Evennia. For example, an enthusiastic new user learned not only about Evennia but also Python for the first time. It was a lot of fun to see him go from having no programming experience except mush softcode to doing advanced Evennia system implementations in the course of a week and offering good feedback on new features in two. Good show! The freedom you get upgrading from something like softcode to Evennia's use of a full modern programming language was seemingly quite eye-opening.

Other discussions have concerned the policies around using clones/branches for development as well as the benefits of some other hosting solution. Nothing has been decided on this. There is however now also an official GitHub mirror of Evennia's main repo to be found here.

Imaginary Realities
The deadline for entering articles for the Imaginary Realities web zine reboot has passed. It's a good initiative to bring this back - the original (archived) webzine remains a useful mud-creation resource to this day. I entered two articles, one about Evennia and another about general mud-roleplaying. It will be fun to see how it comes out, apparently the first issue will appear Nov 13. 

One to Many

As of yesterday, I completed and merged the first of the three upcoming Evennia features I mentioned in my Churning Behind the Scenes blog post: the "Multiple Characters per Player" feature.

Evennia makes a strict division between Player (this is an object storing login-info and represents the person connecting to the game) and their Character (their representation in-game; Characters are just Objects with some nice defaults). When you log into the game with a client, a Session tracks that particular connection.

Previously the Player class would normally only handle one Session at a time. This made for an easy implementation and this behavior is quite familiar to users of many other mud code bases. There was an option to allow more than one Session, but each were then treated equally: all Sessions would see the same returns and the same in-game entities were controlled by all (and giving the quit command from one would kick all out).

What changed now is that the Player class will manage each Session separately, without interfering with other Sessions connected to the same Player. Each Session can be connected, through the Player, to an individual Character. So multiple Characters could in principle be controlled simultaneously by the same real-world player using different open mud clients. This gives a lot of flexibility for games supporting multi-play but also as a nice way to transparently puppet temporary extras in heavy roleplaying games.

It is still possible to force Evennia to accept only one Session per Player just like before, but this is now an option, not a limitation. And even in hardcore one-character-at-a-time roleplaying games it is nice for builders and admins to be able to have separate staff or npc characters without needing a separate account for each.

This feature took a lot more work than I anticipated - it consitutes a lot of under-the-hood changes. But it also gave me ample opportunity to fix and clean up older systems and fix bugs. The outcome is more consistency and standardization in several places. There are plenty of other noteworthy changes that were made along the way in the dev branch along with some API changes users should be aware of.
So if you are an Evennia game developer you should peek at the more detailed mailing list  announcement on what has changed. The wiki is not updated yet, that will come soon.

Now onward to the next feature!

Churning behind the scenes

At the moment there are several Evennia projects churning along behind the scenes, none of which I've yet gotten to the point of pushing into a finished state.  Apart from bug fixes and other minor things happening, these are the main updates in the pipeline at the moment.

Multiple Characters per Player/Session

Evennia has for a long time enforced a clean separation between the Player and the Character. It's a much appreciated feature among our users. The Player is "you", the human playing the game. It knows your password, eventual user profile etc. The Character is your avatar in-game. This setup makes it easy for a Player to have many characters, and to "puppet" characters - all you need to do is "disconnect" the Player object from the Character object, then connect to another Character object (assuming you are allowed to puppet that object, obviously). So far so good. 

What Evennia currently doesn't support is being logged in with different client sessions to the same Player/account while puppeting multiple characters at the same time. Currently multiple client sessions may log into the same Player account, but they will then all just act as separate views of the same action (all will see the same output, you can send commands from each but they will end up with the same Character). 

Allowing each session to control a separate Character suggests changing the way the session is tracked by the player and Character. This turns out to be more work than I originally envisioned when seeing the feature request in the issue tracker. But if my plan works out it will indeed become quite easy to use Evennia to both allow multi-play or not as you please, without having to remember separate passwords for each Character/account.

Webserver change to Server level

Evennia consists of two main processes, the Portal and the Server. The details of those were covered in an earlier blog post here. Evennia comes with a Twisted-based webserver which is currently operating on the Portal level. This has the advantage of not being affected by Server-reboots. The drawback is however that being in a different process from the main Server, accessing the database and notably its server-side caches becomes a problem - changing the database from the Portal side does not automatically update the caches on the Server side, telling them that the database has changed. Also writing to the database from two processes may introduce race conditions. 

