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Actor

An actor in Ergo Framework implements the low-level gen.ProcessBehavior interface. To launch a process based on act.Actor, you need to create an object with an embedded act.Actor and implement a factory function for it. For example:

type MyActor struct {
    act.Actor
    i int
}
func factoryMyActor() gen.ProcessBehavior {
    return &MyActor{}
}

act.Actor uses the act.ActorBehavior interface to interact with your object. This interface defines a set of callback methods:

type ActorBehavior interface {
	gen.ProcessBehavior
	
	// main callbacks
	Init(args ...any) error
	HandleMessage(from gen.PID, message any) error
	HandleCall(from gen.PID, ref gen.Ref, request any) (any, error)
	Terminate(reason error)
	
	// extended callbacks used if SplitHandle was enabled
	HandleMessageName(name gen.Atom, from gen.PID, message any) error
	HandleMessageAlias(alias gen.Alias, from gen.PID, message any) error
	HandleCallName(name gen.Atom, from gen.PID, ref gen.Ref, request any) (any, error)
	HandleCallAlias(alias gen.Alias, from gen.PID, ref gen.Ref, request any) (any, error)
	
	// specialized callbacks
	HandleInspect() map[string]string
	HandleLog(message gen.MessageLog) error
	HandleEvent(message gen.MessageEvent) error
	
}

All methods in the act.ActorBehavior interface are optional, allowing you to implement only the necessary callbacks in your object. Since act.Actor embeds the gen.Process interface, you can directly use its methods from within your actor object.

Example:

func (a *MyActor) Init(args...) error {
    // get the gen.Log interface using Log method of embedded gen.Process interface
    a.Log().Info("starting process %s", a.PID())
    // initialize value
    a.i = 100
    // sending message to itself with methods Send and PID of embedded gen.Process
    a.Send(a.PID(), "hello")
    return nil
}

func (a *MyActor) HandleMessage(from gen.PID, message any) error {
    a.Log().Info("got message from %s: %s", from, message)
    ...
    // handling message
    ...
    return nil
}

func (a *MyActor) Terminate(reason error) {
    a.Log().Info("%s terminated with reason: %s", a.PID(), reason)
}

Process Initialization

Process initialization begins when act.Actor invokes the Init method of the act.ActorBehavior interface, passing the args provided during Spawn (or SpawnRegister). If Init completes successfully, the node registers the process, allowing it to receive messages or synchronous requests. If Init fails, Spawn (or SpawnRegister) returns the error. It's important to note that during initialization, the process is not yet registered with the node, limiting access to certain methods of the embedded gen.Process interface until the process is fully initialized.

Handling Asynchronous Messages and Synchronous Requests

The process mailbox contains several queues: Main, System, Urgent, and a special Log queue. In act.Actor, messages are processed in the following order: Urgent, System, Main, and then Log.

Asynchronous messages are sent using the Send method of the gen.Node or gen.Process interface. Upon receiving such a message, act.Actor calls the HandleMessage method of the gen.ActorBehavior interface.

For synchronous requests, the HandleCall callback method is invoked in an actor based on act.Actor. The result returned by this method is sent as the response to the request.

A process can send asynchronous messages to itself, but attempting to make a synchronous request to itself will return the error gen.ErrTimeout..

act.Actor also allows handling synchronous requests asynchronously. This feature lets you delegate the processing of synchronous requests to other processes or delay the response. To do this, the HandleCall method should return nil, and you should use the SendResponse method of the gen.Process interface to send the response. You must provide the gen.PID of the requesting process and the gen.Ref reference of the request.

For debugging or monitoring the internal state, the Inspect method allows high-priority synchronous requests. In act.Actor, this triggers the HandleInspect method. This feature is widely used in the Observer tool.

If the actor process is registered as a logger, the HandleLog method is called when log messages (gen.MessageLog) are received, generated using the gen.Log interface. If the actor process has subscribed to events using the LinkEvent or MonitorEvent methods of gen.Process, the HandleEvent method is called when an event message is received.

Process Termination

To stop a process, simply return a non-nil error value from the message-handling callback. After termination, the Terminate callback method will be called, with the reason argument being the returned error value.

Additionally, you can terminate a process using predefined constants like:

  • gen.TerminateReasonNormal for a normal (non-failure) termination.

  • gen.TerminateReasonKill when the process is forcibly stopped using the Kill method from the gen.Node interface.

In Ergo Framework, two additional termination reasons are defined:

  • gen.TerminateReasonPanic: Occurs when a panic happens during message processing.

  • gen.TerminateReasonShutdown: This termination reason can be sent by the parent process or the node when the node is stopped using the StopForce method of the gen.Node interface. It is not considered a failure.

At the moment the Terminate callback is invoked, the process has already been removed from the node. Therefore, most methods of the gen.Process interface will return the gen.ErrNotAllowed error.

SplitHandle

The SplitHandle option allows an actor to call the callback methods of the act.ActorBehavior interface based on the process identifier used to send a message or make a synchronous call to your process. You can enable this option using the SetSplitHandle(split bool) method defined in act.Actor. To check the current value of this option, use the SplitHandle method. Here is an example usage:

func (a *MyActor) Init(args...) error {
    a.SetSplitHandle(true)
    ...
    return nil
}

If your process has a registered associated name and receives a message using that name, act.Actor will call the HandleMessageName callback. The name argument will be the value used to send the message. For synchronous requests, the HandleCallName callback will be invoked.

If a process alias (gen.Alias) was used, the callbacks HandleMessageAlias and HandleCallAlias will be triggered for messages and synchronous requests, respectively. This enables customized handling based on the way the message or request was sent.

TrapExit - handling exit-signal

In Ergo Framework, an exit signal can be sent to your process by another process or node using the SendExit method from the gen.Process or gen.Node interfaces. It can also be generated by the node if your process had a link created via the Link* methods in the gen.Process interface. By default, when an actor receives an exit signal, it terminates and calls the Terminate callback with the reason specified in the exit signal.

To prevent your actor from terminating upon receiving an exit signal, you can enable the option to intercept such signals. This can be done using the SetTrapExit(trap bool) method in act.Actor. The default value is false, but you can enable it at any time, for example, during initialization:

func (a *MyActor) Init(args...) error {
    a.SetTrapExit(true)
    ...
    return nil
}

When the TrapExit option is enabled, act.Actor converts exit signals into standard gen.MessageExit* messages:

  • gen.MessageExitPID: The source of the exit signal was a process.

  • gen.MessageExitProcessID: The source was gen.ProcessID (a link created using LinkProcessID).

  • gen.MessageExitAlias: The source was gen.Alias (link created using LinkAlias).

  • gen.MessageExitEvent: The source was gen.Event (link created using LinkEvent).

  • gen.MessageExitNode: The source was a network connection with a node (link created using LinkNode).

Scenarios where the exit signal cannot be intercepted:

  • Parent Process Sends Exit: When the parent process sends an exit signal via SendExit using the gen.Process interface, it cannot be intercepted.

  • Parent Process Terminates: If the parent process, with which a link was created via gen.PID (using Link or LinkPID), terminates, or if the parent process used the gen.ProcessOptions.LinkParent option during child process startup.

In these cases, the actor will terminate, and the Terminate callback will be called, even with TrapExit enabled. All other exit signals can be intercepted when TrapExit is active.

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