How to make a Saga handler Reentrant

Question

I have a task that can be started by the user, that could take hours to run, and where there's a reasonable chance that the user will start the task multiple times during a run. I've broken the processing of the task up into smaller batches, but the way the data looks it's very difficult to tell what's still to be processed. I batch it using messages that each process a bite sized chunk of the data.

I have thought of using a Saga to control access to starting this process, with a Saga property called Processing that I set at the start of the handler and then unset at the end of the handler. The handler does some work and sends the messages to process the data. I check the value at the start of the handler, and if it's set, then just return.

I'm using Azure storage for Saga storage, if it makes a difference for the next bit. I'm also using NSB 6

I have a few questions though:

• Is this the correct approach to re-entrancy with NSB?
• When is a change to Saga data persisted? (and is it different depending on the transport?)
• Following on from the above, if I set a Saga value in a handler, wait a while and then reset it to its original value will it change the persistent storage at all?

Answer 1

Seem to be cross posted in the Particular Software google group:

• https://groups.google.com/forum/#!topic/particularsoftware/p-qD5merxZQ

Sagas are very often used for such patterns. The saga instance would track progress and guard that the (sub)tasks aren't invoked multiple times but could also take actions if the expected task(s) didn't complete or is/are over time.

The saga instance data is stored after processing the message and not when updating any of the saga data properties. The logic you described would not work.

The correct way would be having a saga that orchestrates your process and having regular handlers that do the actual work.

In the saga handle method that creates the saga check if the saga was already created or already the 'busy' status and if it does not have this status send a message to do some work. This will guard that the task is only initiated once and after that the saga is stored. The handler can now do the actual task, when it completes it can do a 'Reply' back to the saga When the saga receives the reply it can now start any other follow up task or raise an event and it can also 'complete'.

Optimistic concurrency control and batched sends

If two message are received that create/update the same saga instance only the first writer wins. The other will fail because of optimistic concurrency control.

However, if these messages are not processed in parallel but sequential both fail unless the saga checks if the saga instance is already initialized.

The following sample demonstrates this: https://github.com/ramonsmits/docs.particular.net/tree/azure-storage-saga-optimistic-concurrency-control/samples/azure/storage-persistence/ASP_1

The client sends two identical message bodies. The saga is launched and only 1 message succeeds due to optimistic concurrency control.

Due to retries eventually the second copy will be processed to but the saga checks the saga data for a field that it knows would normally be initialized by by a message that 'starts' the saga. If that field is already initialized it assumes the message is already processed and just returns:

It also demonstrates batches sends. Messages are not immediately send until the all handlers/sagas are completed.

Saga design

The following video might help you with designing your sagas and understand the various patterns:

Integration Patterns with NServiceBus: https://www.youtube.com/watch?v=BK8JPp8prXc

Keep in mind that Azure Storage isn't transactional and does not provide locking, it is only atomic. Any work you do within a handler or saga can potentially be invoked more than once and if you use non-transactional resources then make sure that logic is idempotent.

Answer 2

So after a lot of testing

I don't believe that this is the right approach. As Archer says, you can manipulate the saga data properties as much as you like, they are only saved at the end of the handler.

So if the saga receives two simultaneous messages the check for Processing will pass both times and I'll have two processes running (and in my case processing the same data twice). The saga within a saga faces a similar problem too.

What I believe will work (and has done during my PoC testing) is using a database unique index to help out. I'm using entity framework and azure sql, so database access is not contained within the handler's transaction (this is the important difference between the database and the saga data). The database will also operate across all instances of the endpoint and generally seems like a good solution.

The table that I'm using has each of the columns that make up the saga 'id', and there is a unique index on them.

At the beginning of the handler I retrieve a row from the database. If there is a row, the handler returns (in my case this is okay, in others you could throw an exception to get the handler to run again). The first thing that the handler does (before any work, although I'm not 100% sure that it matters) is to write a row to the table. If the write fails (probably because of the unique constraint being violated) the exception puts the message back on the queue. It doesn't really matter why the database write fails, as NSB will handle it.

Then the handler does the work.

Then remove the row. Of course there is a chance that something happens during processing of the work, so I'm also using a timestamp and another process to reset it if it's busy for too long. (still need to define 'too long' though :) )

Maybe this can help someone with a similar problem.