org.axonframework.eventsourcing.eventstore

Class SequenceEventStorageEngine

java.lang.Object

org.axonframework.eventsourcing.eventstore.SequenceEventStorageEngine

All Implemented Interfaces:

EventStorageEngine

public class SequenceEventStorageEngine
extends Object
implements EventStorageEngine

EventStorageEngine implementation that combines the streams of two event storage engines. The first event storage engine contains historic events while the second is used for active event storage. If a stream of events is opened this storage engine concatenates the stream of the historic and active storage.

New events and snapshots are stored in the active storage.

When fetching snapshots, if a snapshot cannot be found in the active storage it will be obtained from the historic storage.

No mechanism is provided to move events from the active storage to the historic storage engine so clients need to take care of this themselves.

Author:

Rene de Waele, Allard Buijze

Constructor Summary

Constructors

Constructor and Description
SequenceEventStorageEngine(EventStorageEngine historicStorage, EventStorageEngine activeStorage) Initializes a new SequenceEventStorageEngine using given historicStorage and activeStorage.

Method Summary

Modifier and Type	Method and Description
void	appendEvents(List<? extends EventMessage<?>> events) Append a list of events to the event storage.
TrackingToken	createHeadToken() Creates a token that is at the head of an event stream - that tracks all new events.
TrackingToken	createTailToken() Creates a token that is at the tail of an event stream - that tracks events from the beginning of time.
TrackingToken	createTokenAt(Instant dateTime) Creates a token that tracks all events after given dateTime.
Optional<Long>	lastSequenceNumberFor(String aggregateIdentifier) Returns the last known sequence number for the given aggregateIdentifier.
DomainEventStream	readEvents(String aggregateIdentifier, long firstSequenceNumber) Get a DomainEventStream containing all events published by the aggregate with given aggregateIdentifier starting with the first event having a sequence number that is equal or larger than the given firstSequenceNumber.
Stream<? extends TrackedEventMessage<?>>	readEvents(TrackingToken trackingToken, boolean mayBlock) Open an event stream containing all events stored since given tracking token.
Optional<DomainEventMessage<?>>	readSnapshot(String aggregateIdentifier) Try to load a snapshot event of the aggregate with given aggregateIdentifier.
void	storeSnapshot(DomainEventMessage<?> snapshot) Store an event that contains a snapshot of an aggregate.

Methods inherited from class java.lang.Object

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

Methods inherited from interface org.axonframework.eventsourcing.eventstore.EventStorageEngine

appendEvents, readEvents

Constructor Detail

SequenceEventStorageEngine

public SequenceEventStorageEngine(EventStorageEngine historicStorage,
                                  EventStorageEngine activeStorage)

Initializes a new SequenceEventStorageEngine using given historicStorage and activeStorage.

Parameters:

historicStorage - the event storage engine that contains historic events. This can be backed by a read-only database

activeStorage - the event storage engine that contains 'new' events and to which new events and snapshots will be written

Method Detail

appendEvents

public void appendEvents(List<? extends EventMessage<?>> events)

Description copied from interface: EventStorageEngine

Append a list of events to the event storage. Events will be appended in the order that they are offered in.

Note that all events should have a unique event identifier. When storing domain events events should also have a unique combination of aggregate id and sequence number.

Specified by:

appendEvents in interface EventStorageEngine

Parameters:

events - Events to append to the event storage

storeSnapshot

public void storeSnapshot(DomainEventMessage<?> snapshot)

Description copied from interface: EventStorageEngine

Store an event that contains a snapshot of an aggregate. If the event storage already contains a snapshot for the same aggregate, then it will be replaced with the given snapshot.

Specified by:

storeSnapshot in interface EventStorageEngine

Parameters:

snapshot - The snapshot event of the aggregate that is to be stored

readEvents

public Stream<? extends TrackedEventMessage<?>> readEvents(TrackingToken trackingToken,
                                                           boolean mayBlock)

Description copied from interface: EventStorageEngine

Open an event stream containing all events stored since given tracking token. The returned stream is comprised of events from aggregates as well as other application events. Pass a trackingToken of null to open a stream containing all available events.

If the value of the given mayBlock is true the returned stream is allowed to block while waiting for new event messages if the end of the stream is reached.

Specified by:

readEvents in interface EventStorageEngine

Parameters:

trackingToken - Object describing the global index of the last processed event or null to create a stream of all events in the store

mayBlock - If true the storage engine may optionally choose to block to wait for new event messages if the end of the stream is reached.

Returns:

A stream containing all tracked event messages stored since the given tracking token

readEvents

public DomainEventStream readEvents(String aggregateIdentifier,
                                    long firstSequenceNumber)

Description copied from interface: EventStorageEngine

Get a DomainEventStream containing all events published by the aggregate with given aggregateIdentifier starting with the first event having a sequence number that is equal or larger than the given firstSequenceNumber.

The returned stream is finite, i.e. it should not block to wait for further events if the end of the event stream of the aggregate is reached.

Specified by:

readEvents in interface EventStorageEngine

Parameters:

aggregateIdentifier - The identifier of the aggregate

firstSequenceNumber - The expected sequence number of the first event in the returned stream

Returns:

A non-blocking DomainEventStream of the given aggregate

readSnapshot

public Optional<DomainEventMessage<?>> readSnapshot(String aggregateIdentifier)

Description copied from interface: EventStorageEngine

Try to load a snapshot event of the aggregate with given aggregateIdentifier. If the storage engine has no snapshot event of the aggregate, an empty Optional is returned.

Specified by:

readSnapshot in interface EventStorageEngine

Parameters:

aggregateIdentifier - The identifier of the aggregate

Returns:

An optional with a snapshot of the aggregate

lastSequenceNumberFor

public Optional<Long> lastSequenceNumberFor(String aggregateIdentifier)

Description copied from interface: EventStorageEngine

Returns the last known sequence number for the given aggregateIdentifier.

While it's recommended to use the sequence numbers from the DomainEventStream, there are cases where knowing the sequence number is required, without having read the actual events. In such case, this method is a viable alternative.

Specified by:

lastSequenceNumberFor in interface EventStorageEngine

Parameters:

aggregateIdentifier - The identifier to find the last sequence number for

Returns:

an optional with the highest sequence number, or an empty optional if the aggregate identifier wasn't found

createTailToken

public TrackingToken createTailToken()

Description copied from interface: EventStorageEngine

Creates a token that is at the tail of an event stream - that tracks events from the beginning of time.

Specified by:

createTailToken in interface EventStorageEngine

Returns:

a tracking token at the tail of an event stream, if event stream is empty null is returned

createHeadToken

public TrackingToken createHeadToken()

Description copied from interface: EventStorageEngine

Creates a token that is at the head of an event stream - that tracks all new events.

Specified by:

createHeadToken in interface EventStorageEngine

Returns:

a tracking token at the head of an event stream, if event stream is empty null is returned

createTokenAt

public TrackingToken createTokenAt(Instant dateTime)

Description copied from interface: EventStorageEngine

Creates a token that tracks all events after given dateTime. If there is an event exactly at the given dateTime, it will be tracked too.

Specified by:

createTokenAt in interface EventStorageEngine

Parameters:

dateTime - The date and time for determining criteria how the tracking token should be created. A tracking token should point to very first event before this date and time.

Returns:

a tracking token at the given dateTime, if there aren't events matching this criteria null is returned