Strict Ordered Message Handling wth Wolverine

The feature was built for a current JasperFx Software client, and came with a wave of developments across both Marten and Wolverine to support a fairly complex, mission critical set of application integrations. The PostgreSQL transport new to Wolverine was part of this wave. Some time next week I’ll be blogging about the Marten event subscription capabilities that were built into Marten & Wolverine to support this client as well. The point being, JasperFx is wide open for business and we can help your shop succeed with challenging project work!

Wolverine now has the ability to support strict messaging order with its message listeners. Given any random listening endpoint in Wolverine, just add this directive below to make the message processing be strictly sequential (with the proviso that your error handling policies may impact the order on failures):

var host = await Host.CreateDefaultBuilder().UseWolverine(opts =>
{
    opts.UseRabbitMq().EnableWolverineControlQueues();
    
opts.PersistMessagesWithPostgresql(Servers.PostgresConnectionString, "listeners");

    opts.ListenToRabbitQueue("ordered")
        
        // This option is available on all types of Wolverine
        // endpoints that can be configured to be a listener
        .ListenWithStrictOrdering();
}).StartAsync();

Some notes about the ListenWithStrictOrdering() directive you might have:

It’s supported with every external messaging broker that Wolverine supports, including Kafka, Azure Service Bus, AWS SQS, and Rabbit MQ. It is also supported with the two database backed transports (we have both kinds, Sql Server and PostgreSQL!)
When this directive is applied, Wolverine will only make the listener for each endpoint (in the case above, the Rabbit MQ named “ordered”) be active on a single node within your application. Today that distribution is just crudely spreading out the “exclusive listeners” evenly across the whole application cluster. Definitely note that the strict ordering comes at the cost of reduced throughput, so use this feature wisely! Did I mention that JasperFx Software is here and ready to work with your company on Critter Stack projects?
Every exclusive listener will quickly start up on a single node if WolverineOptions.Durability.Mode = DurabilityMode.Solo, and you may want to do that for local testing and development just to be a little quicker on cold starts
The ListenWithStrictOrdering will make the internal worker queue (Wolverine uses an internal TPL Dataflow ActionBlock in these cases) for “buffered” or “durable” endpoints be strictly sequential
You will have to have a durable message store configured for your application in order for Wolverine to perform the leadership election and “agent tracking” (what’s running where)

Summary

This is a powerful tool in the continually growing Wolverine tool belt. The strict ordering may also be used to alleviate some concurrency issues that some users have hit with event sourcing using Marten when a single stream may be receiving bursts of commands that impact the event stream. The leadership election and agent distribution in Wolverine, in conjunction with this “sticky” listener assignment, gives Wolverine a nascent ability for virtual actors that we will continue to exploit. More soon-ish!