AWS Solutions Architect Exam > AWS Solutions Architect Notes > : Associate Level > Cheat Sheet: SQS & SNS — Decoupling Patterns
Cheat Sheet: SQS & SNS — Decoupling Patterns
1. Amazon SQS Overview
1.1 Core Concepts
| Concept | Description |
|---|---|
| SQS | Fully managed message queuing service for decoupling application components |
| Message Retention | Default 4 days, configurable from 1 minute to 14 days |
| Message Size | 1 byte to 256 KB per message |
| Payload Extension | Use S3 for messages >256 KB with SQS Extended Client Library |
| Queue Limit | Unlimited number of messages in queue |
| Delivery | At-least-once delivery (Standard), exactly-once processing (FIFO) |
1.2 Standard Queue
| Feature | Details |
|---|---|
| Throughput | Nearly unlimited transactions per second |
| Ordering | Best-effort ordering, no guarantee |
| Delivery | At-least-once delivery, occasional duplicates possible |
| Use Case | High throughput applications where order is not critical |
1.3 FIFO Queue
| Feature | Details |
|---|---|
| Throughput | 300 messages/sec (3000 with batching) |
| High Throughput Mode | 3000 messages/sec (30,000 with batching) |
| Ordering | Strict ordering preserved, first-in-first-out |
| Delivery | Exactly-once processing, no duplicates |
| Naming | Queue name must end with .fifo suffix |
| Message Group ID | Required tag to ensure ordering within group |
| Deduplication ID | Token for 5-minute deduplication interval |
| Content-Based Deduplication | SHA-256 hash of message body for automatic deduplication |
2. SQS Message Processing
2.1 Visibility Timeout
| Parameter | Details |
|---|---|
| Definition | Period when message is invisible to other consumers after being received |
| Default | 30 seconds |
| Range | 0 seconds to 12 hours |
| ChangeMessageVisibility API | Extend timeout if more processing time needed |
| Timeout Expiration | Message becomes visible again if not deleted |
2.2 Long Polling
| Concept | Details |
|---|---|
| Purpose | Reduce empty responses and API calls by waiting for messages |
| Wait Time | 1 to 20 seconds (ReceiveMessageWaitTimeSeconds) |
| Benefit | Decreases costs, reduces empty receives |
| Configuration | Queue-level or API-level (ReceiveMessage call) |
| Preferred Over | Short polling (WaitTimeSeconds = 0) |
2.3 Dead Letter Queue (DLQ)
| Aspect | Details |
|---|---|
| Purpose | Store messages that fail processing after maximum receive count |
| MaximumReceives | Threshold after which message moves to DLQ |
| Queue Type Match | DLQ must be same type as source (Standard or FIFO) |
| Retention | Set DLQ retention longer than source queue for debugging |
| Redrive Policy | JSON configuration specifying DLQ ARN and MaximumReceives |
2.4 Delay Queue
| Feature | Details |
|---|---|
| Definition | Postpone delivery of messages to consumers |
| Range | 0 to 15 minutes |
| Default | 0 seconds (immediate delivery) |
| Queue-Level | DelaySeconds parameter affects all messages |
| Message-Level | Override queue default with message timer (Standard only) |
3. SQS APIs and Operations
3.1 Key APIs
| API Call | Function |
|---|---|
| SendMessage | Add single message to queue |
| SendMessageBatch | Add up to 10 messages in single request |
| ReceiveMessage | Poll messages (up to 10) from queue |
| DeleteMessage | Remove message after successful processing |
| DeleteMessageBatch | Remove up to 10 messages in single request |
| ChangeMessageVisibility | Modify visibility timeout for received message |
| PurgeQueue | Delete all messages in queue |
3.2 Message Attributes
- Metadata attached to message as name-value pairs
- Do not count toward 256 KB message size limit
- Types: String, Number, Binary
- Maximum 10 attributes per message
4. Amazon SNS Overview
4.1 Core Concepts
| Concept | Description |
|---|---|
| SNS | Fully managed pub/sub messaging and mobile notifications service |
| Publisher | Application or service that sends messages to topic |
| Topic | Communication channel with access control and delivery policies |
| Subscriber | Endpoint that receives messages from topic |
| Message Size | Maximum 256 KB per notification |
| Fan-Out Pattern | One message to multiple subscribers simultaneously |
4.2 Topic Types
| Type | Details |
|---|---|