For our simple default setup (like a website just listing some database statistics) this is not a terrible problem, but as more users start to use Evennia, there is a growing interest in more advanced uses of the webserver. Several developers want to use the webserver to build game-related rich website experiences for their games - online character generation, tie-in forums and things like that. Out-of-sync caches then becomes a real concern. 

One way around this could be to implement a framework (such as memcached) for homogenizing caches across all Evennia processes. After lots of IRC discussions I'm going with what seems to be the more elegant and clean solution though - moving the webserver into the Server process altogether. The Portal side will thus only hold a web proxy and the webclient protocol. This way all database access will happen from the same process simplifying things a lot. It will make it much easier for users to use django to create rich web experiences without having to worry about pesky behind the scenes things like caches and the like. 

Out-of-band communication 

This has been "brewing" for quite some time, I've been strangely unmotivated to finalize it. Out of band communication means the MUD client can send and receive data to/from the server directly, without the player having to necessesarily enter an active command or see any immediate effect. This could be things like updating a health bar in a client-side GUI, redirect text to a specific client window but also potentially more advanced stuff. I created the Evennia-side oob-handler over Christmas; it allows for client sessions to "sign up" for "listening" to attribute updates, do scheduled checks and so on. It's already in the codebase but is not activated nor tested yet.

On the protocol side (for serializing data to the client) I have a MSDP implementation ready for telnet subnegotiation, it should be simple to add also GMCP once everything is tested. A JSON-based side channel for the webclient is already in place since a long time if I remember correctly, it just need to be connected to the server-side oob-handler once that's finished. 

Evennia changes to BSD license

As of today, Evennia changes to use the very permissive BSD license.

Now, our previous "Artistic License" was also very friendly. One main feature was that it made sure that changes people made to the core Evennia library (i.e. not the game-specific files) were also made available for possible inclusion upstream. A good notion perhaps, but the licensing text was also quite long and it was clear some newcomers parsed it as more restrictive than it actually was.

... And let's be honest, it's not like I would have come hunting down anyone not complying fully with the Artistic license's terms. Changing to the much simpler and more well-known BSD license better clarifies the actual licensing situation.

After all, far too many older MUD-code bases are weighted by a legacy of licensing issues. Anything we can do to avoid this is better in the long run. Indeed we hope this change in licensing will remove eventual licensing doubts for new adopters and have more people join and contribute to the project.

Community interest

It's fun to see a growing level of activity in the Evennia community. The last few months have seen an increase in the number of people showing up in our IRC channel and mailing list. With this has come a slew of interesting ideas, projects and MUD-related discussion (as well as a few inevitable excursions into interesting but very non-mud-related territory - sorry to those reading our chat logs).

One sign of more people starting to actually use Evennia "for real" is how the number of bugs/feature requests have been increasing. These are sometimes issues related to new things being implemented but also features that have been there for some time and which people are now wanting to use in new and creative ways - or systems which noone has yet really used "seriously" before. This is very encouraging, especially since a lot of good alternative solutions, variations and edge cases can be ironed out this way. So keep submitting those Issues, people!


The budding Evennia community consists of people with a wide variety of interests, skillset and ambition.

There are quite a few people who sees Evennia as a great stepping stone for learning Python, or for getting experience with creating a bigger programming project in general. Some are skilled programmers in other languages but we also have a few with only limited prior coding experience. From the experience in chat, it's really quite striking how fast members pick up the ropes. I'd like to think our documentation is at least partially helping here, but of course it helps that Python is inherently very easy a language to learn and use in the first place.
Not all are participating with the goal of building a specific game. The general flow of patches and clone repository merges have also picked up. We have some users which are primarily interested in a coding challenge, to help with fixing bugs and features, or which uses Evennia as a starting point for exploring various web- and technical solutions that may or may not be a part of Evennia in the future.


The proposed Evennia game projects are just as varied as its users - and none are yet open to the public. As is common with these things, it's of course hard to determine who actually has the time and discipline to bring their plans to fruition. But I should really start to keep some sort of record of who works on what, I'm terrible with remembering this stuff ... so below is just some sort of summary of my impressions, not a comprehensive listing.

