Overview

• CQRS often goes hand in hand with event sourcing.
• This is a pattern where you don’t store the current state of your model in the database, but rather the events that happened to the model.
  • When the name of a customer changes, you won’t store the value in a “Name” column.
  • You will store a “NameChanged” event with the new value (and possibly the old one too).
  • When you need to retrieve a model, you retrieve all its stored events and reapply them on a new object.
• This is caleed "rehydrating an object".
• A real-life analogy of event sourcing is accounting.
  • When you add an expense, you don’t change the value of the total.
  • A new line is added with the operation to be performed.
  • If an error was made, you simply add a new line.
  • To make your life easier, you could calculate the total every time you add a line.
  • This total can be regarded as the read model.
• The event sourcing works in a wayt that you never remove events, because they have undeniably happened in the past.
  • To correct situations, we add new events.
• Event sourcing is often combined with CQRS because rehydrating an object can have a performance impact.
  • A fast read model can significantly improve the response time of the application.
• A microservice architecture based on events and commands helps with resilience, scalability, traceability, and loose coupling in the system.
• Events represent facts of information about something that has happened in the past (for example, OrderCreated, ProductShipped).
  • Events can be disregarded but they cannot be retracted or deleted.
  • New events/facts arrive in the system that can invalidate the existing facts.
• Commands, on the other hand, are the object form of a method/action.
  • They are imperative.
  • Examples of commands are CreateOrder and ShipProduct.

Advantages

• This software architecture pattern can provide an audit log out of the box.
• Each event represents a manipulation of the data at a certain point in time.

Disadvantages

• It requires some discipline because you can’t just fix wrong data with a simple edit in the database.
• It’s not a trivial task to change the structure of an event.
  • If you add a property, the database still contains events without that data.
  • Your code will need to handle this missing data graciously.

Scenarios

• Need to publish events to external systems.
• Will be built with CQRS.
• Have complex domains.