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.
A delayed message store implements the
IDelayedMessageStore interface. Delayed message delivery has two parts:
- Storing delayed messages via the
- Polling and dispatching the 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
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
To create a custom storage, implement the
IDelayedMessageStore interface and pass an instance to the
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.
TransactionScope transaction mode, the delayed message store is expected to enlist in the
TransactionScope to ensure exactly once behavior.
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.
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.