As can be expected, most proposed Evennia projects concern relatively standard MUD-style games. A few people are into building traditional hack-and-slash variety games, but most want to expand on the concept considerably. There was even one user exploring using Evennia for a RobotWars kind of experience (where you "program" robot programs in a custom language and battle them). Another project (Avaloria, also blogging on the MUD-dev rss feed) aims for a sort of base-building/strategy mechanic combined with more traditional MUD elements. There are at least two zombie-survival concepts floating around and a few large-scale procedural-content-driven science-fiction text games. One user has apparently a working Smaug->Evennia importer.

It seems that most Evennia users want to offer some sort of roleplaying environment, or at least a "roleplay-friendly" one. Currently we have at least two MUCK admins who aim to convert their existing, running games to Evennia. Whereas the initial idea was to implement parsers for MUCK's MUF language, it seems the conclusion has now shifted to it being faster and easier to just rewrite the MUF-coded functionality in Python (and maybe use something like Evlang for player scripting instead). Several people have announced their interest in creating "RPI"-style games (Armageddon seems to be a big inspiration here), but there was also a MOO admin and even a writer of Interactive Fiction who dropped into the mailing list to see if Evennia could be used for their style of game.

How many of these projects actually reach a point of maturity remains to be seen. But that people are wanting to use the system and is really putting it through its paces is encouraging and very helpful for general Evennia development.

Combining Twisted and Django

Franco Nero as the twisted gunslinger Django

Newcomers to Evennia sometimes misunderstand it as being a "Django mud codebase somehow using Twisted". The correct description is rather that Evennia is a "Twisted-based mud server using Django". Allow me to elaborate.

A mud/mux/moo/mu* is per definition a multi-user online game system. All these users need to co-exist on the server. If one player does something, other players shouldn't have to (noticeably) wait for that something to end before they can do anything. Furthermore it's important for the database schema to be easy to handle and upgrade. Finally, in a modern game, internet presence and web browser access is becoming a must. We combine two frameworks to achieve this.

Two frameworks combined

Twisted is a asynchronous Python framework. "Asynchronous" in this context means, very simplified, that Twisted chops up code execution into as small bits as the code lets it. It then flips through these snippets rapidly, executing each in turn. The result is the illusion of everything happening at the same time. The asynchronous operation is the basis for the framework, but it also helps that twisted makes it easy to support (and create) a massive range of different network protocols.

Django implements a very nice abstract Python API for accessing a variety of SQL-like databases. It makes it very convenient to maintain the database schema (not to mention that django-South gives us easy database migrations). The fact that Django is really a web framework also makes it easy to offer various web features. There is for example an "admin site" that comes with Django. It allows to modify the database graphically (in Evennia's case the admin site is not quite as polished as we would like yet, but it's coming).

Here are some highlights of our architecture:

• Portal - This is a stand-alone Twisted process talking to the outside world. It implements a range of communication protocols, such as telnet (traditional in MUD-world), ssh, ssl, a comet webclient and others. It is an auto-connecting client to Server (below).
• Server - This is the main MUD server. This twisted server handles everything related to the MUD world. It accesses and updates the database through Django models. It makes the world tick. Since all Players connect to the Server through the Portal's AMP connection, it means Server can be restarted without any players getting kicked off the game (they will re-sync from Portal as soon as Server is back up again).
• Webserver - Evennia optionally starts its own Twisted webserver. This serves the game's website (using the same database as the game for showing game statistics, for example). The website is of course a full Django project, with all the possibilities that entails. The Django admin site allows for modifying the database via a graphical interface. 
• Webclient - There is a stand-alone MUD web client existing in a page on the default website. This uses Twisted to implement a long-polling ("comet") connection to a javascript client. As far as Evennia's concerned, this is just another outgoing protocol served by the Portal.
• Other protocols - Since it's easy to add new connectivity, Evennia also offers a bunch of other connectivity options, such as relaying in-game channels to IRC and IMC2 as well as RSS feeds and some other goodies. 

On the joining of the two

An important thing to note about Twisted's asynchronous model is that there is no magic at work here: Each little snippet of code Twisted loops over is blocking. It's just hopefully not blocking long enough for you to notice. So if you were to put sleep(10) in one of those snippets, then congratulations, you just froze the entire server for ten seconds.

Profiling becomes very important here. Evennia's main launcher takes command arguments to run either of its processes under Python's cProfile module. It also offers the ability to connect any number of dummy Players doing all sorts of automated random actions on the server. Such profile data is invaluable to know what is a bottleneck and what is not.

