Explain the difference between events, commands, and queries.

These are three fundamental message types in distributed systems:

Events:

• Describe something that has already happened
• Named in past tense (OrderPlaced, PaymentProcessed)
• Multiple consumers can react to the same event
• Fire-and-forget nature

Commands:

• Represent an intention to do something
• Named in imperative form (PlaceOrder, ProcessPayment)
• Typically have one handler
• Can be rejected or fail

Queries:

• Request for information
• Don't change system state
• Return data to the caller
• Can be cached and optimized

Example:

Command: PlaceOrder -> Handler processes -> Event: OrderPlaced
Query: GetOrderStatus -> Returns current order state