Specs

terminate(reason, state :: term()) :: term()
when reason: :normal | :shutdown | {:shutdown, term()} | term()

Invoked when the server is about to exit. It should do any cleanup required.

reason is exit reason and state is the current state of the GenServer. The return value is ignored.

terminate/2 is called if the GenServer traps exits (using Process.flag/2) and the parent process sends an exit signal, or a callback (except init/1) does one of the following:

• returns a :stop tuple
• raises (via Kernel.raise/2) or exits (via Kernel.exit/1)
• returns an invalid value

If part of a supervision tree, a GenServer will receive an exit signal when the tree is shutting down. The exit signal is based on the shutdown strategy in the child's specification, where this value can be:

• :brutal_kill: the GenServer is killed and so terminate/2 is not called.

• a timeout value, where the supervisor will send the exit signal :shutdown and the GenServer will have the duration of the timeout to terminate. If after duration of this timeout the process is still alive, it will be killed immediately.

For a more in-depth explanation, please read the "Shutdown values (:shutdown)" section in the Supervisor module.

If the GenServer receives an exit signal (that is not :normal) from any process when it is not trapping exits it will exit abruptly with the same reason and so not call terminate/2. Note that a process does NOT trap exits by default and an exit signal is sent when a linked process exits or its node is disconnected.

Therefore it is not guaranteed that terminate/2 is called when a GenServer exits. For such reasons, we usually recommend important clean-up rules to happen in separated processes either by use of monitoring or by links themselves. There is no cleanup needed when the GenServer controls a port (for example, :gen_tcp.socket) or File.io_device/0, because these will be closed on receiving a GenServer's exit signal and do not need to be closed manually in terminate/2.

If reason is neither :normal, :shutdown, nor {:shutdown, term} an error is logged.

This callback is optional.