Getting Started

Outbox with SQL persistence

Component: Sql Persistence
Target Version: NServiceBus 8.x

The outbox feature requires persistence in order to store the messages and enable deduplication.


To keep track of duplicate messages, the SQL persistence implementation of outbox requires the creation of dedicated outbox tables. The names of the outbox tables are generated automatically according to the rules for a given SQL dialect, for example the maximum name length limit.

See scripts used for table creation to learn more: MS SQL Server, Oracle, MySQL and PostgreSQL.

Concurrency control

By default the outbox uses optimistic concurrency control. That means that when two copies of the same message arrive at the endpoint, both messages are picked up (if concurrency settings of the endpoint allow for it) and processing begins on both of them. When the message handlers are completed, both processing threads attempt to insert the outbox record as part of the transaction that includes the application state change.

At this point, one of the transactions succeeds and the other fails due to a unique index constraint violation. When the copy of the message that failed is picked up again, it is discarded as a duplicate.

The outcome is that the application state change is applied only once (the other attempt has been rolled back) but the message handlers have been executed twice. If the message handler contains logic that has non-transactional side effects (e.g. sending an e-mail), that logic may be executed multiple times.

Pessimistic concurrency control

The pessimistic concurrency control mode can be activated using the following API:

var outboxSettings = endpointConfiguration.EnableOutbox();


In the pessimistic mode the outbox record is inserted before the handlers are executed. As a result, when using a database that creates locks on insert, only one thread is allowed to execute the message handlers. The other thread, even though it picked up the second copy of a message, is blocked on a database lock. Once the first thread commits the transaction, the second thread is interrupted with an exception as it is not allowed to insert the outbox. As a result, the message handlers are executed only once.

The trade-off is that each message processing attempt requires additional round trip to the database.

Transaction type

By default the outbox uses the ADO.NET transactions abstracted via DbTransaction. This is appropriate for most situations.

Transaction Scope

In cases where the outbox transaction spans multiple databases, the TransactionScope support has to be enabled:

var outboxSettings = endpointConfiguration.EnableOutbox();

// OR

In this mode the SQL persistence creates a TransactionScope that wraps the whole message processing attempt and within that scope it opens a connection, that is used for:

  • storing the outbox record
  • persisting the application state change applied via SynchronizedStorageSession

In addition to that connection managed by NServiceBus, users can open their own database connections in the message handlers. If the underlying database technology supports distributed transactions managed by the Microsoft Distributed Transaction Coordinator (MSDTC) (e.g. SQL Server, Oracle or PostgreSQL), the transaction is escalated to a distributed transaction.

The TransactionScope mode is most useful in legacy scenarios such as when migrating from the MSMQ transport to a messaging infrastructure that does not support MSDTC. In this scenario, it is no longer possible to use a distributed transaction which includes the transport and the database. To maintain consistency, the outbox must be used instead. If the outbox table cannot be added to the legacy database, it may be placed in a separate database, but access to both databases must be included in distributed transactions.

Transaction Isolation Level

If required, the outbox transaction isolation level may be adjusted:

var outboxSettings = endpointConfiguration.EnableOutbox();


A change in the isolation level affects all data access included in transactions. This should be done only if the business logic executed by message handlers within outbox transactions requires higher than the default isolation level (Read Committed) to guarantee correctness (e.g. Repeatable Read or Serializable). The isolation level may also be adjusted when not using TransactionScope mode.

Deduplication record lifespan

By default, the SQL persistence implementation keeps deduplication records for 7 days and runs the purge every minute.

These values can be changed using the following settings:

var outboxSettings = endpointConfiguration.EnableOutbox();


The cleanup task can be disabled by calling the DisableCleanup method:

var outboxSettings = endpointConfiguration.EnableOutbox();


In scaled-out environments, endpoint instances compete to execute outbox cleanup, which can result in occasional conflicts. There are a few options available to minimize this:

  • Run the cleanup on only a single instance.
  • Increase the cleanup interval so that, on average, one endpoint instance cleans up as often as a single instance would normally. The cleanup timer isn't strict and over time these will drift and will cause less overlap. For example, for 10 endpoint instances, let cleanup run every 10 minutes instead of every minute.
  • Disable cleanup on all instances and have cleanup run as a scheduled job in the database.

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