I never found Twisted asynchronous paradigms much harder to understand than other code. But there are sure ways to write stupid blocking code that will come back and bite you. For example, much of  Evennia's workload is spent in the Server, most notably in its command handler. This is not so strange; the command handler takes care of parsing and executing all input coming from Players, often modifying the game world in various ways (see my previous post for more info about the command handler).
The command handler used to be a monolithic, single method. This meant that Twisted had to let it run its full course before letting anyone else do their turn. Using Twisted's inlineCallbacks instead allowed for yielding at many, many places in this method, giving Twisted ample possibilities to split execution. The effect on multi-user performance was quite impressive. Far from all code can  be rewritten like this though.

Another important bottleneck on asynchronous operations is database operations. Django, as opposed to Twisted, is not an asynchronous framework. Accessing the database is a blocking operation and can be potentially expensive. It was never extremely bad in testing, to be honest. But for large database operations (e.g. many Players) database access was a noticeable effect.

I have read of some people using Twisted's deferToThread to do database writes. The idea sounds reasonable - just offload the operation to another thread and go on your merry way. It did not help us at all though - rather it made things slower. I don't know if this is some sort of overhead (or error) in my test implementation - or an effect of Python just not being ideal with using threading for concurrency (due to the GIL). Either way, certain databases like SQlite3 doesn't support multiple threads very well anyway, and we prefer to keep giving plenty of options with that. So no deferToThread for database writes. I also did a little testing with parallel processes but found that even slower, at least once the number of writes started to pile up (we will offer easy process-pool offloading for other reasons though).

As many have found out before us, caching is king here. There is not so much to do about writes, but at least in our case the database is more often read than written to. Caching data and accessing the cache instead of accessing a field is doing much for performance, sometimes a lot. Database access is always going to cost, but it does not dominate the profile. We are now at a point where one of the most expensive single operations a Player (even a Builder) performs during an entire gaming session is the hashing of their password during login. I'd say that's good enough for our use case anyway.

Django + MUD?

It's interesting that whereas Twisted is a pretty natural fit for a Python MUD (I have learned that Twisted was in fact first intended for mudding, long ago), many tend to be intrigued and/or surprised about our use of Django. In the end these are only behind-the-scenes details though. The actual game designer using Evennia don't really see any of this. They don't really need to know neither Django nor Twisted to code their own dream MUD. It's possible the combination fits less for some projects than for others. But at least in our case it has just helped us to offer more features faster and with less headaches. 

Taking command

Commands are the bread and butter of any game. Commands are the instructions coming in from the player telling the game (or their avatar in the game) to do stuff. This post will outline the reasoning leading up to Evennia's somewhat (I think) non-standard way of handling commands.

In the case of MUDs and other text games commands usually come in the form of entered text. But clicking on a graphical button or using a joystick is also at some level issuing a command - one way or another the Player instructs the game in a way it understands. In this post I will stick to text commands though. So open door with red key is a potential command.

Evennia, being a MUD design system, needs to offer a stable and extensive way to handle new and old commands.  More than that, we need to allow developers pretty big freedom with developing their own command syntax if they so please (our default is not for everyone). A small hard-coded command set is not an option.

Identifying the command

First step is identifying the command coming in. When looking at open door with red key it's probably open that is the unique command. The other words are "options" to the command, stuff the open command supposedly knows what to do with. If you know already at this stage exactly how the command syntax looks, you could hard-code the parsing already here. In Evennia's case that's not possible though - we aim to let people define their command syntax as freely as possible. Our identifier actually requires no more than that the uniquely identifying command word (or words) appear first on the input line. It is hard to picture a command syntax where this isn't true ... but if so people may freely plug in their own identifyer routine.

So the identifyer digs out the open command and sends it its options ... but what kind of code object is open?

 

 The way to define the command

A common variant I've seen in various Python codebases is to implement commands as functions. A function maps intuitively to a command - it can take arguments and it does stuff in return. It is probably more than enough for some types of games.

