Worker Restart Options

Up till now, we’ve seen how the supervision strategy tells the supervisor to deal with the death of a child process.

There’s a second level of configuration that applies to individual workers. The most commonly used of these is the :restart option.

Previously, we said that a supervisor strategy (such as :one_for_all) is invoked when a worker dies. That’s not strictly true. Instead, the strategy is invoked when a worker needs restarting. And the conditions when a worker should be restarted are dictated by its restart: option:

• If :permanent, this worker should always be running. it’s permanent. This means that the supervision strategy will be applied whenever this worker terminates, for whatever reason.

• If :temporary, this worker should never be restarted, so the supervision strategy is never applied if this worker dies.

• If :transient, it’s expected that this worker will at some point terminate normally, and this termination shouldn’t result in a restart. However, should this worker die abnormally, then it should be restarted by running the supervision strategy. The simplest way to specify the restart option for a worker is in the worker module. And we add it to the use GenServer (or use Supervisor) statement:

  defmodule Convolver do
     use GenServer, restart: :transient 
     #...
  

A little more detail

We don’t have to know the information in this section the first time around, but as someone always asks, “We’ve seen that you start a supervisor by passing it a list of children. But what is that list?”

At the very lowest level, it’s a list of child specifications. A child spec is an Elixir map. It describes which function to call to start the worker, how to shut the worker down, the restart strategy, the worker type, and any modules apart from the main module that forms part of the worker. We can create a child spec map using the Supervisor.child_spec/2 function.