The Outbox is an infrastructure feature which simulates the reliability of distributed transactions without requiring use of the Distributed Transaction Coordinator (DTC).
The DTC is used by NServiceBus to guarantee consistency between messaging operations and data persistence. Messaging operations include the receipt and successful processing of an incoming message, and the sending of any outgoing messages as part of that processing. Data persistence includes any business data persisted by a message handler, as well as any NServiceBus saga or timeout data stored at the same time. The DTC ensures that these operations either all complete successfully or are all rolled back.
The Outbox feature can be used instead of the DTC, to mimic the same level of consistency without using distributed transactions. This is done by storing outgoing messages in the database using the same local (non-distributed) transaction which is used to store business and NServiceBus data. After that transaction is successfully committed, the stored outgoing messages are dispatched to their destinations as a separate operation.
Every time an incoming message is processed, a copy of that message is stored in the persistent Outbox storage. Whenever a new message is received, the framework determines whether that message has been processed already by checking for its presence in Outbox storage.
If the message is not found in Outbox storage then it is processed in the regular way, shown in the following diagram:
Processing a new incoming message consists of the following steps:
- The handlers responsible for processing the message are invoked.
- The downstream messages (messages sent while processing the message, e.g. from inside handlers) are stored in Outbox storage and business data is saved. Both operations are executed within a single (non-distributed) transaction.
- The downstream messages are sent and then marked as dispatched in Outbox storage.
If an incoming message is found in Outbox storage, then it is treated as a duplicate and is not processed again.
Even though an incoming message has been processed and business data has been saved, the framework might fail to immediately send the downstream messages. For example, the message queueing system may be unavailable, or the NServiceBus endpoint may be stopped before the messages can be dispatched. However, whenever an NServiceBus endpoint is started or is already running, and the framework detects downstream messages in Outbox storage which are not marked as dispatched, it will attempt to dispatch them at that time.
at-least-oncemessage delivery, meaning downstream messages may be sent and processed multiple times. At the handler level, however, the Outbox guarantees
exactly-oncemessage processing, similar to distributed transactions. This higher guarantee level is due to the deduplication that happens on the receiving side.
- The Outbox feature works only for messages sent from NServiceBus message handlers.
- Endpoints using DTC can only communicate with endpoints using the Outbox if either of the following conditions are satisfied:
- Endpoints using the Outbox do not send messages to endpoints using DTC. However, endpoints using DTC can send messages to endpoints using the Outbox.
- If endpoints using the Outbox send messages to endpoints using DTC, then the handlers processing those messages are idempotent.
- The Outbox may generate duplicate messages if outgoing messages are successfully dispatched but the Mark as dispatched phase fails. This may happen for a variety of reasons, including Outbox storage connectivity issues and deadlocks.
In order to enable the Outbox, use the following code API:
ReceiveOnly. A different level can be explicitly specified in configuration.
To learn about Outbox configuration options such as time to keep deduplication data, or deduplication data clean up interval, refer to the dedicated pages for NHibernate, RavenDB or ServiceFabric persistence.
When converting a system from using the DTC to the Outbox, care must be taken to ensure the system does not process duplicate messages incorrectly.
Because the Outbox feature uses an "at least once" consistency guarantee at the transport level, endpoints that enable the Outbox may occasionally send duplicate messages. These duplicate messages will be properly handled by deduplication in other Outbox-enabled endpoints, but will be processed more than once by endpoints which still use the DTC.
In order to gradually convert an entire system from the DTC to the Outbox:
- Enable the Outbox on any endpoints that receive messages but do not send or publish any messages.
- Enable the Outbox on any endpoints that only send or publish messages to already-converted endpoints, where the Outbox will be able to properly handle any duplicate messages.
- Progress outward until all endpoints are converted.
The Outbox feature requires persistence in order to perform deduplication and store outgoing downstream messages.