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socketsasync-await.net-4.5tcplistener

.Net 4.5 TCP Server scale to thousands of connected clients

I need to build a TCP server using C# .NET 4.5+, it must be capable of comfortably handling at least 3,000 connected clients that will be send messages every 10 seconds and with a message size from 250 to 500 bytes.

The data will be offloaded to another process or queue for batch processing and logging.

I also need to be able to select an existing client to send and receive messages (greater then 500 bytes) messages within a windows forms application.

I have not built an application like this before so my knowledge is based on the various questions, examples and documentation that I have found online.

My conclusion is:

  1. non-blocking async is the way to go. Stay away from creating multiple threads and blocking IO.
  2. SocketAsyncEventArgs - Is complex and really only needed for very large systems, BTW what constitutes a very large system? :-)
  3. BeginXXX methods will suffice (EAP).
  4. Using TAP I can simplify 3. by using Task.Factory.FromAsync, but it only produces the same outcome.
  5. Use a global collection to keep track of the connected tcp clients

What I am unsure about:

  1. Should I use a ManualResetEvent when interacting with the TCP Client collection? I presume the asyc events will need to lock access to this collection.
  2. Best way to detect a disconnected client after I have called BeginReceive. I've found the call is stuck waiting for a response so this needs to be cleaned up.
  3. Sending messages to a specific TCP Client. I'm thinking function in custom TCP session class to send a message. Again in an async model, would I need to create a timer based process that inspects a message queue or would I create an event on a TCP Session class that has access to the TcpClient and associated stream? Really interested in opinions here.
  4. I'd like to use a thread for the entire service and use non-blocking principals within, are there anythings I should be mindful of espcially in context of 1. ManualResetEvent etc..

Thank you for reading. I am keen to hear constructive thoughts and or links to best practices/examples. It's been a while since I've coded in c# so apologies if some of my questions are obvious. Tasks, async/await are new to me! :-)

Solution

  • I need to build a TCP server using C# .NET 4.5+

    Well, the first thing to determine is whether it has to be base-bones TCP/IP. If you possibly can, write one that uses a higher-level abstraction, like SignalR or WebAPI. If you can write one using WebSockets (SignalR), then do that and never look back.

    Your conclusions sound pretty good. Just a few notes:

    SocketAsyncEventArgs - Is complex and really only needed for very large systems, BTW what constitutes a very large system? :-)

    It's not so much a "large" system in the terms of number of connections. It's more a question of how much traffic is in the system - the number of reads/writes per second.

    The only thing that SocketAsyncEventArgs does is make your I/O structures reusable. The Begin*/End* (APM) APIs will create a new IAsyncResult for each I/O operation, and this can cause pressure on the garbage collector. SocketAsyncEventArgs is essentially the same as IAsyncResult, only it's reusable. Note that there are some examples on the 'net that use the SocketAsyncEventArgs APIs without reusing the SocketAsyncEventArgs structures, which is completely ridiculous.

    And there's no guidelines here: heavier hardware will be able to use the APM APIs for much more traffic. As a general rule, you should build a barebones APM server and load test it first, and only move to SAEA if it doesn't work on your target server's hardware.

    On to the questions:

    Should I use a ManualResetEvent when interacting with the TCP Client collection? I presume the asyc events will need to lock access to this collection.

    If you're using TAP-based wrappers, then await will resume on a captured context by default. I explain this in my blog post on async/await.

    There are a couple of approaches you can take here. I have successfully written a reliable and performant single-threaded TCP/IP server; the equivalent for modern code would be to use something like my AsyncContextThread class. It provides a context that will cause await to resume on that same thread by default.

    The nice thing about single-threaded servers is that there's only one thread, so no synchronization or coordination is necessary. However, I'm not sure how well a single-threaded server would scale. You may want to give that a try and see how much load it can take.

    If you do find you need multiple threads, then you can just use async methods on the thread pool; await will not have a captured context and so will resume on a thread pool thread. In this case, yes, you'd need to coordinate access to any shared data structures including your TCP client collection.

    Note that SignalR will handle all of this for you. :)

    Best way to detect a disconnected client after I have called BeginReceive. I've found the call is stuck waiting for a response so this needs to be cleaned up.

    This is the half-open problem, which I discuss in detail on my blog. The best way (IMO) to solve this is to periodically send a "noop" keepalive message to each client.

    If modifying the protocol isn't possible, then the next-best solution is to just close the connection after a no-communication timeout. This is how HTTP "persistent"/"keep-alive" connections decide to close. There's another possibile solution (changing the keepalive packet settings on the socket), but it's not as easy (requires p/Invoke) and has other problems (not always respected by routers, not supported by all OS TCP/IP stacks, etc).

    Oh, and SignalR will handle this for you. :)

    Sending messages to a specific TCP Client. I'm thinking function in custom TCP session class to send a message. Again in an async model, would I need to create a timer based process that inspects a message queue or would I create an event on a TCP Session class that has access to the TcpClient and associated stream? Really interested in opinions here.

    If your server can send messages to any client (i.e., it's not just a request/response protocol; any part of the server can send messages to any client without the client requesting an update), then yes, you'll need a proper queue of outgoing requests because you can't (reliably) issue multiple concurrent writes on a socket. I wouldn't have the consumer be timer-based, though; there are async-compatible producer/consumer queues available (like BufferBlock<T> from TPL Dataflow, and it's not that hard to write one if you have async-compatible locks and condition variables).

    Oh, and SignalR will handle this for you. :)

    I'd like to use a thread for the entire service and use non-blocking principals within, are there anythings I should be mindful of espcially in context of 1. ManualResetEvent etc..

    If your entire service is single-threaded, then you shouldn't need any coordination primitives at all. However, if you do use the thread pool instead of syncing back to the main thread (for scalability reasons), then you will need to coordinate. I have a coordination primitives library that you may find useful because its types have both synchronous and asynchronous APIs. This allows, e.g., one method to block on a lock while another method wants to asynchronously block on a lock.

    You may have noticed a recurring theme around SignalR. Use it if you possibly can! If you have to write a bare-bones TCP/IP server and can't use SignalR, then take your initial time estimate and triple it. Seriously. Then you can get started down the path of painful TCP with my TCP/IP FAQ blog series.