App Integration — Step Functions, EventBridge, API Gateway

The glue stack (mental models)

Step Functions = workflow director (like n8n) — steps A→B→C in order, retries, human approval
EventBridge   = post office — delivers events to whoever cares, with FILTERING rules + cron
SNS           = megaphone — broadcast to subscribers (topic fan-out)
SQS           = inbox — hold messages until consumer ready
Lambda        = worker — does the actual job
API Gateway   = front door/receptionist — accepts outside HTTP requests

Step Functions

• Visual state machine: validate → charge → ship → notify
• Steps: Lambda, ECS, API GW, SNS, SQS, DynamoDB, HTTP, WAIT, human approval
• vs Lambda: Lambda = one function; SFN = orchestrate MANY functions
• Max runtime 1 year (wait/approval steps)
• “orchestrate complex multi-step workflow” → Step Functions

EventBridge (event bus)

• AWS services publish events NATIVELY (S3 upload, EC2 state, DynamoDB) + your apps + CRON
• Rules FILTER by content → route to matching targets (Lambda/SNS/SQS/SFN/ECS)
• Two jobs: service-to-service routing + scheduled tasks (cron(0 2 * * ? *))
• Replaced CloudWatch Events (renamed)
• Replay/archive window ~14 days

vs SNS: SNS = broadcast all subscribers (no filtering). EventBridge = rules filter by content + scheduling + cross-account. EventBridge = SNS + filtering + cron.

API Gateway

• Managed REST / HTTP / WebSocket API endpoint in front of Lambda/services
• Features: auth (IAM/Cognito/API keys/Lambda authorizer), throttling, caching, stages, CloudWatch
• Types: REST API, HTTP API (cheaper), WebSocket API
• “expose Lambda as REST API” → API Gateway

EventBridge vs RabbitMQ

	RabbitMQ	EventBridge
Sources	YOUR apps only	AWS services native + yours + cron
Routing	Exchanges/bindings	Rules (content filter)
Transport	AMQP	HTTPS/JSON
Guarantees	Strong (persistent queues)	At-least-once-ish, 24h retry
Managed	You run / Amazon MQ	Fully AWS
Scheduling	No	Yes (cron)

• RabbitMQ = your message router (apps both sides)
• EventBridge = AWS event router (AWS services already plugged in)
• Amazon MQ = literally managed RabbitMQ/ActiveMQ

Full pattern (how they combine)

User → API Gateway → Lambda (validate) → Step Functions (charge→ship→notify)
  → EventBridge (analytics + audit + email events) → SQS → Lambda (async workers)

Exam mapping

Question	Answer
Orchestrate multi-step with retries	Step Functions
Route events with filtering	EventBridge
Schedule Lambda daily 2AM	EventBridge rule (cron)
Expose Lambda as REST API	API Gateway
Human approval in workflow	Step Functions
WebSocket real-time API	API Gateway WebSocket

Related

• [[aws/index]]
• [[aws/lambda-usage/index]]
• [[aws/messaging-trio/index]]