Commands and Events

Commands and events are the fundamental building blocks of a messaging system.

Commands and events are both kinds of messages. There is nothing material that differentiates commands and events from any kind of message. There isn't a special base class or mixin that causes a message to act either as a command or an event. Messages play the role of either a command or an event, but their implementation is largely identical except for the presence of attributes that may record the different timestamps of the processes that they participate in.

A command is a message that represents an instruction sent to a component. A command is an input message. Commands are sent directly to components. They are a unidirectional transmission from one endpoint directly to another.

An event is a message that represents the effects of processing a command.

When a component processes a command, the result of that process is the writing of one or more events. Those events are the source material that event sourcing and pub/sub patterns are built upon.