Migrating to Microservice Databases

Chapter 4. CRUD and CQRS

Two of the most popular patterns for dealing with data manipulation are Create, Read, Update, and Delete (CRUD) and Command and Query Responsibility Segregation (CQRS). Most developers are familiar with CRUD because the majority of the material and tools available try to support this pattern in one way or another. Any tool or framework that is promoted as a fast way to deliver your software to market provides some sort of scaffolding or dynamic generation of CRUD operations.

Things start to get blurry when we talk about CQRS. Certainly the subject of microservices will usually invoke CQRS in many different discussions between people at conferences and among members of development teams, but personal experience shows that we still have plenty of room for clarification. If you look for the term “CQRS” on a search engine, you will find many good definitions. But even after reading those, it might be difficult to grasp exactly the “why” or even the “how” of CQRS.

This chapter will try to present clear distinction between and motivation behind using both CRUD and CQRS patterns. And any discussion about CQRS won’t be complete if we do not understand the different consistency models that are involved in distributed systems—how these systems handle read and write operations on the data state in different nodes. We’ll start our explanation with these concepts.

Consistency Models

When we’re talking about consistency in distributed systems, we are referring to the ...