Example of Amazon SQS in Action

Let's walk through a simple example of using AWS SQS, using the line at the DMV (Department of Motor Vehicles) as the example subject matter. The DMV is notorious for long waits, forcing people to corral themselves into some form of a line. While this isn't an actual use case anyone would (presumably) solve using AWS SQS, it will allow us to quickly demo their capabilities, with a real-world situation most are all too familiar with.

While AWS SQS has SDK libraries for almost any language you may want to use, I'll be using their REST interface for this exercise (with my trusted REST side kick Postman!).

Authorization

Postman makes it easy to setup all the necessary authorization using Collections. Configure the AWS authorization in the parent collection with the Access Key and Secret Access Key found in the AWS Console:

Then reference that authorization in each request:

Using this pattern, it's easy to quickly spin up requests and put AWS SQS through its paces.

Creating a Queue

When people first walk in the door, any DMV worth their salt will give them a number to begin the arduous process. This is your main form of identification for the next few minutes/hours (depending on that day's "volume"), and it's how the DMV employees think of you ("Number 14 over there sure seems a bit testy!").

Let's create our "main queue" now, with the following REST invocation:

Request:

GET https://sqs.us-east-1.amazonaws.com?Action=CreateQueue&DefaultVisibilityTimeout=0&QueueName=MainLine&Version=2012-11-05

Response:

https://sqs.us-east-1.amazonaws.com/612055710376/MainLine

fa178e12-3178-5318-8d90-da20904943f0

Good deal. Now we've got a mechanism to track people as they come through the door.

Standard vs FIFO

One important detail that should be mentioned - there are two types of queues within AWS SQS:

• Standard - higher throughput, with "at least once delivery", and "best effort ordering".
• FIFO (First-In-First-Out) - not as high throughput, but guarantees on "exactly once" processing, and preserving the ordering of messages.

Long story short, if you need things super fast, can tolerate messages out of order, and possibly sent more than once, Standard queues are the answer. If you need absolute guarantees on order of operations, no duplication of work, and don't have huge throughput needs, then FIFO queues are the best choice.

We'd better make sure we create our MainLine queue using FIFO! While a "mostly in order" guarantee might suffice in some situations, you'd have a riot on your hands at the DMV if people started getting called out of order. Purses swinging, hair pulling - it wouldn't be pretty. Let's add "FifoQueue=true" to the query string to indicate that the queue should be FIFO:

Request

https://sqs.us-east-1.amazonaws.com?Action=CreateQueue&DefaultVisibilityTimeout=0&QueueName=MainLineFIFO&Version=2012-11-05&FifoQueue=true

Send Message

Now that we've got a queue, let's start adding "people" to it, using the "SendMessage" action. Note that when using REST, we need to URL encode the payload. So something like this:

{
"name": "Ronnie Van Zandt",
"drivers_license_number": "1234"
}

Becomes this:

%7B%0A%20%20%20%22name%22%3A%20%22Ronnie%20Van%20Zandt%22%2C%0A%20%20%20%22drivers_license_number%22%3A%20%221234%22%0A%7D

There are many ways of accomplishing this, I find the urlencoder site to be easy and painless.

Here's the final result:

Request

https://sqs.us-east-1.amazonaws.com/612055710376/MainLineFIFO?Version=2012-11-05&Action=SendMessage&MessageBody=%7B%0A%20%20%20%22name%22%3A%20%22Ronnie%20Van%20Zandt%22%2C%0A%20%20%20%22drivers_license_number%22%3A%20%221234%22%0A%7D

Response:

00ad4e10-4394-450f-8902-4a9cf4b96b95

b9f28edc9c6dc9fe2a86f5ae8efb2364

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After this call, we've got young Ronnie standing in line at the DMV. Thanks to AWS's massive scale and performance, we can leave Ronnie there as long as we'd like. And we can add as many people as we'd like - with AWS SQS's capacity, we could have a line around the world. But that's horrible customer service, someone needs to find out what Ronnie needs!

Receive Message

At the DMVs I've been to, there's usually a large electronic sign on the counter that will display the next lucky person's number. You feel a brief pulse of joy when your number displays, and rush to the counter on a pillow of euphoria, eager to get on with your life. How do we recreate this experience in AWS SQS?

Why, "ReceiveMessage", of course! (Note we are invoking it using the actual QueueUrl passed back by the CreateQueue call above)

Request

https://sqs.us-east-1.amazonaws.com/612055710376/MainLineFIFO?Action=ReceiveMessage&Version=2012-11-05

Response

00ad4e10-4394-450f-8902-4a9cf4b96b95

AQEBjq8apWDfLXE0pCbpABh6Wdx70ZbszY0k38t9u8Mrny1Jz+Q522Vwvvf4xLqzQHfjoHQd56JJJEM67LJG5tQ/YSCibFSNCg8jfadyNMbqBH48/WxmpYunI3w1+GbDCL2tlKkDz/Lm9akGasgDZEBtw6U9jw1Bu6XbzNuNiw5jfVzjC99E38KSvxvZMHfmSi3Wo2XOBAcfU0oTpLmGMwccGiRUOp4XtS38nMXHhBdtKSS+U11N38cJAtlnxHQJkXmTAk7ZdvpxJNtnOrXmeGN00vtf6OSyLJzRJJieYHNtxIyxojcGZcnJQ6dTveMWQ1A1FOzschRuavl3wtftDS/YSt5sDNeBcjEOE+Y0QE+18qiWaDZc+nlaetcBvqmt6Hbt

b9f28edc9c6dc9fe2a86f5ae8efb2364

{
"name": "Ronnie Van Zandt",
"drivers_license_number": "1234"
}

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One thing to keep in mind - ReceiveMessage doesn't actually REMOVE the item from the queue - the item will remain there until explicitly removed. Visibility Timeout can be used to ensure multiple readers don't attempt to process the same message.

So how do we permanently mark the item as "processed"? By deleting it from the queue!

Delete Message

The DeleteMessage action is what removes items from a queue. There's not really a good analogy with the DMV here (thankfully, DMV employees can't "delete" us), so we'll just go with an example. DeleteMessage takes the ReceiptHandle returned by the ReceiveMessage endpoint as a parameter (once again, encoded):

Request

https://sqs.us-east-1.amazonaws.com/612055710376/MainLineFIFO?Action=DeleteMessage&Version=2012-11-05&ReceiptHandle=AQEBjq8apWDfLXE0pCbpABh6Wdx70ZbszY0k38t9u8Mrny1Jz%2BQ522Vwvvf4xLqzQHfjoHQd56JJJEM67LJG5tQ%2FYSCibFSNCg8jfadyNMbqBH48%2FWxmpYunI3w1%2BGbDCL2tlKkDz%2FLm9akGasgDZEBtw6U9jw1Bu6XbzNuNiw5jfVzjC99E38KSvxvZMHfmSi3Wo2XOBAcfU0oTpLmGMwccGiRUOp4XtS38nMXHhBdtKSS%2BU11N38cJAtlnxHQJkXmTAk7ZdvpxJNtnOrXmeGN00vtf6OSyLJzRJJieYHNtxIyxojcGZcnJQ6dTveMWQ1A1FOzschRuavl3wtftDS%2FYSt5sDNeBcjEOE%2BY0QE%2B18qiWaDZc%2BnlaetcBvqmt6Hbt

Response

a69c7042-d0e2-546a-bdf7-2476a30b89df

And just like that, Ronnie is able to leave the DMV with his newly printed license, all thanks to AWS SQS!