Evennia chooses to let the command be defined as a class instead. There are a few reasons. Most predominantly, classes can inherit and require less boiler plate (there are a few more reasons that has to do with storing the results of a command between calls, but that's not as commonly useful). Each Evennia command class has two primary methods:

• parse() - this is responsible for parsing and splitting up the options part of the command into easy-to use chunks. In the case of open door with red key, it could be as simple as splitting the options into a list of strings. But this may potentially be more complex. A mux-like command, for exampe, takes /switches to control its functionality. They also have a recurring syntax using the '=' character to set properties. These components could maybe be parsed into a list switches and two parameters lhs and rhs holding the left- and right hand side of the equation sign. 
• func() - this takes the chunks of pre-parsed input and actually does stuff with it. 

One of of the good things with executing class instances is that neither of these methods need to have any arguments or returns. They just store the data on their object (self.switches) and the next method can just access them as it pleases. Same is true when the command system instantiates the command. It will set a few useful properties on the command for the programmer to make use of in their code (self.caller always references the one executing the command, for example). This shortcut may sound like a minor thing, but for developers using Evennia to create countless custom commands for their game, it's really very nice to not have to have all the input/output boilerplate to remember. 

... And of course, class objects support inheritance. In Evennia's default command set the parse() function is  only implemented once, all handling all possible permutations of the syntax. Other commands just inherit from it and only needs to implement func(). Some advanced build commands just use a parent with an overloaded and slightly expanded parse().

 Commands in States

So we have individual commands. Just as important is how we now group and access them. The most common way to do this (also used in an older version of Evennia) is to use a simple global list. Whenever a player enters a command, the identifier looks the command up in the list. Every player has access to this list (admin commands check permissions before running). It seems this is what is used in a large amount of code bases and thus obviously works well for many types of games. Where it starts to crack is when it comes to game states.

• A first example is an in-game menu. Selecting a menu item means an instruction from the player - i.e. a command. A menu could have numbered options but it might also have named options that vary from menu node to menu node. Each of these are a command name that must be identified by the parser. Should you make all those possible commands globally available to your players at all times? Or do you hide them somehow until the player actually is in a menu? Or do you bypass the command system entirely and write new code only for handling menus...?
• Second example: Picture this scenario: You are walking down a dark hallway, torch in hand. Suddenly your light goes out and you are thrown into darkness. You cannot see anything now, not even to look in your own backpack. How would you handle this in code? Trivially you can put if statements in your look and inventory commands. They check for the "dark" flag. Fair enough. Next you knock your head and goes 'dizzy'. Suddenly your "navigation" skill is gone and your movement commands may randomly be turned around. Dizziness combined with darkness means your inventory command now returns a strange confused mess. Next you get into a fight ... the number of if statements starts piling up.  
• Last example: In the hypothetical FishingMUD,. you have lots of detailed skills for fishing. But different types of fishing rods makes different types of throws (commands) available. Also, they all work differently if you are on a shore as compared to being on a boat. Again, lots of if statements. It's all possible to do, but the problem is maintenance; your command body keep growing to handle edge cases. Especially in a MUD, where new features tend to be added gradually over the course of years, this gives lots of possibilities for regressions.

All of these are examples of situation-dependent (or object-dependent) commands. Let's jointly call them state-dependent commands. You could picture handling the in-game menu by somehow dynamically changing the global list of commands available. But then the global bit becomes problematic - not all players are in the same menu at the same time. So you'll then have to start to track who has which list of commands available to them. And what happens when a state ends? How do you get back to the previous state - a state which may itself be different from the "default" state (like clearing your dizzy state while still being in darkness)? This means you have to track the previous few states and ...

A few iterations of such thinking lead to what Evennia now uses: a non-global command set system. A command set (cmdset) is a structure that looks pretty much like a mathematical set. It can contain any number of (unique) command objects, and a particular command can occur in any number of command sets.

• A cmdset stored on an object makes all commands in that cmdset available to the object. So all player characters in the game has a "default cmdset" stored on them with all the common commands like look, get and so on.
• Optionally, an object can make its cmdset available to other objects in the same location instead. This allows for commands only applicable with a given object or location, such as wind up grandfather clock. Or the various commands of different types of fishing rods. 
• Cmdsets can be non-destructively combined and merged like mathematical sets, using operations like "Union", "Intersect" and a few other cmdset-special operations. Each cmdset can have priorities and exceptions to the various operations applied to them. Removing a set from the mix will dynamically rebuild the remaining sets into a new mixed set.

