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
5.3.1 one_for_one
If a child process terminates, only that process is restarted.
One_For_One Supervision5.3.2 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.
One_For_All Supervision5.3.3 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
andMaxT
in the start specification returned by the callback functioninit
:init(...) -> {ok, {{RestartStrategy, MaxR, MaxT}, [ChildSpec, ...]}}.If more than
MaxR
number of restarts occur in the lastMaxT
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 asapply(M, F, A)
.
It should be (or result in) a call tosupervisor:start_link
,gen_server:start_link
,gen_fsm:start_link
orgen_event:start_link
. (Or a function compliant with these functions, seesupervisor(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.
- A
transient
child process is restarted only if it terminates abnormally, i.e. with another exit reason thannormal
.
Shutdown
defines how a child process should be terminated.
brutal_kill
means the child process is unconditionally terminated usingexit(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 usingexit(Child, kill)
.
- If the child process is another supervisor, it should be set to
infinity
to give the subtree enough time to shutdown.
Type
specifies if the child process is a supervisor or a worker.
Modules
should be a list with one element[Module]
, whereModule
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 bedynamic
.
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 butbrutal_kill
should be sufficient.error_man
may need some time for the event handlers to clean up, thusShutdown
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 functionsupervisor: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 theinit
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)
orsupervisor: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 callingapply(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)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.