# Events, Commands, Queries and the Four Event Patterns — Event-Driven Architecture & CQRS

Source: https://www.skillbyai.com/en/event-driven-architecture/f-terms

> Distinguish events from commands and name the main event-driven patterns.

## Words that change the design

A **command** is a request to do something, addressed to one handler, which may refuse it: `PlaceOrder`, `ChargeCard`. An **event** is a statement of a **fact that already happened**, named in the past tense and addressed to nobody in particular: `OrderPlaced`, `PaymentFailed`. A **query** asks for data without changing anything. Martin Fowler describes four patterns that people loosely call "event-driven". **Event notification**: a thin event says something happened, and consumers call back for details. **Event-carried state transfer**: the event includes the data consumers need, so they can keep a local copy and stop calling the source. **Event sourcing**: the sequence of events is the system of record, and current state is derived from it. **CQRS**: separate models for writing and reading. They combine well, but each has different costs, so name which one you mean in design discussions.

## A command and the events it may cause

A command can be rejected; an event cannot be undone, only followed by another event.

```json
// command: addressed to the Orders service, may fail validation
{ "type": "PlaceOrder", "customerId": "c-42", "items": [{ "sku": "pen", "qty": 2 }] }

// events: facts published after the decision
{ "type": "OrderPlaced", "orderId": "o-1001", "customerId": "c-42", "total": "40.00" }
{ "type": "OrderRejected", "customerId": "c-42", "reason": "OUT_OF_STOCK" }
```

## Commands in disguise

An "event" named `SendWelcomeEmail` is really a command aimed at one consumer. Publish `CustomerRegistered` instead and let the email service decide to send a welcome email. That keeps the publisher ignorant of its consumers.

**Quiz:** Which of these is correctly named as an event?

- [ ] ChargeCard
- [ ] GetOrderStatus
- [ ] ReserveStock
- [x] PaymentCaptured

*Answer:* PaymentCaptured. Events describe facts in the past tense; the others are commands or a query.