| Standard | Best-effort ordering, at-least-once delivery, high throughput |
| FIFO | Strict ordering, exactly-once delivery, 300 msg/sec (3000 with batching) |
| FIFO Naming | Topic name must end with .fifo suffix |
| FIFO Subscribers | Only SQS FIFO queues can subscribe to SNS FIFO topics |
4.3 Subscription Protocols
| Protocol | Details |
|---|---|
| HTTP/HTTPS | Deliver JSON-encoded message to web server endpoint |
| Send message to registered email address | |
| Email-JSON | Send JSON-formatted message to email |
| SQS | Deliver message to SQS queue |
| Lambda | Invoke Lambda function with message payload |
| SMS | Send text message to phone number |
| Platform Application Endpoint | Mobile push notifications (APNs, FCM, etc.) |
5. SNS Features
5.1 Message Filtering
| Feature | Details |
|---|---|
| Filter Policy | JSON policy applied to subscription to filter messages |
| Attributes | Filter based on message attributes (not body) |
| Default Behavior | Without filter, subscription receives all topic messages |
| Operators | Exact match, prefix, numeric ranges, exists check |
5.2 Message Attributes
- Name-value pairs sent with message
- Types: String, String.Array, Number, Binary
- Used for message filtering at subscription level
- Maximum 10 attributes per message
5.3 Delivery Retry Policy
| Phase | Details |
|---|---|
| Immediate Retry | No delay between retries, 3 attempts |
| Pre-Backoff | 1 second delay, 10 attempts |
| Backoff | Exponential backoff from 1 sec to 20 sec |
| Post-Backoff | 20 second delay, 100,000+ attempts |
| Total Duration | HTTP/S endpoints: retries over several hours |
5.4 Dead Letter Queue for SNS
- Configure DLQ at subscription level (not topic level)
- SQS queue captures messages that fail delivery after retries
- Client-side errors (4xx) sent directly to DLQ without retries
- Server-side errors (5xx) trigger retry policy before DLQ
6. Decoupling Patterns
6.1 SNS + SQS Fan-Out
| Aspect | Details |
|---|---|
| Pattern | SNS topic delivers message to multiple SQS queues |
| Benefit | Fully decoupled, no data loss, parallel async processing |
| Use Case | Multiple downstream systems need same data |
| Permissions | SQS queue policy must allow SNS SendMessage |
| Cross-Region | SNS can deliver to SQS queues in different regions |
6.2 SQS as Buffer
- Queue absorbs traffic spikes between producers and consumers
- Consumers process at their own pace
- Prevents overloading downstream systems
- Auto Scaling can scale consumers based on queue depth (ApproximateNumberOfMessages)
6.3 Message Processing Patterns
| Pattern | Implementation |
|---|---|
| Load Leveling | SQS queue smooths variable load to steady consumption rate |
| Priority Queue | Use separate queues for different priority levels |
| Request-Response | Use message attributes and correlation ID with two queues |
| Batch Processing | Accumulate messages in queue then process in batches |
6.4 Ordering Considerations
| Scenario | Solution |
|---|---|
| No Order Required | Use Standard SQS for maximum throughput |
| Strict Order Required | Use FIFO SQS with Message Group ID |
| Partial Order | FIFO queue with multiple Message Group IDs for parallel processing |
| Order with SNS | SNS FIFO to SQS FIFO preserves order |
7. Security and Access Control
7.1 Encryption
| Type | Details |
|---|---|
| In-Transit | HTTPS API endpoints, TLS encryption |
| At-Rest (SQS) | SSE using KMS keys (AWS managed or customer managed) |
| At-Rest (SNS) | SSE using KMS keys for messages stored temporarily |
| Key Types | AWS managed key (aws/sqs or aws/sns) or customer managed CMK |
7.2 Access Policies
| Policy Type | Use Case |
|---|---|
| IAM Policy | Control user/role access to SQS/SNS APIs |
| SQS Queue Policy | Grant cross-account access, allow SNS to send messages |
| SNS Topic Policy | Control who can publish or subscribe to topic |
| VPC Endpoint | Private connection from VPC without internet gateway |
7.3 Cross-Account Access
- Use resource-based policies (queue or topic policy)
- Specify Principal as ARN of other account or IAM entity
- For SQS: grant sqs:SendMessage, sqs:ReceiveMessage permissions
- For SNS: grant sns:Publish or sns:Subscribe permissions
8. Monitoring and Troubleshooting
8.1 CloudWatch Metrics for SQS
