Since the mod community seems to be moving full speed ahead and you've already got some basic mods up and running I thought I’d explain the way the Empire modding system is intended to work. This should enable you to work with it, not against it, and hopefully avoid breaking things released by CA in the future.
First of all, the ETW data is meant to stay packed. The pack file system and the virtual file system that it builds internally within the game are central to the patching and modding systems, so please don’t ask users to unpack their data in order to run with your mods.
So how should you mod the game? Well, it works like this. Empire loads –all– files of type .pack in the data directory in a certain order determined by the pack’s type. There are three basic types – release, patch and mod. The fifth byte in the pack file header is an numerical id which tells the game what a pack is, release packs are 0 or 1, patch packs are 2, mod packs are 3. The game then loads release packs first, then patch packs, then mod packs.
At game startup, the file entries in all of these packs are mounted into a virtual file system, and if the game finds an entry in a later pack which has the same name and location as an already existing entry, the new entry replaces the old. This means that a mod pack can replace any file in the vfs and add new ones as well, without asking the person who downloaded the mod to overwrite files on disk and destroy data.
So basically, a mod is intended to be a single “mod pack” file, and installing a mod is just placing that pack inside the game’s data directory, and uninstalling it is removing it. There’s a little bit more to it than that, for example there is a mechanisms to control mod pack load order, but that’s it in a nutshell.
Now, a note on how to work with the databases. You’ll have noticed that each table in the databases (inside the db folder) is both a folder and a binary file. When the game instantiates the databases, which happens after the virtual file system is built, it scans each directory and parses –all– of the binary files it finds. This means that a mod pack can add a new binary data fragment inside the db/unit_stats_land_table folder which adds several new units, and the game will add them to its land units database. The fragments are processed in the same order as the pack files, and any new database records with the same name as an old record will cause the new to replace the old. This means that you can replace any record within the database without touching the rest of the data or modifying the CA-supplied files on disk.
For example, say the CA packs contain a unit record for ‘french_dragoons’, which is one entry out of hundreds inside the shipped unit_stats_land_table data fragment, which is used to build the unit table that was available at launch. Now say you create a mod pack which places a new data fragment, say called my_mods_unit_stats_land inside db/ unit_stats_land_table, which contains just one data record for ‘french_dragoons’. What will happen is your new record will end up replacing the current record for that unit in the game data, and the rest will stay as it was originally.
So… hopefully you’ll now have a good idea of how the modding system was architected, and how you should build tools to work with it. There’s also another interesting consequence of how the system works, which is that it opens the door for mods built on top of other mods…
One more thing to keep in mind is that we've not yet finished testing the mod support fully - that's part of what's happening here before the mod dev package gets released. Also we’re aware that there is currently an upper bound of about 6000 files in a single pack. And you should know that there is some overhead associated with mounting a pack, and a data directory filled with scores of them will make the game startup time significantly longer.
The other thing I should mention – since you’re going to be experimenting with this – is that the game checks the final database integrity after loading mods. So if you add a pack file which includes a new unit record which doesn’t include a display model or is invalid in some other way, the game will unload that mod pack (though it keeps the rest). This isn’t the main error checking mechanism in the ETW data pipeline, that’s done elsewhere, but as modders you should be aware of it. Unfortunately there currently isn’t any log output on the reasons why the mod failed to load, we’re looking into adding that when the modding dev kit is released.
Armed with this info you’ll at least be able to work with the system as it was intended, without having to wait for the mod dev package. I have to say it's great seeing you guys making headway so quickly. The pack and database inspection tool was a great start, I think, and really bodes well for the future of the Empire modding scene.