A command is a message that instructs another system to perform a specific action. In an online-store workflow, a promotion-monitoring service can send commands to an independent notification service, which may serve many other services. Systems commonly combine commands with states, deltas, and events. section 3.2.1 section 3.3
Customer-notification commands
After a promotion starts, a producer can publish one command per customer: {"command":"notify_customer","customer_id":123456,"message":"The promotion for Coke Zero started. It's now 1.99"}. Each message requests the notification service to perform the same action for a different customer. section 3.2.1
# One message per line
{"command": "notify_customer", "customer_id": 123456,
"message": "The promotion for Coke Zero started. It's now 1.99"}
{"command": "notify_customer", "customer_id": 123457,
"message": "The promotion for Coke Zero started. It's now 1.99"}
{"command": "notify_customer", "customer_id": 123458,
"message": "The promotion for Coke Zero started. It's now 1.99"}
Browse the line-oriented command messages used to request customer notifications. section 3.2.1
In Kafka, commands are exchanged as messages in topics. Topic partitions support distribution and performance, while schemas help maintain consistent command data. section 3.3
