What is the practical / hard limit on socket connections per server

I have a number of client devices that open socket connection exposed by a service running o开发者_StackOverflow社区n a Windows 2008 R2 server. I'm wondering if what is hard limit on the number of concurrent client connections.

According to this article, one hard limit is (was) 16,777,214. The practical limit depends on your application also: for example, if you create a thread per connection, then the practical limit comes from the limitation in the number of threads more than from the network stack. There is also a limit on the number of handles any process may have, and so on.

Assuming you select a sensible architecture for your server then the limit will be memory and cpu related. IMHO you'll never reach the hard limit that Martin mentions :)

So, rather than worrying about a theoretical limit that you'll never hit you should, IMHO, be thinking about how you will design your application and how you will test it to determine the current maximum number of client connections that you can maintain for your application on given hardware. The important thing for me is to run your perf tests from Day 0 (see here for a blog posting where I explain this). Modern operating systems and hardware allow you to build very scalable systems but simple day to day coding and design mistakes can easily squander that scalability and so you simply MUST run perf tests all the time so that you know when you are building in road blocks to your performance. You simply cannot go back and fix these kind of mistakes at the end of the project.

As an aside, I ran some tests on Windows 2003 Server with a low spec VM and easily achieved more than 70,000 concurrent and active connections with a simple server based on an overlapped I/O (I/O completion port) based design. See this answer for more details.

My personal approach would be to get a shell of a server put together quickly using whatever technology you decide on (I favour unmanaged C++ using I/O Completion Ports and minimal threads), see this blog posting for more details. Then build a client or series of clients that can stress test the application and keep updating and running the test clients as you implement your server logic. You would expect to see a gradually declining curve of maximum concurrent clients as you add more complexity to your server; large drops in scalability should cause you to examine the latest check ins to look for unfortunate design decisions.