5 Supervisor Behaviour

This section should be read in conjunction with supervisor(3), where all details about the supervisor behaviour is given.

5.1 Supervision Principles

A supervisor is responsible for starting, stopping and monitoring its child processes. The basic idea of a supervisor is that it should keep its child processes alive by restarting them when necessary.

Which child processes to start and monitor is specified by a list of child specifications. The child processes are started in the order specified by this list, and terminated in the reversed order.

5.2 Example

The callback module for a supervisor starting the server from the gen_server chapter could look like this:

-module(ch_sup).
-behaviour(supervisor).

-export([start_link/0]).
-export([init/1]).

start_link() ->
    supervisor:start_link(ch_sup, []).

init(_Args) ->
    {ok, {{one_for_one, 1, 60},
          [{ch3, {ch3, start_link, []},
            permanent, brutal_kill, worker, [ch3]}]}}.

one_for_one is the restart strategy.

1 and 60 defines the maximum restart frequency.

The tuple {ch3, ...} is a child specification.

5.3 Restart Strategy

one_for_one

If a child process terminates, only that process is restarted.

Figure 5.1: One_For_One Supervision

one_for_all

If a child process terminates, all other child processes are terminated and then all child processes, including the terminated one, are restarted.

Figure 5.2: One_For_All Supervision

rest_for_one

If a child process terminates, the 'rest' of the child processes -- i.e. the child processes after the terminated process in start order -- are terminated. Then the terminated child process and the rest of the child processes are restarted.

5.4 Maximum Restart Frequency

The supervisors have a built-in mechanism to limit the number of restarts which can occur in a given time interval. This is determined by the values of the two parameters MaxR and MaxT in the start specification returned by the callback function init:

init(...) ->
    {ok, {{RestartStrategy, MaxR, MaxT},
          [ChildSpec, ...]}}.

If more than MaxR number of restarts occur in the last MaxT seconds, then the supervisor terminates all the child processes and then itself.

When the supervisor terminates, then the next higher level supervisor takes some action. It either restarts the terminated supervisor, or terminates itself.

The intention of the restart mechanism is to prevent a situation where a process repeatedly dies for the same reason, only to be restarted again.

5.5 Child Specification

This is the type definition for a child specification:

{Id, StartFunc, Restart, Shutdown, Type, Modules}
    Id = term()
    StartFunc = {M, F, A}
        M = F = atom()
        A = [term()]
    Restart = permanent | transient | temporary
    Shutdown = brutal_kill | integer()>0 | infinity
    Type = worker | supervisor
    Modules = [Module] | dynamic
        Module = atom()

Id is a name that is used to identify the child specification internally by the supervisor.
StartFunc defines the function call used to start the child process. It is a module-function-arguments tuple used as apply(M, F, A).

It should be (or result in) a call to supervisor:start_link, gen_server:start_link, gen_fsm:start_link or gen_event:start_link. (Or a function compliant with these functions, see supervisor(3) for details.
Restart defines when a terminated child process should be restarted.
- A permanent child process is always restarted.
- A temporary child process is never restarted (not even when the supervisor's restart strategy is rest_for_one or one_for_all and a sibling's death causes the temporary process to be terminated).
- A transient child process is restarted only if it terminates abnormally, i.e. with another exit reason than normal, shutdown or {shutdown,Term}.
Shutdown defines how a child process should be terminated.
- brutal_kill means the child process is unconditionally terminated using exit(Child, kill).
- An integer timeout value means that the supervisor tells the child process to terminate by calling exit(Child, shutdown) and then waits for an exit signal back. If no exit signal is received within the specified time, the child process is unconditionally terminated using exit(Child, kill).
- If the child process is another supervisor, it should be set to infinity to give the subtree enough time to shutdown. It is also allowed to set it to infinity, if the child process is a worker.
Warning

Be careful by setting the Shutdown strategy to infinity when the child process is a worker. Because, in this situation, the termination of the supervision tree depends on the child process, it must be implemented in a safe way and its cleanup procedure must always return.
Type specifies if the child process is a supervisor or a worker.
Modules should be a list with one element [Module], where Module is the name of the callback module, if the child process is a supervisor, gen_server or gen_fsm. If the child process is a gen_event, Modules should be dynamic.

This information is used by the release handler during upgrades and downgrades, see Release Handling.

