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Dead-Letter Queue (DLQ)

Overview​

FormKiQ uses a message-driven architecture where components communicate through asynchronous message passing. This approach:

  • Decouples system components
  • Increases system resilience
  • Prevents cascading failures
  • Enables graceful handling of high load conditions

SQS Dead Letter Queue

Understanding Dead-Letter Queues​

Amazon SQS Dead-Letter Queues (DLQs) provide a mechanism for handling messages that fail processing. FormKiQ implements a centralized DLQ that:

  • Captures failed messages from all queues
  • Enables investigation of processing failures
  • Allows message reprocessing when appropriate
  • Provides monitoring and alerting capabilities

Monitoring and Alerts​

CloudWatch Alarms​

FormKiQ automatically configures Amazon CloudWatch Alarms to monitor the DLQ. These alarms:

  • Track failed message metrics
  • Trigger notifications when messages enter the DLQ
  • Integrate with Amazon SNS for alerting

CloudWatch DLQ Alert

Setting Up Email Notifications​

To receive email alerts when messages fail processing:

  1. Access the Amazon SNS console

  2. Locate the DLQ topic SNS DLQ Topic

  3. Create a subscription:

    • Select the topic
    • Click the Subscriptions tab
    • Click Create Subscription SNS DLQ Topic

  4. Configure the subscription:

    • Verify the Topic ARN
    • Select Email as the Protocol
    • Enter your email address
    • Click Create subscription SNS DLQ Topic

  5. Confirm the subscription:

    • Check your email for a confirmation message
    • Click the verification link
    You have chosen to subscribe to the topic:
    arn:aws:sns:us-east-2:1111111111:FormKiQ-DLQ-dev-Alert

    To confirm this subscription, click or visit the link below 
    (If this was in error no action is necessary):
    Confirm subscription

Message Reprocessing (Redrive)​

When messages fail processing, you can redirect them back to their original queue using the DLQ redrive feature:

  1. Access the DLQ:

    • Find your FormKiQ installation's dead letter queue SQS Dead Letter Queue

  2. Start the redrive process:

    • Click Start DLQ redrive CloudWatch DLQ Alert

  3. Review messages:

    • Click Poll for messages with default settings CloudWatch DLQ Alert
    • Examine message contents by clicking individual messages
    • Delete invalid messages if needed

  4. Reprocess messages:

    • Select messages for reprocessing
    • Click DLQ redrive
    • Messages return to their original queue
note

Messages that fail processing again will return to the DLQ. This helps identify persistent issues that require investigation.

Best Practices​

  1. Regular Monitoring

    • Set up email notifications
    • Review DLQ contents periodically
    • Track failure patterns

  2. Investigation

    • Examine message contents
    • Review system logs
    • Identify root causes

  3. Reprocessing

    • Fix underlying issues before redriving
    • Monitor reprocessed messages
    • Document persistent failures