Nine Nines: Internals

Internals

This chapter may not apply to embedded Ranch as embedding allows you +to use an architecture specific to your application, which may or may +not be compatible with the description of the Ranch application.

Note that for everything related to efficiency and performance, +you should perform the benchmarks yourself to get the numbers that +matter to you. Generic benchmarks found on the web may or may not +be of use to you, you can never know until you benchmark your own +system.

Architecture

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.

Number of acceptors

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.

Platform-specific TCP features

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.

Using raw transport options

{raw, 6, 9, << 30:32/native >>}

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

99s

Internals

Architecture

Number of acceptors

Platform-specific TCP features

+ Ranch + 1.4 + + User Guide +

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