A warning of sorts: this will contain **TECHNICAL INFORMATION** so if you are scared of, or allergic to acronyms with three letters consult your doctor before reading on. No, please do not trouble your doctor with such trivialities; but seriously, it's dangerously close to a 'dev diary' and it's going to be full of words about how internet systems work. although I consider myself a 'good' writer this may not be interesting to you at all and don't expect me to be giving away hats or something at the end, it's just an interesting talk about the technical structure of the Steam economy.

One of the quirks of Geothermal that allows it to work is that it uses Valve's Community APIs (an API, by the by is a construct that communicates data between applications) – the APIs used by the trading, inventory and offers system. These APIs work pretty much completely differently to the normal APIs that other less underground applications use, but support all games and all languages Valve supports. However, they are not documented at all and speak in arcane tongues using terms such as "classid", "instanceid" and "assetid".

One of the important aspects of Geothermal is software aimed at real people rather than the quasi-real programmer people who become confused and enraged when you quote such character sequences as "while (tRue { print {1)" even now I am having to surpress a visceral urge to fix the bugs in that quote. Regardless, catering to real people means having a real UI, a list of items to select from. The websites you're familiar with do this for TF2, DotA 2 and CS:GO by inspecting the 'item schema' for each game, which gives a comprehensive list of the archetypal items on which qualities, strange parts and other seven letter words are applied.

The community APIs have no kind of schema. In fact, they have no way of uniquely identifying a class of items in the sense of the 'definition indexes' you find with the normal APIs that represent, uniquely, the item by its order of definition in the game code, either.

Instead, you have these mysterious "classid"s, which don't represent an item definition in the game code, but are actually dynamically generated and represent only a unique set of item parameters upon item creation. When the item is created, the item servers check if an item with those parameters already exists; if one does not, a new classid is blessed unto us by the powers that be.

This means that there is no real notion of an 'item' in the TF2 sense; there are only different bundles of attrubutes, over 20 million just for TF2 to be exact, and over 200 million of the yet unmentioned instanceids that handle slight deviations such as strange counts, which is several hundreds of thousands more definitions than with the classical schema-based method of item storage.

This seems like lunacy, and it is, but look through this kaleidoscope and you will understand why things are as they are: this setup means that the economy can support any kind of item with any kind of attributes, allowing game creators to throw whatever they want into the Steam economy without having to do any kind of preparatory work.

From the sounds of this archetecture, this change likely came with a transition to a Key/Value store – a key-value store being the simplest and fastest type of database: one with just keys and values, and by the sounds of it, it just might be Mongo (please don't use Mongo, it has a R/W mutex and is difficult to scale for medium large datasets).

Pushing the intricate internals of Valve's systems aside with a sigh of relief, let's dive back into our original question: if the most terse list of items Valve could provide (and they don't for obvious reasons) is over 20 million items in length, how do I make a list of items for users to select from? Actually, scratch that, let me toss another stone at the beehive: if classids are bound not to the concept we know as an item, but instead to attributes, we can't compare these numbers to see if the items we're looking at are the same regardless of attributes: you know, maybe sometimes you don't care about an item's level. How do we express trades for 'a hat called The Hat With No Name'?

I will allow those questions to turns slowly and crisp in a garnish of honey and cloves in your mind while we discuss how TF2O and bazaar.tf, the trading websites which defiant of the problems I just posed, allow you to trade games, which are Steam items and therefore schema-less (as previously mentioned, this means there is no list of all Steam items that are tradable from Valve). These websites work by canvassing, collecting the inventories of their users and using those to build their own comprehensive lists of all the Steam items, avoiding having to use the Community APIs in all but loading inventories, and even when loading inventories the mysterious class and instanceids are ignored.

I considered doing this, but Geothermal is not a website; it is a Chrome plugin, and I don't want my chrome plugin having to rely on, and report to (which has privacy concerns, since I have your Steam account) my server.

So, my decision was to bite the bullet and enjoy it– it probably won't present you with all the items, but it will allow you to load inventories and use that to supplement your item listings. But, I'm not going to use the items themselves for trades, as I mentioned before classids are pretty petty when it comes to comparisons with others. The exchanges that Geothermal lists are actually a series of checks which will be shown to the user and will be editable. Thus, a trade for a key is defined, and can be defined as a trade for an item with the icon of a key.

I'm currently working on the interface for this, and it kills many birds with that one stone we tossed into the beehive before: it allows support for all Valve supported languages, games past present and future, doesn't need to be updated when games update, and allows much greater user control (for example looking for an item with a given level or set of levels).

You might be wondering how I did this before I moved onto this idea. I asked the Valve servers for the prices of all the items on the Steam marketplace, then used those identifiers to lookup the information on those items.

You got this far. I am impressed.

Also I might have lied[tf2r.com].