Supervisor

A supervisor in Ergo Framework is a specialized actor responsible for managing child processes. Its main tasks include starting child processes, monitoring their life cycles, and restarting them if necessary according to the chosen restart strategy.

act.Supervisor implements the low-level gen.ProcessBehavior interface. Similar to act.Actor, it has the embedded gen.Process interface. This means that an object based on act.Supervisor has access to all the methods provided by the gen.Process interface.

To create a supervisor actor, you simply embed act.Supervisor into your object. Additionally, you need to create a factory function for the supervisor. Here is an example:

type MySupervisor struct {
    act.Supervisor
}

func factoryMySupervisor() gen.ProcessBehavior {
    return &MySupervisor{}
}

To work with your object, act.Supervisor uses the act.SupervisorBehavior interface. This interface defines a set of callback methods for initialization, handling asynchronous messages, and handling synchronous calls:

type SupervisorBehavior interface {
	gen.ProcessBehavior

	// Init invoked on a spawn Supervisor process. 
	// This is a mandatory callback for the implementation
	Init(args ...any) (SupervisorSpec, error)

	// HandleChildStart invoked on a successful child process starting if option EnableHandleChild
	// was enabled in gen.SupervisorSpec
	HandleChildStart(name gen.Atom, pid gen.PID) error

	// HandleChildTerminate invoked on a child process termination 
	// if option EnableHandleChild was enabled in gen.SupervisorSpec
	HandleChildTerminate(name gen.Atom, pid gen.PID, reason error) error

	// HandleMessage invoked if Supervisor received a message sent with gen.Process.Send(...).
	// Non-nil value of the returning error will cause termination of this process.
	// To stop this process normally, return gen.TerminateReasonNormal or
	// gen.TerminateReasonShutdown. Any other - for abnormal termination.
	HandleMessage(from gen.PID, message any) error

	// HandleCall invoked if Supervisor got a synchronous request made with gen.Process.Call(...).
	// Return nil as a result to handle this request asynchronously and
	// to provide the result later using the gen.Process.SendResponse(...) method.
	HandleCall(from gen.PID, ref gen.Ref, request any) (any, error)

	// Terminate invoked on a termination supervisor process
	Terminate(reason error)

	// HandleInspect invoked on the request made with gen.Process.Inspect(...)
	HandleInspect(from gen.PID) map[string]string
}

The Init method is mandatory for implementation, while the other methods are optional. This method returns the supervisor specification act.SupervisorSpec, which includes the following options:

Type: The type of supervisor (act.SupervisorType).
Children: The list of child process specifications ([]act.SupervisorChildSpec). This parameter also determines the order of child process startups.
Restart: Defines the restart strategy for child processes.
EnableHandleChild: Enables the invocation of the callback methods HandleChildStart and HandleChildStop during the startup and shutdown of child processes.
DisableAutoShutdown: Prevents the supervisor from stopping if all child processes terminate non-faultily.

Here is an example implementation of the act.SupervisorBehavior interface for the supervisor MySupervisor with one child process MyActor:

//
// Child process implementation
//

func factoryMyActor() gen.ProcessBehavior {
    return &MyActor{}
}

type MyActor struct {
    act.Actor
}

func (a *MyActor) Init(args ...any) error {
    a.Log().Info("starting child process MyActor")
    return nil
}

//
// gen.SupervisorBehavior implementation
//

func (s *MySupervisor) Init(args ...any) (act.SupervisorSpec, error) {
    s.Log().Info("initialize supervisor...")
    spec := act.SupervisorSpec{
        Children: []act.SupervisorChildSpec{
            act.SupervisorChildSpec{
                Name: "myChild",
                Factory: factoryMyActor,
            },
        }
    }
    return spec, nil
}
...
pid, err := node.Spawn(factoryMySupervisor, gen.ProcessOptions{})
node.Log().Info("MySupervisor is started succesfully with pid %s", pid)
...

Supervisor Types

The type of supervisor in the SupervisorSpec defines its behavior. In Ergo Framework, several supervisor types are implemented:

act.SupervisorTypeOneForOne

This is the default supervisor type. If the type is not specified in the SupervisorSpec, this one will be used. Upon startup, the supervisor sequentially starts child processes as specified in act.SupervisorSpec.Children. If an error occurs while starting the child processes, the supervisor process will terminate.

Each child process is launched with the associated name defined in the Name option of the child process specification act.SupervisorChildSpec. This means that only one process can be started for each child process specification. If this field is empty, the supervisor will use the Spawn method from the gen.Process interface. Otherwise, the SpawnRegister method will be used.

Stopping a child process triggers the restart strategy for that process. Depending on the restart strategy defined by act.SupervisorSpec.Restart.Strategy, the process will either be restarted or stopped. The restart strategy does not affect other processes.

The child process is restarted with the same set of arguments that were used in its previous startup.

The KeepOrder option in act.SupervisorSpec.Restart is ignored for this supervisor type. When a new child process specification is added via the AddChild method (provided by act.Supervisor), the supervisor automatically starts the new child process. When a child specification is disabled (using method) DisableChild, the supervisor stops the corresponding child process with the reason gen.TerminateReasonShutdown. The supervisor will automatically terminate if no child processes remain (with the reasons gen.TerminateReasonNormal or gen.TerminateReasonShutdown). You can enable the DisableAutoShutdown option in act.SupervisorSpec to prevent this.

act.SupervisorTypeOneForAll

When starting, this supervisor sequentially launches child processes according to the list of child process specifications. If an error occurs during the startup of child processes, the supervisor process terminates.

