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MSMQ transport delayed delivery

Component: MSMQ Transport
NuGet Package: NServiceBus.Transport.Msmq (2.x)
Target Version: NServiceBus 8.x

Because MSMQ lacks a mechanism for sending delayed messages, the MSMQ transport uses an external store for delayed messages. Messages that are to be delivered later (e.g. saga timeouts or delayed retries) are persisted in the delayed message store until they are due. When a message is due, it is retreived from the store and dispatched to its destination.

The MSMQ transport requires explicit configuration to enable delayed message delivery. 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 = new MsmqTransport
{
    DelayedDelivery = new DelayedDeliverySettings(messageStore)
    {
        NumberOfRetries = 7,
        MaximumRecoveryFailuresPerSecond = 2,
        TimeToTriggerStoreCircuitBreaker = TimeSpan.FromSeconds(20),
        TimeToTriggerDispatchCircuitBreaker = TimeSpan.FromSeconds(15),
        TimeToTriggerFetchCircuitBreaker = TimeSpan.FromSeconds(45)
    }
};
endpointConfiguration.UseTransport(transport);

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 of delayed messages

A delayed message is stored using the Store method.

Polling and dispatching of 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 (see next paragraph), 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. If another delayed message is persisted in the meantime, the Store method wakes up the polling thread.

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. In case of an unexpected exception during forwarding the failure is registered using IncrementFailureCount. If the configured number of retries is exhausted the message is forwarded to the configured error queue.

Configuration

The settings described in this section allow changing the default behavior of the built-in delayed delivery store.

NumberOfRetries

Number of retries when trying to forward due delayed messages.

Defaults to 0.

TimeToTriggerStoreCircuitBreaker

Time to wait before triggering the circuit breaker that monitors the storing of delayed messages in the database.

Defaults to 30 seconds.

TimeToTriggerFetchCircuitBreaker

Time to wait before triggering the circuit breaker that monitors the fetching of due delayed messages from the database.

Defaults to 30 seconds.

TimeToTriggerDispatchCircuitBreaker

Time to wait before triggering the circuit breaker that monitors the dispatching of due delayed messages to the destination.

Defaults to 30 seconds.

MaximumRecoveryFailuresPerSecond

Maximum number of failed attempts per second to increment the per-message failure counter that triggers the recovery circuit breaker.

Defaults to 1 per sec.

Using a custom delayed message store

Create a class which implements the IDelayedMessageStore interface and pass an instance to the DelayedDeliverySettings constructor.

If the custom store needs to set up some infrastructure (create tables, etc.) then it must implement IDelayedMessageStoreWithInfrastructure. This interface extends IDelayedMessageStore adding a method for setting up the infrastructure. This new method is called before IDelayedMessageStore.Initialize().

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.

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.