Ranch is an OTP application.

Like all OTP applications, Ranch has a top supervisor. It is responsible for supervising the ranch_server process and all the listeners that will be started.

The ranch_server gen_server is a central process keeping track of the listeners and their acceptors. It does so through the use of a public ets table called ranch_server. The table is owned by the top supervisor to improve fault tolerance. This way if the ranch_server gen_server fails, it doesn’t lose any information and the restarted process can continue as if nothing happened.

Ranch uses a custom supervisor for managing connections. This supervisor keeps track of the number of connections and handles connection limits directly. While it is heavily optimized to perform the task of creating connection processes for accepted connections, it is still following the OTP principles and the usual sys and supervisor calls will work on it as expected.

Listeners are grouped into the ranch_listener_sup supervisor and consist of three kinds of processes: the listener gen_server, the acceptor processes and the connection processes, both grouped under their own supervisor. All of these processes are registered to the ranch_server gen_server with varying amount of information.

All socket operations, including listening for connections, go through transport handlers. Accepted connections are given to the protocol handler. Transport handlers are simple callback modules for performing operations on sockets. Protocol handlers start a new process, which receives socket ownership, with no requirements on how the code should be written inside that new process.

The second argument to ranch:start_listener/5 is the number of processes that will be accepting connections. Care should be taken when choosing this number.

First of all, it should not be confused with the maximum number of connections. Acceptor processes are only used for accepting and have nothing else in common with connection processes. Therefore there is nothing to be gained from setting this number too high, in fact it can slow everything else down.

Second, this number should be high enough to allow Ranch to accept connections concurrently. But the number of cores available doesn’t seem to be the only factor for choosing this number, as we can observe faster accepts if we have more acceptors than cores. It might be entirely dependent on the protocol, however.

Our observations suggest that using 100 acceptors on modern hardware is a good solution, as it’s big enough to always have acceptors ready and it’s low enough that it doesn’t have a negative impact on the system’s performances.

Some socket options are platform-specific and not supported by inet. They can be of interest because they generally are related to optimizations provided by the underlying OS. They can still be enabled thanks to the raw option, for which we will see an example.

One of these features is TCP_DEFER_ACCEPT on Linux. It is a simplified accept mechanism which will wait for application data to come in before handing out the connection to the Erlang process.

This is especially useful if you expect many connections to be mostly idle, perhaps part of a connection pool. They can be handled by the kernel directly until they send any real data, instead of allocating resources to idle connections.

To enable this mechanism, the following option can be used.

It means go on layer 6, turn on option 9 with the given integer parameter.