SQS FIFO Queues and Dead-Letter Queues

When message ordering matters or when messages consistently fail processing, you need more than a Standard queue. FIFO queues guarantee the order of message processing, while dead-letter queues (DLQs) capture messages that cannot be processed after repeated attempts. Together, they form the backbone of robust, fault-tolerant messaging systems.

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FIFO Queues in Depth

FIFO stands for First-In, First-Out. Messages are delivered in the exact order they were sent and are processed exactly once.

When to Use FIFO Queues

Scenario	Why FIFO?
Financial transactions	Debits and credits must be applied in order
Inventory management	Stock updates must be processed sequentially
User action sequences	Login, action, logout must happen in order
Command processing	Commands must execute in the sequence issued
Workflow steps	Each step depends on the previous one completing

Creating a FIFO Queue

FIFO queue names must end with the .fifo suffix:

my-order-queue.fifo
payment-processing.fifo

Message Group ID

FIFO queues use a Message Group ID to define the scope of ordering. Messages within the same group are strictly ordered. Messages in different groups can be processed in parallel.

sqs.send_message(
    QueueUrl='https://sqs.eu-west-2.amazonaws.com/123456789012/orders.fifo',
    MessageBody='{"orderId": "ORD-001", "action": "process"}',
    MessageGroupId='customer-123',
    MessageDeduplicationId='ORD-001-process'
)

Think of Message Group ID like a lane on a motorway. Each lane (group) maintains its own order, but different lanes run in parallel.

Group: customer-123  →  [Msg1] [Msg2] [Msg3]  →  processed in order
Group: customer-456  →  [MsgA] [MsgB] [MsgC]  →  processed in order (parallel with above)

Message Deduplication

FIFO queues prevent duplicate messages using one of two mechanisms:

1. Content-Based Deduplication

Enable this on the queue, and SQS generates a hash of the message body. If an identical message is sent within the 5-minute deduplication window, it is discarded.

2. Explicit Deduplication ID

Provide a MessageDeduplicationId with each message. SQS uses this ID to detect duplicates within the 5-minute window.

Method	How It Works	Best For
Content-based	SHA-256 hash of message body	Messages with unique bodies
Explicit ID	You provide a unique deduplication ID	Messages that may have identical bodies but different intent

FIFO Throughput

Mode	Throughput
Standard FIFO	300 messages/second per queue
High-throughput FIFO	3,000 messages/second per queue (with batching)
With batching	Each batch counts as 1 request, so 10 messages per batch = 3,000 effective messages/second

To enable high-throughput mode, set the queue attribute DeduplicationScope to messageGroup and FifoThroughputLimit to perMessageGroupId.

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Dead-Letter Queues (DLQs)

A dead-letter queue is a separate SQS queue that receives messages which have failed processing after a specified number of attempts. DLQs prevent "poison messages" from blocking your main queue indefinitely.

How DLQs Work

1. A message is received from the main queue
2. The consumer fails to process it and does not delete it
3. The visibility timeout expires, and the message becomes visible again
4. After the message has been received maxReceiveCount times without being deleted, SQS automatically moves it to the DLQ