Architecture Patterns with Python

Like events, commands are a type of message—instructions sent by one part of a system to another. We usually represent commands with dumb data structures and can handle them in much the same way as events. The differences between commands and events, though, are important. Commands are sent by one actor to another specific actor with the expectation that a particular thing will happen as a result. When we post a form to an API handler, we are sending a command. We name commands with imperative mood verb phrases like “allocate stock” or “delay shipment.” Commands capture intent. They express our wish for the system to do something. As a result, when they fail, the sender needs to receive error information. Events are broadcast by an actor to all interested listeners. When we publish BatchQuantityChanged, we don’t know who’s going to pick it up. We name events with past-tense verb phrases like “order allocated to stock” or “shipment delayed.” We often use events to spread the knowledge about successful commands. Events capture facts about things that happened in the past. Since we don’t know who’s handling an event, senders should not care whether the receivers succeeded or failed.