Ordering messages in an Amazon SQS Queue

Last November, AWS released a very useful feature in its FIFO support for Amazon Simple Queue Service (SQS). FIFO stands for “first in first out”, which essentially means that a FIFO queue maintains the order of how messages are delivered out of an SQS queue. The FIFO queue feature also guarantees one-time delivery of a message, which is a pretty big deal if you’ve had to design systems where one-time delivery was a “must have” feature.

Prior to having FIFO queue support for SQS, the random order of SQS messages was not a deal breaker but any ordering needed to be handled outside of SQS. You could create your own ordering key and parse it in the message attributes. Features such as the message timeout window and dead letter queues helped manage the issue of message duplication if needed. This approach didn’t always suit if you needed to implement a queue where the order and frequency of messages were mission critical.

Enter First In First Out queue support for Amazon SQS in US West and US East.

How FIFO for Amazon SQS works

You interact with a FIFO SQS queue the same way that you interact with a standard SQS queue. With a FIFO queue, however, the order in which messages are sent and received is strictly preserved. A message is delivered once and remains available until a consumer processes and deletes it. This means that duplicates will not be created in the queue. This is pretty cool, so let’s take a look at how we can use it.

An SQS queue is recognized as a FIFO queue if it has the FifoQueue attribute set to true and the queue name ends with the .fifo suffix. FIFO queues use the same API actions as standard queues.

The methods for receiving messages, deleting messages, and changing the visibility timeout are the same as for a standard SQS queue. However, when you send a message from a FIFO queue you must specify a message group ID.

Every message sent to a FIFO queue requires a MessageGroupId value. If you don’t add a MessageGroupId value, the send action fails.

How Amazon SQS uses MessageGroupId

Amazon SQS uses the MessageGroupId value to group messages together. Messages within a group are ordered within that group. So if we add a MessageGroupId=1 value to 10 messages, those 10 messages are treated as a message group and will be processed one by one, in a strict order relative to the group MessageGroupId=1. If we send another five messages with a new MessageGroupId=2, those five messages will be ordered within the group MessageGroupId=2.

That means we can have multiple ordered message groups within a single queue. So, our 10 messages in MessageGroupId=1 and our five messages in MessageGroupId=2 will be processed in order relative to each group rather than to the queue as a whole. As ordering only applies to the message group, messages that belong to a different message group might be processed in the same order.

If you have a system that will have multiple senders and multiple recipients, you should generate a unique MessageGroupId for each message. If you don’t need to create multiple ordered message groups, just use the same MessageGroupId for all of your messages.

If your SendMessage action fails, you can retry sending as many times as you want using the same message deduplication ID. Assuming that you receive at least one acknowledgment before the deduplication interval expires (the minimum deduplication window is five minutes), multiple retries will not affect the ordering of messages or create duplications.

When you receive messages, SQS will try to return as many messages with the same MessageGroupId as possible. However, you can’t request to receive messages with a specific MessageGroupId. If you get a failed ReceiveMessage action, you can retry using the same receive request attempt ID. Assuming that you receive at least one acknowledgment before the visibility timeout expires, multiple retries will not affect the ordering of messages.

A FIFO message uses a MessageDeduplicationId key to manage deduplication of sent messages. Every message must have a unique MessageDeduplicationId. If a message with a particular MessageDeduplicationId is sent successfully, any messages sent with the same MessageDeduplicationId will not be delivered during the five-minute deduplication window.

So, if you send 10 messages in succession to a FIFO queue, each with a distinct MessageDeduplicationId, SQS acknowledges the transmission and stores those messages. Then, each message can be received and processed in the exact order in which the messages were transmitted.

How to create a MessageDeduplicationId

There are two ways to create a MessageDeduplicationId. First, you can define your own MessageDeduplicationId for the message. If your application sends messages with identical message bodies, you can modify your code to provide a unique message MessageDeduplicationId for each sent message.

Second, you can just enable content-based deduplication. This tells SQS to use an SHA-256 hash to generate the MessageDeduplicationId using the body of the message. If your application sends messages with unique message bodies, then enabling content-based deduplication will be easiest.

Is a FIFO SQS for you?

Here are five things to keep in mind when considering if a FIFO SQS queue suits your use case.

1. Conversions. You can’t convert an existing standard queue into a FIFO queue. You need to delete your standard queue and then create a new FIFO queue.

2. Transaction rate. FIFO queues are limited to 300 transactions per second (TPS). If your application needs a higher rate than 300 TPS, stick to a standard SQS queue.

3. Delays. FIFO queues support per queue delays but not per message delays. If your application sets a DelaySeconds = 10 on each message, that’s probably not going to work with a FIFO queue. You will need to remove any per message parameter and set a DelaySeconds =10 parameter on the entire queue.

4. Processing time. You need to ensure that there is enough time to process and delete a message. Extend your message visibility timeout to the maximum time it takes to process and delete the message. If you don’t know how long it takes to process a message, specify the initial visibility timeout plus the period of time after which you can check whether the message is processed. If it’s taking a long time to process messages and your visibility timeout is already set to a high value, consider adding a receive request attempt ID to each ReceiveMessage action. This allows you to retry receive attempts in case of networking failures and prevents queues from pausing due to failed receive attempts.

5. Inflight message limit. While Amazon SQS queues can include an unlimited number of messages, there is a 20,000 inflight message limit for inflight FIFO queues (The inflight message limit for standard SQS queues is 120,000.). Messages are inflight after they have been received from the queue by a consumer, but have not yet been deleted from the queue.

Refer main article at

http://cloudacademy.com/blog/ordering-messages-in-an-amazon-simple-queue-service-sqs/

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