CQRS Architecture

• Stands for Command and Query Responsibility Segregation.
• The central concept of this pattern is that an application has read operations and write operations that must be totally separated.
• This also means that the model used for write operations (commands) will differ from the read models (queries).
• Furthermore, the data will be stored in different locations.
  • In a relational database, this means there will be tables for the command model and tables for the read model.
  • Some implementations even store the different models in totally different databases, e.g. SQL Server for the command model and MongoDB for the read model.
• This pattern is often combined with event sourcing.

Mechanics

• There are two types of methods in object-oriented software: commands and queries.
  • Commands are operations that change the application state and return no data.
  • These are the methods that have side effects within the application.
  • Queries are operations that return data but don’t change application state.
• In concrete terms for .NET developers, your application will have two “stacks,” or namespaces.
  • One namespace will be YourApplication.CommandStack and the other YourApplication.QueryStack.
  • This structure gives you a clear picture of what pieces of code change the state of the application.
  • If you like to think of software in terms of domain models, this structure has two of them—a query model and a command model.
  • These are different models that have their own operations and representations of data.
  • These representations can then be optimized for their specific purpose.
• Changes to data tend to cause the most bugs.
  • Having a clear understanding of which parts of the application change data and which do not will help with maintainability and debugging.

Workflow

1. When a user performs an action, the application sends a command to the command service.
2. The command service retrieves any data it needs from the command database, makes the necessary manipulations and stores that back in the database.
3. It notifies the read service so that the read model can be updated.
4. When the application needs to show data to the user, it can retrieve the read model by calling the read service.

CQRS Principles

Basic CQRS

• Following basic CQRS design allows for optimization of reads and writes.
• The design of the query stack represents the most efficient way to read data.