Higher Performance Dynamic Consistency Boundary Development with Marten 9.0

To try to explain “Dynamic Consistency Boundary” usage in Event Sourcing, I’d contrast it to “traditional” Event Sourcing where events are only organized into a stream of related events. For example, all the events related to a single invoice in an invoicing system are an example of an event stream. DCB came about because Axon IQ has weak consistency and couldn’t support transactions across multiple streams the way that Marten or Polecat can it’s often impossible to model a system where every operation only involves a single event stream. To that end, folks created the idea of “Dynamic Consistency Boundary” Event Sourcing where events are more organized by tags and the event stores that support DCB are able to enforce transactional boundaries based on an event tag query (think: all the events of these types that are related to either this class id, student id, or instructor id) so that systems can be much more flexible over time.

Marten has had support for the Dynamic Consistency Boundary approach (DCB) to Event Sourcing for a little while. The Marten 9.0 release last week added a new, potentially more performant option for DCB using the PostgreSQL HSTORE extension — which is supported by all the major cloud providers plus specialized cloud providers for managed PostgreSQL like Neon and Supabase.

Unfortunately, we don’t yet have a way to retroactively switch from “classic” DCB to the HSTORE style DCB in an existing application, but let’s say that you’re starting:

• A greenfield application
• A problem domain where the event stream boundaries either aren’t clear upfront or you think will never cleanly line up neatly in terms of event streams

You might want to adopt DCB style Event Sourcing from the get go, then use the HSTORE flavor of Marten DCB to be more performant. To get started, just opt into that style of DCB storage like this:

var builder = Host.CreateApplicationBuilder();
builder.Services.AddMarten(opts =>
{
    opts.Connection(builder.Configuration.GetConnectionString("postgres"));
    // Marten does need to know about the possible tag types
    // upfront, and we rely on value types for this
    opts.Events.RegisterTagType<StudentId>("student");
    opts.Events.RegisterTagType<CourseId>("course");
    // This is all you need to do, but this does assume
    // that the HSTORE extension is available
    opts.Events.DcbStorageMode = DcbStorageMode.HStore;
});

The HSTORE style of DCB is a big performance improvement if you are querying events by two or more tag values at a time, which I’d probably argue is the only time DCB is worthwhile to use from a logical structure perspective anyway:)

I do owe the Critter Stack community a better YouTube video and blog post on using DCB, but for now, I’ll actually send you to the Wolverine documentation on command handlers using DCB with Marten for more examples and context.

Summary

I was the technical lead of a very successful software project for supply chain management in the early 2000’s. The most popular feature within that early web application was a last minute throw in report that I did as a favor for our business contact that wasn’t even part of our original specification. Once the system was live and folks found out about that report, we actually had to add new servers to the application cluster to keep up with the unexpected load just because that one single report was so popular with supply chain analysts.

My only point there is that I’m not always sure what features will actually resonate with users. Sometimes you know based on reported friction that a new feature will eliminate a pain point, but with the DCB support in Marten and Wolverine, I flat out don’t know. DCB is very popular in the Event Sourcing community outside of the Critter Stack, and it was clear we had to have that feature set just to be competitive from an adoption perspective, but I’m not seeing a lot of interest in DCB from our existing community as Marten is much more able to handle more flexible transactional consistency across event streams than specialist Event Store databases seem to be able to do.

But, if DCB is something you’re interested in or just works much easier for your mental model of how the domain should be modeled in your system, Marten has you covered!