The MongoEventStorageEngine has an @PostConstruct annotated method, called ensureIndexes which will generate the indexes required for correct operation. That means, when running in a container that automatically calls @PostConstruct handlers, the required unique index on "Aggregate Identifier" and "Event Sequence Number" is created when the event store is created.

Note that there is always a balance between query optimization and update speed. Load testing is ultimately the best way to discover which indices provide the best performance.

• Normal operational use

  An index is automatically created on "aggregateIdentifier", "type" and "sequenceNumber" in the domain events (default name: "domainevents") collection. Additionally, a non-unique index on "timestamp" and "sequenceNumber" is configured on the domain events (default name: "domainevents") collection, for the tracking event processors.

• Snapshotting

  An (unique) index on "aggregateIdentifier" and "sequenceNumber" is automatically created in the snapshot events (default name: "snapshotevents") collection.

• Sagas

  Put a (unique) index on the "sagaIdentifier" in the saga (default name: "sagas") collection. Put an index on the "sagaType", "associations.key" and "associations.value" properties in the saga (default name: "sagas") collection.

Configuration in Spring Boot

// The Event store `EmbeddedEventStore` delegates actual storage and retrieval of events to an `EventStorageEngine`.
@Bean
public EmbeddedEventStore eventStore(EventStorageEngine storageEngine, AxonConfiguration configuration) {
    return EmbeddedEventStore.builder()
            .storageEngine(storageEngine)
            .messageMonitor(configuration.messageMonitor(EventStore.class, "eventStore"))
            .build();
}
​
// The `MongoEventStorageEngine` stores each event in a separate MongoDB document
@Bean
public EventStorageEngine storageEngine(MongoClient client) {
    return MongoEventStorageEngine.builder().mongoTemplate(DefaultMongoTemplate.builder().mongoDatabase(client).build()).build();
}