Realm Tactics runs all of its game system through downloaded config files. All terrain, units, attacks, sprites, AI config and map config are just files that need to be loaded for the game to work. The game client itself is completely divorced from all of that – not a single game mode included out of the box.
It may be tempted to shove all of it in a big zip file which can be downloaded easily, unzipped and give all that is needed to play the game; making sure its all perfectly intact and internally consistent. However, that is a very static way of doing it and clients would need to either download the whole lot again when an update is make or to become stale and miss latest updates.
At the other end of the scale, one could download each file individually when its needed. If this get performed each and every time then obviously the config would always be updated but that would be a load of downloads! We clearly need some compromise solution…
Meta Files
Our system is build on a bunch of meta files. These files as the name suggests contain the meta information about other files which should be present. For the most part, there is a meta file at each level of the config folder structure. In reality its fuzzier as meta files can reference other directories and also can reference sub-meta files (meta files in other locations), but for the most part consider that each directory (folder in the games file system) contains a meta file.
These meta files are extremely small comma separated data files that detail what other files should be present – one file reference per line. When the user navigates to the menu that is linked to a directory, the game makes a quick call and downloads the latest meta file. This is a very lightweight call but lets the game check what the state of the game should look like. Each meta file entry has a version number. The existing version number of an entry is compared with the new version number of the new downloaded meta file. If the version has updated, this indicates to the game that the file has changed and it can call and get just the updated files that have changed. As most files will remain unchanged session to session, most files wont have an updated version number so no action is needed. As you can imagine, this hits a good balance with keeping the files updated with not making too many calls for files. But as some of you may be thinking, that is still a lot of individual files that will need downloading when the game is first loaded of if there has been a major change, affecting a bunch of files. There needs to be a way of cutting down the number of downloads for large updates…
Downloading Blobs
As mentioned at the beginning of the article, having a big zip with all of the files is a great way of getting a large number of config files at once. So, we have a happy hybrid approach. The meta system describe above is still in play but an entry in a meta file can represent a zip file to download and extract. This will of course overwrite all files that are conflicts. What is clever though is that the zip file entry will act in exactly the same way as regular files, having a version number and only download if the latest meta file indicates that it has been updated. The zip file itself will also contain all of the files and the associated meta files which has a record of the latest version codes for its contained files.
So imagine a big update has been made. A new zip file is made which contains a bunch of files and meta files – all having their latest version codes. The game client downloads the zip and extracts it and puts in all the files that it needs. The game will no longer re-download the zip as its meta file already shows the latest version code. However, shock horror one of the files in the zip contains an error and an update needs to be maid! No worries, the zip file on the game config server doesn’t need updating, only the individual file that needs fixing and its individual meta version code bumped up. The game client will know that it has the correct zip, but sees the new version of the specific file and only that file is updated.
Win-win! We have a great amount of control to create zip files containing a bunch of config whilst also having the ability to make small tweaks here and there as needed. All the while the game clients only download what is needed and is always kept up to date!