Amazon EventBridge: Event Bus and Rules

Amazon EventBridge (formerly CloudWatch Events) is a serverless event bus that makes it easy to connect applications using data from your own applications, integrated SaaS applications, and AWS services. It is the backbone of modern Event-Driven Architectures (EDA).

The Real-World Analogy

Imagine a Central Post Office. Thousands of packages (events) arrive from different senders (AWS Services, SaaS apps). The Post Office doesn’t open the packages; it looks at the label (metadata/pattern). Based on specific sorting rules, it routes the package to the correct delivery truck (Target) like Lambda, SQS, or an external API. If no rule matches, the package is discarded or sent to a Dead Letter Queue (DLQ).

Core Concepts for SAA-C03

1. Event Buses

• Default Event Bus: Receives events from AWS services (e.g., EC2 state changes, S3 via CloudTrail).
• Custom Event Bus: Created for your own applications to send custom events.
• SaaS Event Bus: Receives events from third-party partners like Zendesk, Datadog, or PagerDuty.

2. Rules and Filtering

Rules evaluate incoming events. If an event matches a JSON Event Pattern defined in a rule, EventBridge routes it to the specified target. A single rule can route to up to 5 targets simultaneously.

3. Targets

Common targets include AWS Lambda, Amazon SNS, Amazon SQS, Step Functions, and even API Destinations (which allow you to send events to any HTTP endpoint outside of AWS).

Comparison: EventBridge vs. SNS vs. SQS

Feature	EventBridge	SNS (Simple Notification Service)	SQS (Simple Queue Service)
Primary Pattern	Event-Driven (Many-to-Many)	Pub/Sub (One-to-Many)	Point-to-Point (Decoupling)
Filtering	Advanced JSON Pattern Matching	Attribute-based only	None (Consumer filters)
SaaS Integration	Native Integration	No	No
Latency	~500ms (Higher than SNS)	Sub-100ms	Sub-100ms

Decision Matrix: If-Then Guide

• IF you need to trigger a process based on an EC2 instance state change… THEN use EventBridge (Default Bus).
• IF you need to fan-out a message to 10,000+ subscribers with ultra-low latency… THEN use SNS.
• IF you need to route events based on the content of the message body… THEN use EventBridge.
• IF you need to retry failed events for up to 24 hours… THEN use EventBridge (default retry policy).

Topics covered:

Summary of key subtopics covered in this guide:

• Default vs Custom vs SaaS Event Buses
• Event Pattern Matching and JSON Filtering
• Targets (Lambda, SQS, API Destinations)
• EventBridge Archive and Replay
• Schema Discovery and Registry
• Cross-account event routing

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