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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.

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

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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 TypeDescription
DefaultAutomatically available; receives events from AWS services.
CustomCreated by you; receives events from your applications.
PartnerIngests 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.

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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"]
  }
}

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How They Work Together

The following table summarizes the roles of each component:
ComponentRoleExample Target
EventPayload describing a change{ "paymentStatus":"SUCCESS" }
Event BusCentral router for eventsDefault or custom bus
Event RuleFilters and forwards matching eventsLambda, SNS, SQS, etc.

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E-Commerce Order Processing Workflow

  1. OrderPlaced event is emitted when a customer completes checkout.
  2. EventBridge sends the event to the default event bus.
  3. An event rule filters for paymentStatus: ["SUCCESS"].
  4. 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"
Ensure your event rule IAM role has permission to invoke the target service (lambda:InvokeFunction, sns:Publish, etc.).

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References

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