SQS - Simple Queue Service

Integration and Messaging

• Deploying multiple applications will inevitable result in the necessity of a communication layer between them
• There are 2 types of integration communication patterns:
  • Synchronous communication
  • Asynchronous communication
• Application decoupling models:
  • SQS: queue model
  • SNS: pub/sub model
  • Kinesis: real-time streaming model

SQS - Standard Queue

• Oldest offering on AWS (over 10 years old)
• Fully managed service, used to decouple applications
• Attributes:
  • Unlimited throughput, unlimited number of messages in the queue
  • Each message is short leaved: default retention period is 4 days, maximum is 14 days
  • Low latency: <10 ms on publish and receive
  • Limitation for message size: maximum size of a message is 256KB
• SQS Standard Queue can have duplicate messages (at least once delivery)
• It also can have out of order messages (best effort ordering)

SQS Standard - Producing Messages

• Producers send messages to the queue using the SDK (SendMessage API)
• The message is persisted on the queue until a consumer deletes it
• Message retention: default 4 days, up to 14 days
• SQS standard has unlimited throughput

SQS Standard - Consuming messages

• Consumers are applications (running on EC2 instances, other servers or AWS Lambda)
• Consumers poll the queue for messages (they can receive up to 10 messages at a time)
• After the messages are processed the consumers delete the messages from the queue using DeleteMessage API
• Multiple consumers:
  • Consumers receive the messages in parallel
  • Each consumer consumes a fraction of the number of the messages sent
  • We can scale the number of the consumers based on the throughput of processing
• SQS with Auto Scaling Group:
  • We can scale based on the ApproximateNumberOfMessages metric by creating a CloudWatch alarm

SQS Security

• Encryption:
  • In-flight encryption using HTTPS
  • At-rest encryption using KMS
  • Client-side encryption if the client wants to perform encryption/decryption itself
• Access Control: IAM policies to regulate access to the SQS API
• SQS Access Policies:
  • Useful for cross-account access to SQS queues
  • Useful for allowing other services (SNS, S3) to write to an SQS queue

Message Visibility Timeout

• After a message is polled by a consumer, it becomes invisible to other consumers
• Default message visibility timeout is 30 seconds, which means the consumer has 30 seconds to process the message
• After the message visibility timeout is over, the message becomes visible to other consumers
• If the processing is not finished during the visibility timeout, there is a chance the message will be processed twice
• If a consumer knows that the processing wont finish in time, it can use the ChangeVisibility API to request more time
• If the message visibility timeout is high and the processing fails, it may take a long time for the message to be processed again
• If the visibility timeout it too short, we may end up processing the same message twice
• Best practice: the visibility timeout should be set to something appropriate. The consumer must be implemented in a way to use the ChangeVisibility API

Dead Letter Queues

• If a consumer fails to process a message within the visibility timeout, the messages goes back to the queue. This can happen multiple times.
• We can set a MaximumReceives threshold, which denotes how many time a message should be able to go back to the queue
• If the MaximumReceives threshold is exceeded, the message is sent to a dead letter queue
• DLQs are useful for debugging
• We have to make sure the messages are processed in DLQ before expiring. It is not a good idea to set a short expiration time for the DLQ

Delay Queue

• Delaying a message means the consumers wont be able to see the message for a period of time after it was sent. Delay time can be up to 15 minutes
• Delay can be set at a queue level or tt also can be set to message level using the DelaySeconds parameter

FIFO Queues

• FIFO - First In First Out
• The messages will be ordered in the queue, meaning that the messages will be consumed in the same order as they were sent
• FIFO queues have limited throughput: 300 msg/s without batching, 3000 msg/s with batching
• Exactly-once send capability (by activating content-based deduplication)
• The name of the FIFO queue must end with the .fifo

SQS With Auto Scaling Group

• Allows scaling the number of EC2 instances based on the available messages in the queue
• In order to accomplish auto scaling we have to create a CloudWatch custom metric representing the number of available messages on the queue divided by the number of EC2 instances. This metric is pushed from an EC2 instance

Data Ordering in SQS

• For standard SQS queues there is no data ordering
• For SQS FIFO, if we don’t use a Group ID, messages are consumed in the order they are sent, with only one consumer
• Messages can be group by specifying a Group ID for the message
• For each group we can have different consumers, which will read messages in order