I am currently building a terminal based chat system where I use :gen_tcp.

I have a TcpServer that accepts connections, and a TcpClientPool, which is an Agent, that saves each client in the TcpClientPool own state. Basically, the TcpServer retrieves the TcpClientPool states to get all of the current connected clients, and broadcasts one messsage from one client to all other clients. The TcpClientPool accepts an unlimited number of clients.

However, in Going low level with TCP sockets and :gen_tcp, Orestis Markou mentions the need for implementing a pool of connections that accepts a limited number of clients. I remember also having read in the Elixir in Action book that implementing a limited number of connections for the to-do application was the way to go.

Why would one do that?

I have read the Elixir School’s post on Poolboy:

Let’s think of a specific example for a moment. You are tasked to build an application for saving user profile information to the database. If you’ve created a process for every user registration, you would create unbounded number of connections. At some point the number of those connections can exceed the capacity of your database server. Eventually your application can get timeouts and various exceptions.

But I still don’t understand:

• I thought the BEAM could handle millions of concurrent processes, so I’m quite confused; if the BEAM can handle millions of concurrent processes, then I don’t need to implement a pool of processes, unless I plan to have millions of them running concurrently right?
• Or is it simply that my TcpServer process cannot handles that many concurrent connections?
• If so how to know the maximum of connections that my TcpServer can handles?