Take a situation where you want to make an actor sleep and it should resume after a period of time. Or you want your actor to perform a task on a recurring basis then you must schedule a message. Here I will discuss how to do that !!
Scheduled delivery of messages
Often, a need exists to schedule the sending of a message to an actor:
a single time after some delay (send a message for once after a period of time and then we never send that message)
periodically with a given interval (e.g after every 5sec, 5min, etc.)
Two Akka facilities exist to achieve this:
The Akka Scheduler service
The akka.actors.Timers trait
Let’s discuss both of them one by one !!
Scheduler Service
import context.dispatcher
import scala.concurrent.duration._
context.system.scheduler.scheduleOnce(
2 seconds, // provide the delay
self, //actor name on which we want a scheduling message
ShradhaSaxena //message that you want to deliver
)
A scheduler is something that is provided by the actor system:
Allows to run a task or send a message to an actor
Scheduling could be once or on a periodic basis
You need an implicit ExecutionContext, e.g. an actor’s dispatcher
Timers Trait
import akka.actor.Timers
import scala.concurrent.duration._
class AkkaLearner(..<elided>..) extends Actor with ActorLogging with Timers {
timers.startSingleTimer(
"Let's start learning akka", // Name for the timer
ShradhaSaxena, // Message to be passed
2 seconds // delay
)
As of AKKA 2.5, to schedule the sending of a message to self after a given time:
Mix-in the akka.actor.Timers trait which will give you access to the timers field
Use methods timers.startSingleTimer (for sending message once) or timers.startPeriodicTimer to schedule events periodically
Name a timer by passing in a key (type Any)
Scheduler Service versus Timers
Timers are bond to the lifecycle of the actor that owns it and thus are canceled automatically when the actor is restarted or stopped. As a consequence the risk is eliminated from having:
‘zombie‘ timer events in case of stopping an actor. As the timer will also die when the actor has died
If we restart an actor then the timer may also get restarted and hence cause doubling of events
Timer guarantees that a message from a canceled timer, will not be received by the actor, even though the message might already be enqueued in the mailbox when cancel is called
The Scheduler Service supports sending timed events to other actors
“KEY POINT” –> When scheduling periodic or single messages in an actor to itself, use the Actor Timers instead of the Scheduler directly.
