Is OCaml suitable to write networking servers?

Question

I was wondering if OCaml will perform well in terms of performance and ease of implementation while dealing with typical client/server interactions over TCP in a multi threaded environment.. I mean something really typical like having a thread per client that receives data, operated changes on game states and send them back to clients.

This because I need to write a server for a game and I always did these things in C but since now I know OCaml I was curious to know if it would be ok or I'll just find myself trying to solve a typical problem in a language that doesn't fit well that.

Answer 1

I'm so sorry this question has been sitting here for eight years with what I consider to be several quite bad answers because they all ignore the elephant in the room.

You say "really typical like having a thread per client" but having an OS thread per client is an extremely bad design. Threads are heavyweight, taking a long time to create and destroy and consuming ~1MB just for the thread stack. If you have one thread per connection then 1,000 simultaneous client connections (which is entirely feasible) will burn 1GB of RAM just for their stacks and the performance of your program (in any language) will be cripppled by the amount of context switching required to get any work done. You don't want to use that design in any language including both C and OCaml. Note that this problem is especially bad in the context of tracing garbage collected languages because the GC also traverses all of those thread stack in order to collate global roots before every GC cycle. I am the first to admit that this anti-pattern is ubiquitous in the real world but please don't copy it! I have seen "low latency" servers in the finance industry written in C++ using one thread per connection and they suffered latency stalls of up to six seconds just from the (Windows) OS servicing those threads.

See: http://people.eecs.berkeley.edu/~sangjin/2012/12/21/epoll-vs-kqueue.html

Let's consider an efficient design instead, like an epoll or kqueue interface to the OS kernel giving the server's code information about incoming and outgoing data buffers. Single threaded servers can attain excellent performance with this design. However, a typical server has serialization work to do per client and some core work that is often performed in serial across all client connections. Therefore, serialization and deserialization can be parallelized but the core server operation cannot. In this context, OCaml is great for everything except the serialization layer because it has poor support for parallelism.

I have personally implemented many servers for various industries with hugely varying performance requirements. In my experience, OCaml is one of the best tools for this because it offers excellent libraries (easy to use and reliable) and excellent serial performance. The only issue I have is around parallelizing the serialization layer but, in practice, I have found that OCaml runs circles around alternatives like Java and .NET even though they can parallelize this. I found typical latencies were ~100us for .NET and 10us for OCaml.

See also: http://prl.ccs.neu.edu/blog/2016/05/24/measuring-gc-latencies-in-haskell-ocaml-racket/

Answer 2

Performance: probably not. OCaml's threads do not provide parallel execution, they are only a way to structure your program. The OCaml runtime itself is not thread-safe, so the only code that could possibly execute in parallel of a single OCaml thread would be interfaced C code (without callbacks to OCaml!).

Implementation-wise, there is a mutex on the run-time, which is released when calling blocking C primitives, and could also be released when calling C functions that do significant work.

Ease of implementation: it wouldn't be world-changing. You would have the comfort of OCaml and a pthread-like library on the side. If you are looking for new things to discover while leveraging what you have learnt of OCaml, I recommend Jocaml. It goes in and out of sync with OCaml, but there was a (re-)re-implementation quite recently, and even when it is slightly out of sync, it is a lot of fun, and a completely new perspective of concurrent programs.

Jocaml is implemented on top of OCaml. What with the run-time not being concurrent and all, I am almost sure it uses separate processes and message-passing. But for the application that you mentioned it should be able to do fine.

Answer 3

OCaml is quite suitable for writing network servers, although as Pascal observes, there are limitations on threading.

Fortunately, however, threading isn't the only way to organize such a program. The Lwt library (for Light Weight Threads) provides an abstraction of asynchronous I/O that is quite easy to use (particularly when combined with a bit of syntax support). Everything actually runs in one thread, but it's all driven by an asynchronous I/O loop (built on the Unix select call), and the programming style lets you write code that looks like direct code (avoiding much of the normal code overhead of doing asynchronous I/O in many other languages). For example:

lwt my_message = read_message socket in
let repsonse = compute_response my_message in
send_response socket response

Both the read and the write happen back in the main event loop, but you avoid the normal "read, calling this function when you're done" manual overhead.

Answer 4

OCaml will work great for networking applications as long as you can live with a relatively small number of threads active at one time—say no more than 100. You could consider MLdonkey as an example, although in the client space, not in the server space.