# 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](http://www.quartz-scheduler.org/) based
3. [Axon Server](https://axon-cn.ahoo.me/axon-server/introduction) 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`](https://axon-cn.ahoo.me/serialization#event-serialization) 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.