EventBridge

Serverless event bus for connecting AWS services, SaaS apps, and custom events

Amazon EventBridge is a serverless event bus that makes it easy to connect AWS services, SaaS applications, and your own applications using events. It routes events from sources to targets based on rules, enabling loosely coupled, event-driven architectures without managing infrastructure. EventBridge has largely superseded CloudWatch Events and is now the primary event routing service in the AWS ecosystem.

Event Buses, Rules, and Targets

EventBridge uses three core concepts: event buses receive events, rules filter and route events, and targets process events.

Concept	Description	Key Points
Event bus	Stream of events from a source	Default bus (AWS services), custom buses, partner buses
Event	JSON object with source, detail-type, detail, and metadata	Max 256 KB, always JSON
Rule	Matches events based on pattern and routes to targets	Up to 5 targets per rule, evaluated in parallel
Target	Destination that processes the event	Lambda, SQS, SNS, Step Functions, Kinesis, HTTP endpoint, etc.
Schema registry	Discovers and stores event schemas	Enables code generation for strongly-typed event handling

bash

# Publish a custom event to EventBridge
aws events put-events \
  --entries '[{
    "Source": "com.mycompany.orders",
    "DetailType": "OrderPlaced",
    "Detail": "{\"orderId\":\"12345\",\"customerId\":\"67890\",\"amount\":99.99}",
    "EventBusName": "my-app-bus"
  }]'

# Create a rule that matches OrderPlaced events
aws events put-rule \
  --name catch-order-placed \
  --event-bus-name my-app-bus \
  --event-pattern '{
    "source": ["com.mycompany.orders"],
    "detail-type": ["OrderPlaced"]
  }' \
  --state ENABLED

EventBridge vs SNS vs SQS: Choosing the Right Service

EventBridge, SNS, and SQS all route messages between components but serve different purposes. This is a very common interview topic.

Dimension	EventBridge	SNS	SQS
Primary use	Event routing across services, SaaS, accounts	Fan-out notifications	Queuing and buffering
Routing	Content-based filtering on any JSON field	Subscription filter policies on attributes	No routing - single consumer group
Schema	Built-in schema registry	No schema management	No schema management
Targets	20+ native targets including cross-account	SQS, Lambda, HTTP, email, SMS	Only Lambda and EC2 consumers
Third-party sources	Native SaaS integrations (Zendesk, Stripe, etc.)	No	No
Cross-account	Resource policy on event bus	Topic policy	Queue policy
Latency	~500ms typical	~10ms typical	Polling adds latency
Throughput	10,000 events/sec/account (increasable)	300M+ messages/sec	Unlimited
Pricing	$1 per million events published	$0.60 per million deliveries	$0.40 per million requests

💡

Use EventBridge when: routing based on event content, integrating with SaaS event sources, routing across AWS accounts, or working with AWS service events. Use SNS when: fan-out to many subscribers with low latency matters. Use SQS when: you need buffering, exactly-once processing, or consumer-paced processing.

Common EventBridge Patterns

EventBridge enables several powerful architecture patterns that are difficult or impossible to implement with SNS/SQS alone.

Pattern	Description	How EventBridge Helps
Event-driven microservices	Services publish events; other services subscribe	Central bus with per-service rules; no direct dependencies
Cross-account event routing	Events from account A processed in account B	Event bus resource policies allow cross-account sends
SaaS event integration	React to Stripe payments, Zendesk tickets, etc.	Native partner event sources - no webhook plumbing needed
Scheduled tasks	Replace cron jobs with managed schedules	EventBridge Scheduler - 1 minute to years, with retry
Audit logging	Log all API calls and state changes	Route CloudTrail events to S3 via Firehose
Event replay	Reprocess past events after fixing a bug	Event archive + replay feature

bash

# EventBridge Scheduler - create a one-time schedule
aws scheduler create-schedule \
  --name send-reminder-tomorrow \
  --schedule-expression "at(2024-12-15T09:00:00)" \
  --flexible-time-window '{"Mode":"OFF"}' \
  --target '{
    "Arn": "arn:aws:lambda:us-east-1:123:function:send-reminder",
    "RoleArn": "arn:aws:iam::123:role/scheduler-role",
    "Input": "{\"userId\":\"abc123\",\"message\":\"Your trial ends tomorrow\"}"
  }'

# Rate-based schedule (every 15 minutes)
aws scheduler create-schedule \
  --name cleanup-job \
  --schedule-expression "rate(15 minutes)" \
  --flexible-time-window '{"Mode":"FLEXIBLE","MaximumWindowInMinutes":5}' \
  --target '{"Arn":"arn:aws:lambda:...","RoleArn":"arn:aws:iam::..."}'

EventBridge Pipes: Point-to-Point Event Processing

EventBridge Pipes (launched 2022) connects sources directly to targets with optional filtering and enrichment - without writing glue code. It is the simplest way to build event processing pipelines.

Component	Options	Notes
Source	SQS, DynamoDB Streams, Kinesis, MSK, RabbitMQ, ActiveMQ	Polls the source automatically
Filter	JSON pattern matching on event content	Reduces downstream processing cost
Enrichment	Lambda, Step Functions, API Gateway, EventBridge ApiDestination	Optional - transform or augment event
Target	14+ targets including EventBridge bus, SQS, Lambda, Kinesis, Step Functions	Final destination

💡

EventBridge Pipes replaces the common pattern of SQS -> Lambda -> EventBridge with a managed pipe that does the same thing. Use Pipes when you need to move events from one service to another with optional transformation - it removes boilerplate Lambda code.

EventBridge Pricing

Feature	Price	Notes
Custom events published	$1.00 per million	Most common charge
AWS service events	Free	CloudTrail, EC2, S3 state changes
Cross-account events	$1.00 per million	Charged on sending account
Schema discovery	$0.10 per million events	Only when enabled
Event replay	$0.10 per GB archived	Plus $0.023 per GB replayed
Scheduler invocations	$1.00 per million scheduled invocations	First 14 million/month free
Pipes	$0.40 per million events processed	Plus source polling costs

💡

At $1 per million events, EventBridge is 2.5x more expensive than SNS for raw delivery. The premium buys you content-based routing, schema registry, SaaS integrations, and cross-account routing. For pure fan-out, SNS is cheaper.

🎯

Interview Focus Points

1What is the difference between EventBridge, SNS, and SQS? When would you use each?
2How does EventBridge content-based filtering differ from SNS filter policies?
3How would you route events from one AWS account to another using EventBridge?
4What is EventBridge Scheduler and how does it differ from CloudWatch Events rate/cron expressions?
5How would you integrate a Stripe webhook into your AWS architecture using EventBridge?
6What is the EventBridge schema registry and why is it useful for teams?
7Explain EventBridge Pipes and how they simplify event processing pipelines.
8How would you implement event replay after discovering a bug in an event consumer?