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deadline-managers.md

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Deadline Managers

Deadlines can be scheduled from sagas and aggregates. The DeadlineManager component is responsible for scheduling deadlines and invoking @DeadlineHandlerwhen the deadline is met. The DeadlineManager can be injected as a resource. It has two flavors: SimpleDeadlineManager and QuartzDeadlineManager

Scheduling a Deadline

A deadline can be scheduled by providing a Duration after which it will be triggered (or an Instant at which it will be triggered) and a deadline name.

Scheduled Events or Scheduled Deadlines

Unlike Event Scheduling, when a deadline is triggered there will be no storing of the published message. Scheduling/Triggering a deadline does not involve an EventBus (or EventStore), hence the message is not stored.

class DeadlineSchedulingComponent {
    void scheduleMyDeadline() {
        String deadlineId = 
            deadlineManager.schedule(Duration.ofMillis(500), "myDeadline");
        // For example store the `deadlineId`
    }
}

As a result we receive a deadlineId which can be used to cancel the deadline. In most cases, storing this deadlineId as a field within your Aggregate/Saga is the most convenient. Cancelling a deadline could come in handy when a certain event means that the previously scheduled deadline has become obsolete (e.g. there is a deadline for paying the invoice, but the client payed the amount which means that the deadline is obsolete and can be canceled).

class DeadlineCancelingComponent {
    void cancelMyDeadline(String deadlineId) {
        deadlineManager.cancelSchedule("myDeadline", deadlineId);
    }
}

Note that there are more options to cancel a deadline next to the previously mentioned:

  • cancelAll(String deadlineName)

    Cancels every scheduled deadline matching the given deadlineName.

    Note that this thus also cancels deadlines from other aggregate and/or saga instances matching the name.

  • cancelAllWithinScope(String deadlineName)

    Cancels a scheduled deadline matching the given deadlineName, within the Scope the method is invoked in.

    For example, if this operation is performed from within "aggregate instance X",

    the ScopeDescriptor from "aggregate instance X" will be used to cancel.

  • cancelAllWithinScope(String deadlineName, ScopeDescriptor scope)

    Cancels a scheduled deadline matching the given deadlineName and ScopeDescriptor.

    This allows canceling a deadline by name from differing scopes then the one it's executed in.

If you need contextual data about the deadline when the deadline is being handled, you can attach a deadline payload when scheduling a deadline:

class DeadlineSchedulingWithPayloadComponent {
    void scheduleMyDeadlineWithPayload() {
        String deadlineId = deadlineManager.schedule(
            Duration.ofMillis(500), "myDeadline", 
            new MyDeadlinePayload(/* some user specific parameters */)
        );
        // For example store the `deadlineId`
    }
}

Handling a Deadline

We have now seen how to schedule a deadline. When the scheduled time is met, the corresponding @DeadlineHandler is invoked. A @DeadlineHandler is a message handler like any other in Axon - it is possible to inject parameters for which ParameterResolvers exist.

The Scope of a Deadline

When scheduling a deadline, the context from where it was scheduled is taken into account. This means a scheduled deadline will only be triggered in its originating context. Thus any @DeadlineHandler annotated function you wish to be called on a met deadline, must be in the same Aggregate/Saga from which is was scheduled.

Axon calls this context a Scope. If necessary, implementing and providing your own Scope will allow you to schedule deadlines in your custom, 'scoped' components.

A @DeadlineHandler is matched based on the deadline name and the deadline payload.

@DeadlineHandler(deadlineName = "myDeadline")
public void on(MyDeadlinePayload deadlinePayload) {
    // handle the deadline
}

If the deadline's name is not defined in the @DeadlineHandler, matching will proceed based on the deadline payload alone.

@DeadlineHandler
public void on(MyDeadlinePayload deadlinePayload) {
    // handle the deadline
}

If we scheduled a deadline without a specific payload, the @DeadlineHandler does not have to specify the payload.

@DeadlineHandler(deadlineName = "payloadlessDeadline")
public void on() {
    // handle the deadline
}

Using Time In Your Application

In cases where applications need to access the clock, they can take advantage of the clock used in the EventMessage, by accessing GenericEventMessage.clock. This clock is set to Clock.systemUTC at runtime, and manipulated to simulate time during testing.

public void handle(PublishTime cmd) {
    apply(new TimePublishedEvent(GenericEventMessage.clock.instant()));
}

Note that the current timestamp is automatically added to the EventMessage. If handlers only need to rely on the timestamp the event was published, they can access that timestamp directly, as described in Handling Events.