A child process is started with the associated name defined in the Name option of act.SupervisorChildSpec.

Stopping a child process in this supervisor triggers the restart strategy for all child processes according to act.SupervisorSpec.Restart.Strategy.

If the KeepOrder option is enabled, child processes are stopped in reverse order. Without KeepOrder, they are stopped simultaneously. After stopping, the supervisor restarts them in the specified order.

Each child process is restarted with the same arguments as in the previous run.

When a new child process specification is added (using AddChild), the supervisor automatically starts the child process. When disabling (DisableChild), the supervisor stops the corresponding child process with the reason gen.TerminateReasonShutdown.

The DisableAutoShutdown option allows the supervisor to continue running even if all child processes have stopped (terminated with gen.TerminateReasonNormal or gen.TerminateReasonShutdown).

act.SupervisorTypeRestForOne

A child process is started with the associated name defined in the Name option of act.SupervisorChildSpec.

When a child process terminates, the restart strategy is activated. All child processes starting from the last one (according to the order in act.SupervisorSpec.Children) to the process that triggered the restart are stopped.

With the KeepOrder option enabled, child processes are stopped sequentially in reverse order. If the option is disabled, child processes stop simultaneously.

After all child processes are stopped, the supervisor restarts them according to the order specified in act.SupervisorSpec.Children.

Each child process is restarted with the same set of arguments used in the previous startup.

Each child process is launched with the associated name defined in the Name option of the child process specification act.SupervisorChildSpec.

act.SupervisorTypeSimpleOneForOne

This type of supervisor is a simplified version of act.SupervisorTypeOneForOne. It does not start child processes on startup. Instead, child processes are started using the StartChild method.

As in act.SupervisorTypeOneForOne, the termination of a child process triggers the restart strategy for that process. Depending on the strategy defined in act.SupervisorSpec.Restart.Strategy, the process will either be restarted or stopped. The restart strategy does not affect other processes.

Child processes in this supervisor are launched without an associated name, and multiple processes can be started from one child process specification.

The KeepOrder option in act.SupervisorSpec.Restart is ignored for this supervisor type.

The DisableAutoShutdown option is also ignored, and stopping all child processes does not result in the termination of the supervisor process.

Child Process Specification

This specification describes the parameters for starting a child process. This is done using act.SupervisorChildSpec:

Name: The specification's name, used as the associated name for the process (except for act.SupervisorTypeSimpleOneForOne).
Factory, Options, Args: Parameters needed to launch a new process.
Significant: Determines the importance of the child process. The termination of a significant process may lead to the supervisor and all child processes stopping, depending on the restart strategy:
- act.SupervisorStrategyTemporary: Termination of a significant child process leads to the termination of the supervisor and all children.
- act.SupervisorStrategyTransient: Non-crash termination (e.g., gen.TerminateReasonNormal or gen.TerminateReasonShutdown) of a significant child process leads to the termination of the supervisor and all children, while a crash triggers the restart strategy.
- act.SupervisorStrategyPermanent: The Significant field is ignored, and termination always triggers the restart strategy.

Additionally, the Significant field is ignored in supervisors of types act.SupervisorTypeOneForOne and act.SupervisorSimpleOneForOne.

Restart Strategy

The Restart option in act.SupervisorSpec is of type act.SupervisorRestart and defines parameters for the restart strategy:

Strategy: Specifies the restart strategy type:
- act.SupervisorStrategyTransient: A crash triggers a restart. If the process terminates normally (e.g., gen.TerminateReasonNormal or gen.TerminateReasonShutdown), the restart strategy is not activated. This is the default strategy.
- act.SupervisorStrategyTemporary: No restart occurs, even in the event of a crash.
- act.SupervisorStrategyPermanent: Any termination triggers a restart, ignoring DisableAutoShutdown.
Intensity and Period: Define the maximum number of restart activations allowed within a given period (in seconds). If this limit is exceeded, the supervisor terminates all child processes and itself, with the reason act.ErrSupervisorRestartsExceeded.
KeepOrder: Specifies the order in which child processes are stopped when the restart strategy is activated. If this option is enabled, processes will be stopped sequentially in reverse order. By default, this option is disabled, meaning that all child processes are stopped simultaneously. Processes are restarted only after all child processes have fully stopped (either all processes for act.SupervisorTypeAllForOne or part of the processes for act.SupervisorTypeRestForOne).

Methods of `act.Supervisor`

StartChild(name gen.Atom, args...): Starts a child process by its specification name. If arguments (args) are provided, they are updated in the specification for future restarts.
AddChild(child act.SupervisorChildSpec): Adds a child specification and automatically starts the process.
EnableChild(name gen.Atom): Enables and starts a previously disabled child process.
DisableChild(name gen.Atom): Disables the specification and stops the running process.
Children(): Returns a list of []act.SupervisorChild with data on specifications and running processes, sorted according to the child process specification list.

During initialization, the supervisor defines the initial set of child processes, the supervisor type, and the restart strategy for the child processes. act.Supervisor provides several methods for managing child processes dynamically and obtaining information about running processes:

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