Example: The child specification to start the server ch3 in the example above looks like:

{ch3,
 {ch3, start_link, []},
 permanent, brutal_kill, worker, [ch3]}

Example: A child specification to start the event manager from the chapter about gen_event:

{error_man,
 {gen_event, start_link, [{local, error_man}]},
 permanent, 5000, worker, dynamic}

Both the server and event manager are registered processes which can be expected to be accessible at all times, thus they are specified to be permanent.

ch3 does not need to do any cleaning up before termination, thus no shutdown time is needed but brutal_kill should be sufficient. error_man may need some time for the event handlers to clean up, thus Shutdown is set to 5000 ms.

Example: A child specification to start another supervisor:

{sup,
 {sup, start_link, []},
 transient, infinity, supervisor, [sup]}

5.6 Starting a Supervisor

In the example above, the supervisor is started by calling ch_sup:start_link():

start_link() ->
    supervisor:start_link(ch_sup, []).

ch_sup:start_link calls the function supervisor:start_link/2. This function spawns and links to a new process, a supervisor.

The first argument, ch_sup, is the name of the callback module, that is the module where the init callback function is located.
The second argument, [], is a term which is passed as-is to the callback function init. Here, init does not need any indata and ignores the argument.

In this case, the supervisor is not registered. Instead its pid must be used. A name can be specified by calling supervisor:start_link({local, Name}, Module, Args) or supervisor:start_link({global, Name}, Module, Args).

The new supervisor process calls the callback function ch_sup:init([]). init is expected to return {ok, StartSpec}:

init(_Args) ->
    {ok, {{one_for_one, 1, 60},
          [{ch3, {ch3, start_link, []},
            permanent, brutal_kill, worker, [ch3]}]}}.

The supervisor then starts all its child processes according to the child specifications in the start specification. In this case there is one child process, ch3.

Note that supervisor:start_link is synchronous. It does not return until all child processes have been started.

5.7 Adding a Child Process

In addition to the static supervision tree, we can also add dynamic child processes to an existing supervisor with the following call:

supervisor:start_child(Sup, ChildSpec)

Sup is the pid, or name, of the supervisor. ChildSpec is a child specification.

Child processes added using start_child/2 behave in the same manner as the other child processes, with the following important exception: If a supervisor dies and is re-created, then all child processes which were dynamically added to the supervisor will be lost.

5.8 Stopping a Child Process

Any child process, static or dynamic, can be stopped in accordance with the shutdown specification:

supervisor:terminate_child(Sup, Id)

The child specification for a stopped child process is deleted with the following call:

supervisor:delete_child(Sup, Id)

Sup is the pid, or name, of the supervisor. Id is the id specified in the child specification.

As with dynamically added child processes, the effects of deleting a static child process is lost if the supervisor itself restarts.

5.9 Simple-One-For-One Supervisors

A supervisor with restart strategy simple_one_for_one is a simplified one_for_one supervisor, where all child processes are dynamically added instances of the same process.

Example of a callback module for a simple_one_for_one supervisor:

-module(simple_sup).
-behaviour(supervisor).

-export([start_link/0]).
-export([init/1]).

start_link() ->
    supervisor:start_link(simple_sup, []).

init(_Args) ->
    {ok, {{simple_one_for_one, 0, 1},
          [{call, {call, start_link, []},
            temporary, brutal_kill, worker, [call]}]}}.

When started, the supervisor will not start any child processes. Instead, all child processes are added dynamically by calling:

supervisor:start_child(Sup, List)

Sup is the pid, or name, of the supervisor. List is an arbitrary list of terms which will be added to the list of arguments specified in the child specification. If the start function is specified as {M, F, A}, then the child process is started by calling apply(M, F, A++List).

For example, adding a child to simple_sup above:

supervisor:start_child(Pid, [id1])

results in the child process being started by calling apply(call, start_link, []++[id1]), or actually:

call:start_link(id1)

A child under a simple_one_for_one supervisor can be terminated with

supervisor:terminate_child(Sup, Pid)

where Sup is the pid, or name, of the supervisor and Pid is the pid of the child.

Because a simple_one_for_one supervisor could have many children, it shuts them all down at same time. So, order in which they are stopped is not defined. For the same reason, it could have an overhead with regards to the Shutdown strategy.

5.10 Stopping

Since the supervisor is part of a supervision tree, it will automatically be terminated by its supervisor. When asked to shutdown, it will terminate all child processes in reversed start order according to the respective shutdown specifications, and then terminate itself.