| Metric | Description |
|---|---|
| ApproximateNumberOfMessages | Messages available for retrieval |
| ApproximateNumberOfMessagesVisible | Messages in queue not in-flight |
| ApproximateNumberOfMessagesNotVisible | Messages in-flight (sent but not deleted) |
| NumberOfMessagesSent | Messages added to queue |
| NumberOfMessagesReceived | ReceiveMessage API calls returning messages |
| NumberOfMessagesDeleted | Messages deleted from queue |
| ApproximateAgeOfOldestMessage | Age of oldest non-deleted message (seconds) |
8.2 CloudWatch Metrics for SNS
| Metric | Description |
|---|---|
| NumberOfMessagesPublished | Messages published to topic |
| NumberOfNotificationsDelivered | Messages successfully delivered |
| NumberOfNotificationsFailed | Messages failed to deliver |
| PublishSize | Size of published messages (bytes) |
8.3 Common Issues
| Problem | Solution |
|---|---|
| Duplicate Messages | Use FIFO queue or implement idempotent processing |
| Message Loss | Check DLQ, verify DeleteMessage only after successful processing |
| Slow Processing | Increase visibility timeout, scale consumers horizontally |
| SNS Delivery Failure | Check endpoint availability, subscription filter policy, IAM permissions |
| High Latency | Enable long polling, use batching APIs, check queue depth |
9. Cost Optimization
9.1 SQS Cost Factors
- Charged per request (64 KB chunk)
- SendMessage, ReceiveMessage, DeleteMessage, ChangeMessageVisibility count as requests
- Long polling reduces costs by decreasing empty ReceiveMessage calls
- Batch operations (up to 10 messages) count as single request
- First 1 million requests per month free tier
9.2 SNS Cost Factors
- Charged per publish request and delivery
- 64 KB per message chunk
- SMS and mobile push have additional per-message fees
- First 1 million publishes per month free tier
- Data transfer charges for cross-region delivery
9.3 Cost Optimization Strategies
| Strategy | Implementation |
|---|---|
| Message Batching | SendMessageBatch and DeleteMessageBatch APIs |
| Long Polling | Reduce empty receives, set ReceiveMessageWaitTimeSeconds to 20 |
| Message Size | Use S3 for large payloads, reference in message |
| Filter Subscriptions | SNS message filtering reduces unnecessary deliveries |
10. Design Best Practices
10.1 SQS Best Practices
- Set visibility timeout to 6x average processing time
- Use DLQ with appropriate MaximumReceives threshold (5-10 recommended)
- Implement idempotent message processing for Standard queues
- Delete messages only after successful processing
- Use long polling to reduce costs and latency
- Monitor ApproximateAgeOfOldestMessage for processing bottlenecks
- Scale consumers based on ApproximateNumberOfMessages metric
- Use FIFO only when strict ordering or deduplication required
10.2 SNS Best Practices
- Use message filtering to reduce unnecessary processing
- Configure DLQ at subscription level for failed deliveries
- Set appropriate retry policies for HTTP/S endpoints
- Use SNS + SQS fan-out for reliable multi-subscriber pattern
- Enable encryption for sensitive data
- Use resource-based policies for cross-account access
- Monitor NumberOfNotificationsFailed for delivery issues
10.3 Resilience Patterns
| Pattern | Implementation |
|---|---|
| Decouple Components | SQS between microservices for loose coupling |
| Handle Failures | DLQ for poison messages, retry logic in consumers |
| Scale Independently | Producers and consumers scale based on different metrics |
| Multi-Region | SNS to SQS queues in multiple regions for DR |
| Event-Driven | SNS triggers Lambda for serverless event processing |
10.4 Choosing Between Standard and FIFO
| Use Standard When | Use FIFO When |
|---|---|
| Throughput is priority (unlimited) | Order must be preserved |
| Order is not critical | Exactly-once processing required |
| Can handle duplicates | Duplicates are unacceptable |
| Best-effort ordering sufficient | Sequential operations needed |
The document Cheat Sheet: SQS & SNS — Decoupling Patterns is a part of the AWS Solutions Architect Course AWS Solutions Architect: Associate Level.
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Apr 19, 2026 Last updated
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