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, using the Store method.
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.
Using a custom delayed message store
Create a class which implements the IDelayedMessageStore interface and pass an instance to the DelayedDeliverySettings constructor.
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.