Package org.axonframework.messaging.eventhandling.sequencing

Class FallbackSequencingPolicy<E extends Exception>

java.lang.Object

org.axonframework.messaging.eventhandling.sequencing.FallbackSequencingPolicy<E>

Type Parameters:

E - The type of exception to catch and handle.

All Implemented Interfaces:

SequencingPolicy

public class FallbackSequencingPolicy<E extends Exception>
extends Object
implements SequencingPolicy

Implementation of SequencingPolicy that provides exception-based fallback behavior. When the delegate policy throws a specified exception type, this implementation will catch it and delegate to a fallback policy.

This allows for composing sequencing strategies where certain policies might fail with exceptions for unsupported event types, falling back to more generic approaches when exceptions occur.

Since:

5.0.0

Author:

Mateusz Nowak

Constructor Summary

Constructors

Constructor	Description
FallbackSequencingPolicy(SequencingPolicy delegate, SequencingPolicy fallback, Class<E> exceptionType)	Initializes a new instance with the given delegate policy, fallback policy, and exceptionType to catch.

Method Summary

Modifier and Type	Method	Description
Optional<Object>	getSequenceIdentifierFor(EventMessage event, ProcessingContext context)	Returns the sequence identifier for the given event.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

FallbackSequencingPolicy

public FallbackSequencingPolicy(@Nonnull
 SequencingPolicy delegate,
 @Nonnull
 SequencingPolicy fallback,
 @Nonnull
 Class<E> exceptionType)

Initializes a new instance with the given delegate policy, fallback policy, and exceptionType to catch.

Parameters:

delegate - The primary policy to attempt sequence identification with first, not null.

fallback - The fallback policy to use when the delegate throws the specified exception, not null.

exceptionType - The type of exception to catch from the delegate policy, not null.

Throws:

NullPointerException - When any of the parameters is null.

Method Details

getSequenceIdentifierFor

public Optional<Object> getSequenceIdentifierFor(@Nonnull
 EventMessage event,
 @Nonnull
 ProcessingContext context)

Description copied from interface: SequencingPolicy

Returns the sequence identifier for the given event. When two events have the same identifier (as defined by their equals method), they will be executed sequentially. A Optional#empty() value indicates that there are no sequencing requirements for the handling of this event.

The Optional#empty() should ONLY be returned when the policy cannot determine a sequence identifier for the given event. This typically happens when the policy is not applicable for the specific event type. When Optional#empty() is returned, it is up to the component using this policy to provide a default behavior, use another policy, or throw an exception / react in any other way - as appropriate.

Specified by:

getSequenceIdentifierFor in interface SequencingPolicy

Parameters:

event - The event for which to get the sequencing identifier.

context - The processing context in which the event is being handled. This instance of the ProcessingContext doesn't allow you to register phases actions by for example ProcessingLifecycle.on(org.axonframework.messaging.core.unitofwork.ProcessingLifecycle.Phase, java.util.function.Function<org.axonframework.messaging.core.unitofwork.ProcessingContext, java.util.concurrent.CompletableFuture<?>>) or retrieving components by ApplicationContext.component(java.lang.Class<C>).

Returns:

A sequence identifier for the given event, or Optional#empty() if this policy cannot determine a sequence identifier for the given event.