Message Correlation

Correlation is the process of finding saga instances based on data in the incoming message. For example, an OrderId property of a CompleteOrder message can be used to find the existing saga instance for that order.

To declare this, use the ConfigureHowToFindSaga method and use the Mapper to specify which saga property each message maps to.

protected override void ConfigureHowToFindSaga(SagaPropertyMapper<MySagaData> mapper)
{
    mapper.ConfigureMapping<MyMessage>(message => message.SomeId)
        .ToSaga(sagaData => sagaData.SomeId);
}

When an instance of MyMessage arrives, NServiceBus asks the saga persistence infrastructure to find an object of the type MySagaData that has a property SomeId whose value is the same as the SomeId property of the message. If found, the saga instance is loaded and the Handle method for the MyMessage message is invoked. Should the saga instance not be found, the saga is not started and the saga not found handlers are be invoked.

Starting from NServiceBus version 7.4, it is also possible to use the inverse API to map the saga. The benefit of this approach is that the fluent API allows mapping multiple messages in one operation. Using this API, it is also possible to add a map to the header values in the mix.

protected override void ConfigureHowToFindSaga(SagaPropertyMapper<MySagaData> mapper)
{
    mapper.MapSaga(saga => saga.SomeId)
        .ToMessage<MyFirstMessage>(msg => msg.SomeId)
        .ToMessage<MySecondMessage>(msg => msg.SomeOtherId)
        .ToMessageHeader<MyThirdMessage>("SomeHeaderKey");
}

Message property expression

If correlating on more than one saga property is necessary or matched properties are of different types use a custom saga finder.

It is possible to specify the mapping to the message using expressions if the correlation information is split between multiple fields.

protected override void ConfigureHowToFindSaga(SagaPropertyMapper<MySagaData> mapper)
{
    mapper.ConfigureMapping<MyMessage>(message => $"{message.Part1}_{message.Part2}")
        .ToSaga(sagaData => sagaData.SomeId);
}

Message header correlation

Sagas can be correlated to messages using a message header instead of a message property.

protected override void ConfigureHowToFindSaga(SagaPropertyMapper<MySagaData> mapper)
{
    mapper.ConfigureHeaderMapping<MyMessage>("HeaderName")
        .ToSaga(saga => saga.SomeId);
}

Auto-correlation

A common usage of sagas is to have them send out a request message to get some work done and receive a response message back when the work is complete. To make this easier NServiceBus automatically correlates those response messages back to the correct saga instance.

Custom saga finder

Full control over how a message is correlated can be achieved by creating a custom saga finder.

Uniqueness

NServiceBus makes sure that all properties used for correlation are unique across all instances of the given saga type. How this is enforced is up to each persister but will most likely translate to a unique key constraint in the database.

Mapping a single message to multiple saga instances is not supported. This can be simulated by using a message handler that looks up all saga instance affected and send a separate message targeting each of those instances using the regular correlation described above.