SQS FIFO queues guarantee message order within a message group, but you need to explicitly define the MessageGroupId for this to work.
Let’s see this in action. Imagine we have a system processing customer orders. Each customer is a distinct group, and we want to ensure their orders are processed sequentially.
import boto3
sqs = boto3.client('sqs', region_name='us-east-1')
queue_url = 'YOUR_FIFO_QUEUE_URL' # Replace with your actual FIFO queue URL
# Sending messages for two different customers
customer_a_order_1 = {
'Id': 'order1_a',
'MessageBody': '{"order_id": "A123", "item": "widget"}',
'MessageGroupId': 'customer-A'
}
customer_a_order_2 = {
'Id': 'order2_a',
'MessageBody': '{"order_id": "A456", "item": "gadget"}',
'MessageGroupId': 'customer-A'
}
customer_b_order_1 = {
'Id': 'order1_b',
'MessageBody': '{"order_id": "B789", "item": "thingamajig"}',
'MessageGroupId': 'customer-B'
}
response = sqs.send_message_batch(
QueueUrl=queue_url,
Entries=[
customer_a_order_1,
customer_a_order_2,
customer_b_order_1
]
)
print(response)
When you send messages to an SQS FIFO queue, you must provide a MessageGroupId. All messages with the same MessageGroupId are routed to the same queue partition and are therefore processed in the order they were sent within that group. Messages with different MessageGroupIds can be processed in parallel.
The core problem this solves is maintaining strict order for related events without creating a single bottleneck for all events. If you had a standard queue and tried to enforce order by, say, adding a sequence number to the message body and having your consumer sort them, you’d run into several issues: the consumer would need to buffer messages, handle out-of-order arrivals, and potentially reprocess messages if it crashed. This would be complex and inefficient. FIFO queues with MessageGroupId offload the ordering responsibility to SQS itself.
Internally, SQS FIFO queues use a distributed log-like structure. When you send messages with the same MessageGroupId, SQS ensures they are written to the same partition within the queue’s log. When consumers poll the queue, SQS guarantees that messages from a given partition are delivered in the order they were written to that partition. A consumer processing messages from a specific partition will receive them strictly in order. If multiple consumers are processing messages from the same FIFO queue but from different partitions (which happens when you have multiple MessageGroupIds), they can operate in parallel.
The MessageDeduplicationId is also crucial for FIFO queues. If you don’t provide it, SQS automatically uses the message body as the deduplication ID. This is useful for preventing duplicate messages from being processed. However, if you have multiple messages with the same body but they are intended to be distinct operations (e.g., two identical order updates that should both be applied), you’d need to provide a unique MessageDeduplicationId for each, perhaps by incorporating a unique transaction ID or timestamp into it.
The most surprising thing about MessageGroupId is that you can use it for more than just user IDs. You could use it to group messages by a specific resource ID, a transaction ID, a job ID, or even a geographical region. This allows you to create independent processing streams for different logical entities within your application, maximizing parallelism while preserving order where it matters. For instance, if you have a microservice architecture, you could use the service name as a MessageGroupId to ensure all messages related to a specific service are processed in order by a dedicated consumer instance, preventing inter-service dependencies from causing ordering issues.
Once you’ve mastered ordered processing per group, you’ll likely want to explore how to handle message visibility timeouts effectively in a distributed processing environment to prevent message loss or duplicate processing in the face of consumer failures.