CQRS without Multiple Data Sources

One of the most common misconceptions about CQRS is it implies Eventual Consistency. That you must have different data sources for your commands and queries. Meaning you will have a use one data source for commands/writes and an entirely different data source for query/reads. This is simply untrue.

This assumption implies that you’re query/read data source will be eventually consistent with the command/write side. This is because the assumption is your commands will write to its data source, then emit events that will be processed and update your query/read database independently.

If you’re unfamiliar with CQRS, I highly recommend checking some other posts I’ve written about CQRS before reading futher.

Different Models

One of the benefits of applying CQRS is that you can have different representations of your data. Your write model may look very different than your read model.

However, this doesn’t mean you need to have different data sources and use event handlers to build your query model.

Views

If you’re just getting into applying CQRS, you can use the exact same underlying data model for both commands/writes and queries/reads. There’s nothing saying you can’t.

However, if you’re using a relational database you can get all the benefits of tailored query models by mapping your queries/reads models to database views. Or if you database supports it, materialized views.

If you’re using Entity Framework Core, this is pretty straight forward by defining your query types in the OnModelCreating method of your DbContext.

Consistentcy

This means you’re command/write model and query/read models are always 100% consistent. You’re not dealing with eventual consistency.

Another bonus is you’re not writing event handlers to update your read/query database which also eliminates a pile of code and complexity.

From my experience, when applied wrong, eventual consistency can be a giant pain and not at all what you’re users are expecting.

Most often users are expecting to click a button and see the results immediately. Obviously, there are many ways to handle this, but if you’re new to CQRS, my initial recommendation is to keep things as simple as possible and that means keeping data consistent.

Start simple:

  • Create a class that changes state (command) and create a separate class that reads state (queries).
  • Use SQL Views (or materialized views) to map tailored queries.
  • Use something like Automapper for compositing the query result.

Atomic

If using Views isn’t an option, and you’re using the same relational database for both reads and writes another option is to wrap the entire operation in a transaction. This means your operation to modify your database records for the command, as well as modify database records for your queries happen within the same transaction.

I’ll elaborate more on this, eventual consistency, event sourcing and more in coming posts.

Fat Controller CQRS Diet

I’ve blogged a bit about how to implement CQRS without any of the other fluff. You can check out my Fat Controller CQRS Diet blog series as well as a related talk:

If you have any questions or comments, please let me know on twitter as I will focus my posts on those questions and comments.

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