The last point is the most interesting aspect of cmdsets. The ability to merge cmdsets allows you to develop your game states in isolation. You then just merge them in dynamically whenever the game state changes. So to implement the dark example above, you would define two types of "look" (the dark version probably being a child of the normal version). Normally you use your "default cmdset" containing the normal look. But once you end up in a dark room the system (or more likely the room) "merges" the dark cmdset with the default one on the player, replacing same-named commands with new ones. The dark cmdset contains the commands that are different (or new) to the dark condition - such as the look command and the changed inventory command.  Becoming dazed just means yet another merger - merging the dazed set on top of the other two. Since all merges are non-destructive, you can later remove either of the sets to rebuild a new "combined" set only involving the remaining ones in any combination. 

Similarly, the menu becomes very simple to create in isolation (in Evennia it's actually an optional contrib). All it needs to do is define the required menu-commands in its own cmdset. Whenever someone triggers the menu, that cmdset is loaded onto the player. All relevant commands are then made available. Once the menu is exited, the menu-cmdset is simply removed and the player automatically returns to whichever state he or she was in before.

Final words

The combination of commands-as-classes and command sets has proved to very flexible. It's not as easy to conceptualize as is the simple functions in a list, but so far it seems people are not having too much trouble. I also think it makes it pretty easy to both create and, importantly, expand a game with interesting new forms of gameplay without drastically rewriting old systems.

Extending time and details

For the fun of it I added an "Extended Room" contrib to Evennia the other night.

("Contribs" are optional code snippets and systems that are not part of the actual codebase. They are intended for you to use or dissect as you like in your game development efforts).

The ExtendedRoom is a room typeclass meant to showcase some more advanced features than the default one. Its functionality is by all means nothing revolutionary in MUD-world, but it was fun and very simple to do using only Evennia's basic building blocks - the whole thing took me some two hours to code, document and clean up for a contrib push. The "ExtendedRoom" contribution has the following features:

• Season-based descriptions. The new Room typeclass will change its overall description based on the time of year (the contrib supports the four seasons, you can hack this as you please). It's interesting from an implementation point of view since it doesn't require any Script or ticker at all - it just checks on-demand, whenever it is being looked at, only updating if the season has actually changed. There is also a general description used as a fallback in case of a missing seasonal one.
• Time-of-day-based descriptions. Within each Season-based description you can embed time-of-day based ones with special tags. The contrib supports four time slots out of the box (morning, afternoon, evening, night). In the description, you just embed time-dependent text within tags, like <morning>Morning sunlight is shining through the windows</morning>. Only time-relevant tags will be shown. This is a simple regular expression substitution, should be easy to expand on if one wants more fine-grained time slots.
• Details. I took the inspiration of these from a MOO tutorial I read a long time ago. "Details" are "virtual" look-targets in the room. It allows you to add visual interest without having to add a bunch of actual objects for that purpose. Details are simply stored in a dictionary on the room. Details don't change with Season in this implementation, but they are parsed for time-of-day based tags!
• Custom commands. The room is supported by extending two of the custom commands. The Details require a slightly modified version of the look command. There is also a new @desc for setting/listing details and seasonal descriptions. The new time command, finally, simply shows the current game time and season in the room.

Installing and testing the snippet is simple - just add the new commands to the default cmdset (they will dynamically replace the same-named default ones), dig a few rooms of the new typeclass and play around! Especially the details do make building interesting rooms a lot more fun (I got hung up playing with them way too long last night).

Coding from the inside

Some time ago, a message on the Evennia mailing list asked about "softcode" support in Evennia. Softcode, a defacto standard in the MUX/MUCK/MUSH/MOO world, is conceptually a "safe" in-game scripting language that allows Players to extend the functionality of things without having access to the server source.

Now, Evennia is meant to be extended by normal Python modules. For coding game systems and advanced stuff, there is really no reason (in my opinion) for a small development team to not use a modern version control system and proper text editors rather than entering things on a command line without formatting.

But there is a potential fun aspect of having an online scripting language - and that is player content creation. Crafters wanting to add some pizazz to their objects, builders getting an extra venue of creativity with their rooms - that kind of thing. I didn't plan to add softcode support to Evennia, but it "sounded like an interesting problem" and one thing led to another.

