Show notes

CQRS (Command-Query Responsibility Segregation)

What’s CRUD?

  • Conceptual model where we can Create, Read, Update, and Delete a record in a datastore for that model.


  • At it’s simplest: using a different model or service from writing and reading information.
  • It’s taking CRUD and turning it into R CUD
  • This allows for multiple differents representations of data, sometimes simultaneously. Different access patterns depending on the domain.


  • For very complex use cases, having the same conceptual model for commands and queries makes things more difficult to follow and requires compromises for both these queries and commands.
  • Much more flexible. Eg. Reads could be run on a completely separate physical server.

Most people take it a few steps further

  • Update and read models are separate but are no longer backed by the same table or datastore.
    • Allows independent scaling, access patterns and optimization strategies for reads and writes
    • Allows complex modeling like reads which depend on multiple models, collapsing multiple writes into a single read, read from different models based on complex validation, etc.
    • Requires some sort of propagation from write system to read system

Why not?

  • More complexity and maintenance (eg. multiple models to know about)
  • If using separate tables or databases, achieving full consistency becomes less straight-forward and can lead to resorting to eventual consistency instead.
    • Highly available, but not guarantees around safety.

General notes:

  • Fits well with event-based programming models, event collaboration, event sourcing