Creating IDs with CQRS and Event Sourcing in Java and .NET

I’m currently working on a cloud-based system that uses microservices with domain driven design and CQRS and Event Sourcing in Java. I love C# and .NET but I decided to do this in Java for several reasons. This could have been done in C# just as well. In fact, this system is a mix of some C# microservices and Java microservices. The important thing is that we are using the same concepts that are applicable when doing DDD and CQRS and Event Sourcing.

Anyways, I wanted to put down my thoughts on what I have been thinking about since last week and I hope this might be useful to anyone who came across the same question. So, my question since last week was this:

“Who creates an entity id in DDD when doing CQRS and Event Sourcing?”

Well, the way I have been doing this was as follows:

  1. When an application service executes a use case via one of its methods, it can create a new entity via a new immutable value object. All my entities’ ids are immutable value objects.
  2. Sometimes, a factory in the domain model creates new entities. Again, the entities’ ids are created via immutable value objects.
  3. Sometimes, a domain service creates new entities with immutable value objects.

This is all fine and good. I started to look at Greg Young‘s sample C# project at his Github here. For this who do not know him, he is a fantastic speaker and mentor for CQRS and Event Sourcing. Check out his videos, papers, and blog posts. Excellent material. He has a free 6 hour video here. And, while you are at it, check out his video about: “7 Reasons Why DDD Projects Fail“.

What poked my interest was this particular line in his CreateInventoryItem command code:

public class CreateInventoryItem : Command {
    public readonly Guid InventoryItemId;
    public readonly string Name;

    public CreateInventoryItem(Guid inventoryItemId, string name)
    {
        InventoryItemId = inventoryItemId;
        Name = name;
    }
}

If you look closely on line 5, you will see that the entity id inventoryItemId is being passed as part of the CreateInventoryItem command. This is a small but important detail.

I had an interesting thread discussion with Greg Young about this here. Greg raised a few good questions:

  1. What about when you want to send 3 commands?
  2. How will you start returning ids from commands?
  3. What if a command creates 3 entities?
  4. How will the domain get those ids back to the client?
  5. My UI is creating an account then the next step is managing some details. How does the UI go from one step to the next without knowing the account id?

As you can see, there are several good reasons why a client would want to create the ids instead of the domain model. Another reason I would want to add to this is that if the domain would create the ids, it would violate the goal of keeping track of all state changes in domain events with the commands as the initiators. In my opinion:

In a CQRS based system with Event Sourcing, commands carry all attributes that are required to execute the command including the creation of identities.

So, a client can create a command with ids that are based on Guids/Uuids and then send these creation commands with those ids. This allows you to do things such as persisting the commands and being able to replay those commands as well with 100% accuracy because the domain model with arrive every time to a deterministic state when you replay the domain events.

I like this very much and it provides a lot of freedom with inductive UIs / task based UIs.

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