Event Schedulers

Event Schedulers

This section will proceed with a suggested course of action when utilizing the EventScheduler for dealing with deadlines.

To help understand this better lets take the scenario of a saga: 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. For invoices, that is typically several weeks, whereas the confirmation of a credit card payment should occur within a few seconds.

Scheduled Events as Deadlines

In Axon, you can use an EventScheduler to schedule an event for publication. In the example of an invoice, you would expect the 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 event. This token can be used to cancel the schedule, for example when a payment of an Invoice has been received.

Axon provides three EventScheduler implementations:

1. Pure Java

2. Quartz based

3. Axon Server based

The 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. The 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).

The 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 "AxonFramework-Events".

The AxonServerEventScheduler uses Axon Server to schedule events for publication. As such, it is a hard requirement to use Axon Server as your Event Store solution to utilize this event scheduler. Just as the QuartzEventScheduler, the AxonServerEventScheduler is a reliable and enterprise-worthy implementation of the EventScheduler interface. Creating a AxonServerEventScheduler can be done through its builder, whose sole requirement is the AxonServerConnectionManager.

It is important to note that QuartzEventScheduler and AxonServerEventScheduler both should use the event Serializer to serialize and deserialize the scheduled event. If the Serializer used by the scheduler does not align with the Seralizer used by the event store, exceptional scenarios should be expected. The Quartz implementation's Serializer can be set by defining a different EventJobDataBinder, whereas the Axon Server implementation allows defining the used Serializer directly through the builder.

One or more components will be listening for scheduled Events. These components might rely on a transaction 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 Configuration

Spring users can use the QuartzEventSchedulerFactoryBean or SimpleEventSchedulerFactoryBean for easier configuration. It allows you to set the PlatformTransactionManager directly.

Spring Boot users which rely on Axon Server do not have to define anything. The auto configuration will automatically create a AxonServerEventScheduler for them.