Idempotent Aggregates

Idempotent AggregatesThis blog post will cover creating Idempotent Aggregates and is apart of a series of posts related.

Please take a look any previous posts as they may give more context to the code samples below.


Idempotence

Idempotence is the property of certain operations in mathematics and computer science, that can be applied multiple times without changing the result beyond the initial application.

In my previous post Idempotent Commands, I looked at creating idempotency at the persistence level if you are using an ACID compliance database.

But what happens if you aren’t?  What other options do you have?

Well if you are building a domain model and are creating Aggregates, you could create idempotent aggregates.

Aggregates

If there is enough value and complexity, I will create a domain model with Aggregates that will contain domain behavior and business logic.

This is example is very trivial and I wouldn’t be creating an aggregate for basic CRUD.

However, if we did have a complex domain and aggregate, we can pass in an OperationId to our Aggregate, prior to calling any behavioral methods in order to check if we have already executed that operation.

Handlers

Now in our Command Handler, we can call SetOperation and pass our MessageId from our Envelope<T>.

And as our previous example, here is our NancyFX Module (similar to ASP.NET Controller) which is sending our Command via MediatR.

Concurrency

I have not brought up concurrency as this is something that will be determined based on your data storage solution.

How do you?

How do you handle idempotent commands? I’d love to hear about other implementations or uses.  Comment below or on twitter.


Other Posts

10 thoughts to “Idempotent Aggregates”

  1. The another approach that comes to my mind is connected with the commands being invoked on the basis of events (an event handler reacting to an event, like a projection). If you had a store that can apply total order, like the EventStore, all you need to do is remember the last position of the applied event per handler. This unfortunately doesn’t cover client generated identifiers but still could be useful to lower the size of the idempotence data.

      1. Yes. I haven’t read your articles yet (I’ve just skimmed through), but I will. Additionally, the projections themselves use a very same notion of checkpoints (in other way, a total order within partition) which can be seen in the projection handler API https://github.com/EventStore/EventStore/blob/release-v3.6.0/src/EventStore.Projections.Core/Services/IProjectionStateHandler.cs#L40 I really do like it that the whole database is only a stream of streams.

    1. I suspect I’ll get into using a service bus sooner rather than later for the project i’m working on. Then just Ack back to the client and deal with any issues during processing as more events.

      What are you using for a service bus?

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