Discarding expired messages

Component: SQL Server Transport
NuGet Package NServiceBus.SqlServer (4-pre)
Target NServiceBus Version: 7.x
This page targets a pre-release version and is subject to change prior to the final release.

Sending

The messages with Time-To-Be-Received (TTBR) set are stored in the queue table just like regular messages. The only difference is that the Expires column is set to the time (in UTC) when the message expires. The calculation of expiry time is done on the database level so that clock drifts between endpoints don't affect the TTBR.

Receiving

The messages with specified Time-To-Be-Received (TTBR) are pushed through the message pump, just like regular messages, regardless if their TTBR already elapsed or not. The receive query compares the current UTC time in the database with the Expires column value and sets a flag indicating if a given message has expired. When the message is received from the database that flag is checked and if it is set, the message is dropped and not passed into the processing pipeline. Setting the flag on the database level prevents the TTBR feature from being affected by potential clock drift of machines running sender and receiver endpoints.

In addition to that, the expired messages can be removed from the queue before the endpoints starts up. In this case the transport removes all expired messages, in batches, using delete statements. Each batch is by default limited to 10'000 messages (that setting is configurable). The receiver is started when there are no messages to remove.

To enable start-up expired message purging use:

var transport = endpointConfiguration.UseTransport<SqlServerTransport>();
transport.PurgeExpiredMessagesOnStartup(purgeBatchSize: 5000);

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