Python is of course an excellent scripting language from the start. Problem is that it's notoriously tricky to make it run safely with untrusted code - like that inserted by careless or even potentially malignant Players. Scanning the Internet on this topic is a pretty intimidating experience - everywhere you hear that it shouldn't be done, and that the various suggested solutions of a "sandbox" are all inherently unsafe. Python's awesome introspection utilities is its own enemy in this particular case.

For Evennia we are however not looking for a full sandbox. We want a Python-like way for Players to influence a few determined systems. Moreover, we expect short, simple scripts that can do without most of Python's functionality (since our policy is that if it's too complex or large, it belongs in an external Python module). We could supply black-box "safe" functions to hide away complex functionality while still letting people change things we expect them to want to script. This willingness to accept heavy restrictions to the language should work to our advantage, I hope.

Evennia actually already has a safe "mini-language" in the form its "lock system", and thus it was a natural way for me to start looking. A "lock string" has a very restricted syntax - it's basically function calls optionally separated by boolean operators, like this:

lockfunc1(*args) and lockfunc(*args, **kwargs) and not lockfunc2()

The result of this evaluation will always be a boolean True/False (if the lock is passed or not). Only certain functions are available to use (controlled by the coder). The interesting thing is that this string can be supplied by the Player, but it is not evaluated - rather it's manually parsed, from left to right. The function names and arguments are identified (as for the rest, only and/or/not are allowed). The functions are then called explicitly (in Python code, not evaluated as a string) and combined to get a result. This should be perfectly safe as long as your functions are well-defined.


For the potential softcode language, I first took this hands-on approach - manually parsing the string into its components. I got a pretty decent demo going, but the possibilities are much larger than in the simple lockstring case. Just parsing would be one thing, but then to also make sure that each part is okay to use too is another matter ... It would probably be doable, but then I got to supplying some sort of flow-control. The code starts to become littered with special cases which is never a good sign.

So eventually I drifted off from the "lock-like" approach and looked into Python's ast module. This allows you to view Python code as an "abstract syntax tree" (AST).  This solves the parsing issues but once you start dealing with the AST tree you are sort of looking at the problem from the other end - rather than parsing and building the script from scratch it more becomes a matter of removing what is already there (an AST tree can be compiled directly back into Python code after all). It nevertheless seemed like the best way forward.

Testing a few different recipes from the web, I eventually settled on an approach which (with some modifications compared to the original) uses a whitelist (and a blacklist for some other things) to only allow a given set of ast nodes and items in the execution environment. It walks the AST tree before execution and kills dangerous Python structures in one large swath. I expanded on this a fair bit, cutting away a lot of Python functionality for our particular use case. Stuff like attribute acces and assignments, while loops and many other Pythonesque things went out the window.

Around this highly stunted execution system I then built the Evennia in-game scripting system. This includes in-game commands as well as scriptable objects with suitable slots defining certain functionality the Player might want to change. Each Evennia developer can also supply any set of "safe" blackbox functions to offer more functionality to their Player-coders.

A drawback is the lack of a functional timeout watchdog in case of a script running too long. I'm using Twisted's deferToThread to make sure the code blocks as little as possible, but alas, whereas I can check timeouts just fine,  the problem lies in reliably killing the errant sub-thread. Internet experts suggest this to be tricky to do safely at the best of times (with threads running arbitrary code at least), and not wanting to kill the Twisted server is not helping things. I pondered using Twisted's subprocess support, but haven't gotten further into that at this point. Fact is that most of the obvious DOS attack vectors (such as the while loop and huge powers etc) are completely disabled, so it should hopefully not be trivial to DOS the system (famous last words).

I've tentatively dubbed the softcode system "Evlang" to differentiate it from our normal database-related "Scripts".
So is Evlang "safe" to use by untrusted evil Players? Well, suffice to say I'm putting it up with a huge EXPERIMENTAL flag, with plenty of warnings and mentions of "on your own risk". Running Evennia in a chroot jail and with minimum permissions is probably to recommend for the security paranoid. Hopefully Evennia coders will try all sorts of nasty stuff with it in the future and report their finding in our Issue tracker!

But implementation details aside, I must admit it's cool to be able to add custom code like this - the creative possibilities really do open up. And Python - even a stunted version of it - is really very nice to work with, also from inside the game.

Dummies doing (even more) dummy things

This is a follow-up to the Dummies doing dummy things post. I originally posted info about this update on the mailing list some time back, but it has been pointed out to me that it might be a nice thing to put on the dev blog too since it's well, related to development!

