Messages: Events versus Commands
Differences
| Aspect | Event | Command |
|---|---|---|
| Contract | Producer of event determines & publishes schema | Producer of commands has an agreed contract with receiver |
| Contract testing | Consumers test against published schema | Subject to a contract which either party (producer & consumer) can test against |
| Relationship | Producer of an event has no knowledge of consumers | Producer of a command knows there is one and only one well known consumer |
| Language and tense | Events are verbs in the past tense | Commands are verbs in the present tense |
| Message payload | Contains the state that relates to the event, not an entire model or graph | Contains all parameters associated with the execution of the command |
| Dead-letter queue (DLQ) | Consumer has a private DLQ in its broker where it may store messages | Consumer is able to place the unprocessed command on a DLQ of the publisher’s broker |
Similarities
Sequencing
It is a burden upon any consumer to process messages in sequence whenever it has scaled horizontally
Discriminant
It is a burden upon any producer to indicate the message intent of event notification or command invocation through a message header or message name