Projection tracks events and project their data for specific purposes.

Communication Guide Table

Triggered by Description
Event Domain events represent business changes which have already happened

Best Practices

{% hint style="success" %} You can/should/must...

  • a projection must be idempotent
  • a projection must not issue new commands or events {% endhint %}

{% hint style="warning" %} You should not...

  • a projection should not query other projections. All the data of a projection must be collected from the Events' data
  • a projection should not establish calls to external systems {% endhint %}