Recently, I’ve been checking out RabbitMQ over C# as a way to implement pub/sub. I’m more used to working with NServiceBus. NServiceBus handles transactions by enlisting MSMQ in a TransactionScope. Other transaction aware operations can also enlist in the same TransactionScope (like MSSQL) so everything is truly atomic. Underneath, NSB brings in MSDTC to coordinate.
I see that in the C# client API for RabbitMQ there is a IModel.TxSelect() and IModel.TxCommit(). This works well to not send messages to the exchange before the commit. This covers the use case where there are multiple messages sent to the exchange that need to be atomic. However, is there a good way to synchronize a database call (say to MSSQL) with the RabbitMQ transaction?
As far as I’m aware there is no way of coordinating the TxSelect/TxCommit with the TransactionScope.
Currently the approach that I’m taking is using durable queues with persistent messages to ensure they survive RabbitMQ restarts. Then when consuming from the queues I read a message off do some processing and then insert a record into the database, once all this is done I ACK(nowledge) the message and it is removed from the queue. The potential problem with this approach is that the message could end up being processed twice (if for example the message is committed to the DB but say the connection to RabbitMQ is disconnected before the message can be ack’d), but for the system that we’re building we’re concerned about throughput. (I believe this is called the “at-least-once” approach).
The RabbitMQ site does say that there is a significant performance hit using the TxSelect and TxCommit so I would recommend benchmarking both approaches.
However way you do it, you will need to ensure that your consumer can cope with the message potentially being processed twice.
If you haven’t found it yet take a look at the .Net user guide for RabbitMQ here, specifically section 3.5