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MSMQ transport delayed delivery

Component: MSMQ Transport
NuGet Package: NServiceBus.Transport.Msmq (1.2)
Target Version: NServiceBus 7.x

The MSMQ transport can use the timeout manager for delayed delivery of messages (e.g. saga timeouts or delayed retries) as well as an external store for delayed messages.

The MSMQ transport requires explicit configuration to enable delayed message delivery using an external store. For example:

var messageStore = new SqlServerDelayedMessageStore(
    connectionString: "database=(local); initial catalog=my_catalog; integrated security=true",
    schema: "my_schema", //optional, defaults to dbo
    tableName: "my_delayed_messages"); //optional, defaults to endpoint name with '.delayed' suffix

var transport = endpointConfiguration.UseTransport<MsmqTransport>();
var delayedDeliverySettings = transport.NativeDelayedDelivery(messageStore);

delayedDeliverySettings.NumberOfRetries = 7;
delayedDeliverySettings.MaximumRecoveryFailuresPerSecond = 2;
delayedDeliverySettings.TimeToTriggerStoreCircuitBreaker = TimeSpan.FromSeconds(20);
delayedDeliverySettings.TimeToTriggerDispatchCircuitBreaker = TimeSpan.FromSeconds(15);
delayedDeliverySettings.TimeToTriggerFetchCircuitBreaker = TimeSpan.FromSeconds(45);

The SQL Server delayed message store (SqlServerDelayedMessageStore) is the only delayed message store that ships with the MSMQ transport.

How it works

A delayed message store implements the IDelayedMessageStore interface. Delayed message delivery has two parts:

  • Storing delayed messages via the Store method
  • Polling and dispatching the delayed messages

Polling and dispatching delayed messages

The message store is polled for due delayed messages in a background task which periodically calls FetchNextDueTimeout. If the method returns a message, the message is sent, and the method is immediately called again. If the method returns null, Next is called, which returns either a DateTimeOffset indicating when the next message will be due, or null if there are no delayed messages.

When a due delayed message is returned by FetchNextDueTimeout, the message is sent to the destination queue and then removed from the store using the Remove method. If an exception occurs when forwarding the message, the failure is registered using IncrementFailureCount. If the configured number of retries is exhausted the message is forwarded to the configured error queue.

Using a custom delayed message store

To create a custom storage, implement the IDelayedMessageStore interface and pass an instance to the DelayedDeliverySettings constructor.

The built-in SQL Server delayed message store takes a pessimistic lock on the delayed message row in the FetchNextDueTimeout operation to prevent other physical instances of the same logical endpoint from delivering the same delayed message. A custom delayed message store must also take some kind of lock to prevent this from happening. For example, a delayed message store using Azure Blog Storage may take a lease lock.

Consistency

In TransactionScope transaction mode, the delayed message store is expected to enlist in the TransactionScope to ensure exactly once behavior. FetchNextDueTimeout, Remove, and sending messages to their destination queues are all executed in a single distributed transaction. The built-in SQL Server store supports this mode of operation.

In lower transaction modes the dispatch behavior is at least once. FetchNextDueTimeout and Remove are executed in the same TransactionScope but sending messages to their destination queues is executed in a separate (inner) transport scope. If Remove fails, the message will be sent to the destination queue multiple times and the destination endpoint must handle the duplicates, using either the outbox feature or a custom de-duplication mechanism.


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