Serverless Event Driven Architecture

Serverless & Event-Driven Architecture Patterns

When to Use This Skill

Triggers — load this skill when:

• An event-driven flow needs routing, ordering, or orchestration designed
• Failures must be captured with DLQs, retries, and idempotent handlers
• Cold starts, concurrency limits, or throttling are causing problems

Route elsewhere when:

• Long-running containerized services -> cloud-native-microservices-patterns
• Per-invocation cost modelling -> finops-framework-inform-optimize-operate
• Tracing across async hops -> prometheus-grafana-otel-tracing

1. EventBridge Rule with Dead-Letter Queue (Terraform)

resource "aws_cloudwatch_event_rule" "order_placed" {
  name        = "order-placed-rule"
  description = "Routes order.created events to invoice processing lambda"

  event_pattern = jsonencode({
    "source"      = ["ecommerce.orders"],
    "detail-type" = ["OrderCreated"]
  })
}

resource "aws_cloudwatch_event_target" "lambda_target" {
  rule      = aws_cloudwatch_event_rule.order_placed.name
  target_id = "InvoiceProcessingLambda"
  arn       = aws_lambda_function.invoice_lambda.arn

  dead_letter_config {
    arn = aws_sqs_queue.eventbridge_dlq.arn
  }

  retry_policy {
    maximum_event_age_in_seconds = 3600
    maximum_retry_attempts       = 5
  }
}

2. Best Practices & Anti-Patterns

• Do: Always configure Dead Letter Queues (DLQ) with alarms on every asynchronous invocation to prevent event loss.
• Do: Make event consumers idempotent using unique idempotency keys stored in DynamoDB/Redis.
• Don't: Never use synchronous Lambda-to-Lambda calls for business workflows; orchestrate with AWS Step Functions or EventBridge.

3. Delivery Guarantees: DLQ Redrive, Idempotency & FIFO Ordering

Nothing is lost, so nothing may be silently dropped. Every asynchronous hop needs its own failure destination — an EventBridge rule, a Lambda invocation, and an SQS consumer each fail differently:

resource "aws_sqs_queue" "orders_dlq" {
  name                      = "orders-dlq"
  message_retention_seconds = 1209600          # 14 days: room to fix and redrive
}

resource "aws_sqs_queue" "orders" {
  name                       = "orders"
  visibility_timeout_seconds = 180             # >= 6x the Lambda timeout
  redrive_policy = jsonencode({
    deadLetterTargetArn = aws_sqs_queue.orders_dlq.arn
    maxReceiveCount     = 5                    # then park it, do not retry forever
  })
}

resource "aws_sqs_queue_redrive_allow_policy" "dlq" {
  queue_url = aws_sqs_queue.orders_dlq.id
  redrive_allow_policy = jsonencode({
    redrivePermission = "byQueue"
    sourceQueueArns   = [aws_sqs_queue.orders.arn]   # enables DLQ redrive back to source
  })
}

Redrive is an operational procedure, not a button: fix the defect, replay a single message, verify, then redrive the batch with a bounded rate.

At-least-once delivery means duplicates are normal. Every handler must be idempotent, keyed on a business identifier rather than the message ID (a retry after a partial write arrives with a new one):

def handler(event, _ctx):
    for record in event["Records"]:
        order = json.loads(record["body"])
        try:
            table.put_item(
                Item={"pk": f"ORDER#{order['order_id']}", "state": "PROCESSED"},
                ConditionExpression="attribute_not_exists(pk)",   # dedupe = the write itself
            )
        except ClientError as e:
            if e.response["Error"]["Code"] == "ConditionalCheckFailedException":
                continue                                          # already processed
            raise

Return batchItemFailures for partial batch failure so one bad message does not replay the whole batch.

Ordering: FIFO buys per-group order, not global order. MessageGroupId is the ordering and concurrency unit — pick the entity whose order matters (customer_id, account_id), never a constant:

A single group serialises the whole queue; order_id as the group gives ordering nobody needed at maximum cost. Group by the entity, keep handlers idempotent anyway, and use MessageDeduplicationId for the 5-minute exactly-once window.