SAA-C03
Deep Dive
Decoupling is key to resilient architectures. SQS adds buffers between components, SNS distributes messages to multiple subscribers, and EventBridge connects services with events. The exam presents scenarios where you must choose the right service.
Contents
SQS is a fully managed message queue service. Producers send messages to the queue and consumers process them at their own pace. It fully decouples producer from consumer.
Key SQS parameters
Visibility Timeout
How long a message stays invisible after being read. Default: 30s. If the consumer doesn't delete it, it goes back to the queue.
Message Retention
How long the message lives. Default: 4 days. Max: 14 days.
Long Polling
WaitTimeSeconds=1-20. Reduces empty calls. More efficient than short polling.
Delay Queue
DelaySeconds=0-900. Delays delivery of new messages.
SQS Standard vs FIFO
Standard Queue
•Unlimited throughput
•At-least-once delivery (can duplicate)
•Best-effort ordering (not guaranteed)
•For most use cases
FIFO Queue
•Up to 3,000 msg/s with batching
•Exactly-once delivery (no duplicates)
•Strict order (First-In-First-Out)
•For payments, inventory, auditing
DLQ (Dead Letter Queue)
Messages that fail N times (maxReceiveCount) are moved to a DLQ for analysis. Set up alarms on the DLQ to detect issues.
SNS is a pub/sub notification service. A producer publishes to a topic and all subscribers receive the message simultaneously. Push-based: SNS pushes to subscribers (subscribers don't poll).
SQS Queue
Lambda
Email / Email-JSON
HTTP/S endpoints
SMS / Mobile Push
Amazon Data Firehose
Subscriber types supported by an SNS Topic
Key difference: SNS vs SQS
SNS = Push — SNS sends the message to all subscribers immediately. There's no persistence in SNS.
SQS = Pull — The consumer polls the queue to get messages. Messages persist for up to 14 days.
EventBridge is a serverless event bus. It connects applications using events from AWS services, third-party SaaS (Zendesk, Shopify) or your own applications. More powerful than SNS for event-driven architectures.
Event Rules
Filters events by pattern (e.g. "only EC2 instance-state-change events to running") and routes them to targets. Multiple targets per rule.
EC2 terminates → EventBridge → Lambda + SNS + SQS simultaneously
Scheduled Events (Cron)
Runs rules on a defined schedule. Replaces CloudWatch Events for scheduling.
"Run Lambda every Monday at 8am UTC"
The Fan-out pattern combines SNS + SQS: a producer publishes to SNS, which distributes the message to multiple SQS queues. Each consumer service has its own queue and processes at its own pace.
Why Fan-out instead of multiple direct queues
Simple producer
The producer publishes to a single SNS topic. It doesn't need to know how many consumers there are.
Buffer per service
Each service has its own independent SQS queue. If one fails, the others keep working.
Scalability
Adding a new consumer = adding an SNS subscription. No changes to the producer.
| Service | Model | Persistence | When to use |
|---|---|---|---|
| SQS | Pull (consumer polls) | Up to 14 days | Buffering, decoupling, processing at your own pace, automatic retry |
| SNS | Push (SNS notifies) | No persistence | Fan-out, alerts, notifications to multiple subscribers |
| EventBridge | Event bus | No persistence (optional archive) | Event-driven, SaaS integration, scheduled tasks, complex routing |
Exam trap
If the scenario mentions "messages must not be lost if the service goes down" — you need SQS (it persists). If it mentions "multiple services must receive the same message" — SNS fan-out or EventBridge. Each service processes at its own pace.
An SNS message distributed to multiple independent SQS queues.
Did you get this topic?
Test what you just learned
An e-commerce application needs to process purchase orders such that: (1) the inventory, billing and notifications services each process every order independently, (2) if one service fails, messages are not lost, (3) each service processes at its own pace. What is the best design?
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