It is easy to make a saga take action when something happens. After all, there is an event to notify the saga. But what if you want your saga to do something when nothing happens? That's what deadlines are used for. In invoices, that is typically several weeks, while the confirmation of a credit card payment should occur within a few seconds.
In Axon, you can use an
EventScheduler to schedule an event for publication. In the example of an Invoice, you would expect that invoice to be paid within thirty days. A saga would, after sending the
CreateInvoiceCommand, schedule an
InvoicePaymentDeadlineExpiredEvent to be published in 30 days. The
EventScheduler returns a
ScheduleToken after scheduling an èvent. This token can be used to cancel the schedule, for example when a payment of an Invoice has been received.
Axon provides two
EventScheduler implementations: a pure Java one and one using Quartz 2 as a backing scheduling mechanism.
This pure-Java implementation of the
EventScheduler uses a
ScheduledExecutorService to schedule event publication. Although the timing of this scheduler is very reliable, it is a pure in-memory implementation. Once the JVM is shut down, all schedules are lost. This makes this implementation unsuitable for long-term schedules.
SimpleEventScheduler needs to be configured with an
EventBus and a
SchedulingExecutorService (see the static methods on the
java.util.concurrent.Executors class for helper methods).
QuartzEventScheduler is a more reliable and enterprise-worthy implementation. Using Quartz as underlying scheduling mechanism, it provides more powerful features, such as persistence, clustering and misfire management. This means event publication is guaranteed. It might be a little late, but it will be published.
It needs to be configured with a Quartz
Scheduler and an
EventBus. Optionally, you may set the name of the group that Quartz jobs are scheduled in, which defaults to
One or more components will be listening for scheduled Events. These components might rely on a Transaction being bound to the thread that invokes them. Scheduled events are published by threads managed by the
EventScheduler. To manage transactions on these threads, you can configure a
TransactionManager or a
UnitOfWorkFactory that creates a transaction bound unit of work.
Spring users can use the
SimpleEventSchedulerFactoryBeanfor easier configuration. It allows you to set the