Do’s and don’ts

Whenever creating an application that uses CQRS+ES there are several things you need to keep in mind to make it easier and minimize the potential bugs. This guide will give you some details on typical problems and how EventFlow can help you minimize the risk.

Business rules

Specifications

Consider moving complex business rules to specifications. This eases both readability, testability and re-use.

Events

Produce clean JSON

Make sure that when your aggregate events are JSON serialized, they produce clean JSON as it makes it easier to work with and enables easier deserialization of events in the future.

• No type information
• No hints of value objects (see value objects)

Here’s an example of good clean event JSON produced from a create user event.

{
  "Username": "root",
  "PasswordHash": "1234567890ABCDEF",
  "EMail": "root@example.org",
}

Keep old event types

Keep in mind that you need to keep the event types in your code for as long as these events are in the event source, which in most cases is forever as storage is cheap and information, i.e., your domain events, are expensive.

However, you should still clean your code. Have a look at how you can upgrade and version your events for details on how EventFlow supports you in this.

Subscribers and out of order events

Be very careful if aggregates emit multiple events for a single command, subscribers will almost certainly receive these out of order.