Event Event Bus and Event Rule

Welcome to this lesson on AWS Events, Event Buses, and Event Rules. By the end of this guide, you’ll understand how these components interact to build decoupled, scalable architectures.

What Is an Event?

An event is a JSON document that captures a state change or activity in your environment. Think of it as a structured message with metadata and payload—whenever something happens (for example, a new order is placed), an event is emitted.

Example event payload:

{
  "version": "0",
  "id": "abcd-1234-efgh-5678",
  "detail-type": "OrderPlaced",
  "source": "ecommerce.app",
  "time": "2024-06-01T12:34:56Z",
  "detail": {
    "orderId": "12345",
    "paymentStatus": "SUCCESS",
    "amount": 99.99
  }
}

Note

Events are immutable and timestamped. Use descriptive detail-type and source fields to simplify filtering.

What Is an Event Bus?

An event bus acts as a central router for all your events. AWS EventBridge provides three types of buses:

Bus Type	Description
Default	Automatically available; receives events from AWS services.
Custom	Created by you; receives events from your applications.
Partner	Ingests events from SaaS partners and third-party services.

The event bus handles ingestion, filtering, and routing—abstracting away the need to maintain custom message brokers.

What Is an Event Rule?

An event rule inspects incoming events on a bus against a pattern or schedule. When an event matches, the rule routes it to one or more targets (Lambda functions, SNS topics, SQS queues, Step Functions, etc.).

Key features:

Pattern matching: Filter by JSON fields, prefixes, or numeric ranges.
Scheduled events: Trigger actions on cron or rate-based schedules.
Multiple targets: Fan out a single event to various downstream services.

Example rule pattern:

{
  "source": ["ecommerce.app"],
  "detail-type": ["OrderPlaced"],
  "detail": {
    "paymentStatus": ["SUCCESS"]
  }
}

How They Work Together

The following table summarizes the roles of each component:

Component	Role	Example Target
Event	Payload describing a change	`{ "paymentStatus":"SUCCESS" }`
Event Bus	Central router for events	Default or custom bus
Event Rule	Filters and forwards matching events	Lambda, SNS, SQS, etc.

E-Commerce Order Processing Workflow

OrderPlaced event is emitted when a customer completes checkout.
EventBridge sends the event to the default event bus.
An event rule filters for paymentStatus: ["SUCCESS"].
Matched events trigger a Lambda function that fulfills the order and sends a confirmation email.

# Create a rule targeting a Lambda function
aws events put-rule \
  --name ProcessSuccessfulOrders \
  --event-pattern '{
    "source": ["ecommerce.app"],
    "detail-type": ["OrderPlaced"],
    "detail": { "paymentStatus": ["SUCCESS"] }
  }'

aws events put-targets \
  --rule ProcessSuccessfulOrders \
  --targets "Id"="1","Arn"="arn:aws:lambda:us-east-1:123456789012:function:FulfillOrder"

Warning

Ensure your event rule IAM role has permission to invoke the target service (lambda:InvokeFunction, sns:Publish, etc.).

References

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