The current technology contenders for the server are Stackless Python (and it's eventual successor PyPy) or Erlang.
In both cases, they provide for concurrent processing using the Actor Model, which we have already determined is our preferred development model (remember, there are no correct models, just more and less useful ones, and this is our useful model). Stackless and Erlang both provide native support for this model, using the tasklet in Stackless and the process in Erlang.
In terms of real-world implementations, Stackless is being successfully used by Eve Online for both the server and the client. Eve Online shares one important philosophy that we want to emulate as well: no sharding of the user population. Python is an incredibly efficient language to program in, sharing strong points of both object oriented and functional languages. Our programmers (me and two others) are also much more familiar with procedural, object oriented programming. All things being equal, we would certainly choose Stackless Python.
But all things aren't equal. Python suffers from a significant barrier in its scalability: the Global Interpreter Lock (the "GIL") prevents Python from scaling to multiple CPUs efficiently. Even without the GIL, multiple processors yield a significant complexity challenge as you deal with locks and concurrent threads.
In fact, that can be seen in Eve Online, as they struggle to deal with very large population centers overwhelming the ability to Stackless to scale horizontally. Stackless Python scales horizontally for multiple processes that don't share state (such as multiple zones in a game), but within a single state construct, you are limited to a single CPU.
This is where Erlang comes in. Built from the ground-up to support concurrency, both on a single machine and on multiple machines, it doesn't suffer from the state construct problems. Erlang is a functional language, and one of the implications of that is there is no state to be shared between processes. Processes can be placed "anywhere", and all state communication between processes is done via messages.
In terms of the real-world, which is an important consideration for me, Erlang came out of the Swedish telecom giant Ericsson as a solution for building telecom switches that required huge scalability and Nine "9's" of reliability. For myself, coming from a Java background, seeing what it takes to hit five "9's", that is a huge motivator. Another large example is Amazon's SimpleDB being written in Erlang.
In terms of gaming, Open Poker was recently re-written using Erlang and Vendetta Online rewrote their AI functionality using Erlang.
So, the downsides...
- There isn't an established body of work to follow in the game
- We don't have any experience writing systems using functional
- There are few to no existing libraries to simplify our lives.
On the other hand, the upsides are significant...
- Highly scalable across multiple processors and multiple
systems, for free.
- Ability to hot-swap code, to patch the system without shutting it
- A database
supports the highly concurrent, distributed environment that Erlang
In short, the benefits are all around the concurrency and
scalability. Personally, I want to see an environment where the
number of actors (people, NPCs, etc) is not arbitrarily limited.
Right now, I'm heavily leaning towards Erlang.