I have been at it with further profiling in Evennia. Notably even more aggressive on-demand caching of objects as well as on-object attributes. I found from profiling that there was an issue with how object access checks were done - they caused the lock handler to hit the database every lock check as it retrieved the needed attributes.

Whereas this was not much of a hit per call, access checks are done all the time, for commands, objects, scripts, well everything that might need restricted access.
After caching also attributes, there is no need to hit the database as often. Some commands, such as listing all command help entries do see this effect (although you still probably wouldn't notice it unless you checked before and after like I did). More importantly, under the hood I'm happy to see that the profile for normal Evennia usage is no longer dominated by Django db calls but by the functional python code in each command - that is, in code that the end user have full control over anyway. I'd say this is a good state of affairs for a mud creation system.

 
In the previous "Dummies ..." post I ran tests with rather extreme conditions - I had dummy clients logging to basically act like heavy builders.  They dug rooms, created and defined objects randomly every five seconds (as well as walking around, reading help files, examining objects and other spurious things). In that post I found that my puny laptop could handle about 75 to 100 such builders at a time without me seeing a slowdown when playing. My old but more powerful desktop could handle some 200 or so.

Now, I didn't re-run these build-heavy tests with the new caches in place. I imagine the numbers will improve a bit, but it's just a guess. By all means, if you expect regularly having more than 100 builders on your game continuously creating 250 new rooms/objects per minute, do get back to me ...

... Instead I ran similar tests with more "normal" client usage. That is, I connected dummy clients that do what most players would do - they walk around, look at stuff, read help files and so on. I connected clients in batches of 100 at a time, letting them create accounts and logging in fully before connecting the next set of 100.

All in all I added 1000 dummy clients this way before I saw a noticeable lag on my small laptop. I didn't find it necessary to try the desktop at this point. Whereas this of course was with a vanilla Evennia install, I'd say it should be reasonable room for most realistic mud concepts to grow in.

With the rather extensive caching going on, it is interesting to know what the memory consumption is.
 
This graph shows memory info I noted down after adding each block of 100 players. The numbers fluctuated up and down a bit between readings (especially what the OS reported as total usage), which is why the lines are not perfectly straight.

In the end the database holds 1000 players (which also means there are 1000 Character objects), about as many rooms and about twice as many attributes.  The "idmapper cache" is the mapper that makes sure all Django model instances retain their references between accesses (as opposed to normal Django were you can never be sure of this). "Attribute cache" is a cache storing the attribute objects themselves on the Objects, to avoid an extra database lookup. All in all we see that keeping the entire database in memory takes about 450MB.

Evennia's caching is on-demand (so e.g. a room would not be loaded/cached until someone actually accessed it somehow). One could in principle run a script to clean all cached regularly if one was short on RAM - time will tell if this is something any user needs to worry about on modern hardware.

Shortcuts to goodness

Evennia, being a MUD-design system, needs to take some special considerations with its source code - its sole purpose is after all to be read, understood and extended.
Python is of course very readable by default and we have worked hard to give extensive comments and documentation. But for a new user looking into the code for the first time, it's still a lot of stuff to take in. Evennia consists of a set of Django-style "applications" interacting and in some cases inheriting from each other so as to avoid code duplication. For a new user to get an overview could therefore mean diving into more layers of code than one would like.

I have now gone through the process of making Evennia's API (Application Programming Interface) "flatter". This has meant exposing some of the most commonly used methods and classes at a higher level and fully documenting exactly what they inherit av every layer one looks at. But I have also added a new module ev.py to the root directory. It  implements "shortcuts" to all the most commonly used parts of the system, forming a very flat API. This means that what used to be

      from src.objects.objects import Object

can now be done as 

      from ev import Object

Not only should it be easier to find things (and less boilerplate code to write) but I like that one can also easier explore Evennia interactively this way.  Using a Python interpreter (I recommend ipython) you can just import ev and easily inspect all the important object classes, tab to their properties, helper functions and read their extensive doc strings.

Creating this API, i.e. going through and identifying all the useful entry points a developer will need, was also interesting in that it shows how small the API really is. Most of the ev interface is really various search functions and convenient options to inspect the database in various ways. The MUD-specific parts of the API is really lean, as befits a barebones MUD server/creation system.