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Complex saga finding logic

Component: NServiceBus
NuGet Package: NServiceBus (9.x)

A saga can handle multiple messages. When NServiceBus receives a message that should be handled by a saga, it uses the configured mapping information to determine the correct saga instance that should handle the incoming message. In many cases the correlation logic is simple and can be specified using the provided mapping function, which is the recommended default approach. However, if the correlation logic is very complex it might be necessary to define a custom saga finder.

Custom Saga Finders are created by implementing IFindSagas.

public class MySagaFinder :
    ISagaFinder<MySagaData, MyMessage>
{
    public Task<MySagaData> FindBy(MyMessage message, ISynchronizedStorageSession storageSession, IReadOnlyContextBag context, CancellationToken cancellationToken)
    {
        // SynchronizedStorageSession will have a persistence specific extension method
        // For example GetDbSession is a stub extension method
        var dbSession = storageSession.GetDbSession();
        return dbSession.GetSagaFromDB(message.SomeId, message.SomeData);
        // If a saga can't be found Task.FromResult(null) should be returned
    }
}

Many finders may exist for a given saga or message type. If a saga can't be found and if the saga specifies that it is to be started for that message type, NServiceBus will know that a new saga instance is to be created.

Samples

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