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So now let's talk about an important concept,

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which is called the message visibility timeout.

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So when a message is polled by a consumer,

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it becomes invisible to other consumers.

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So let's take an example.

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We have the time going from left to right,

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and we have a consumer doing a ReceiveMessage request,

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and therefore, a message will be returned from the queue.

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Now the visibility timeout begins.

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And by default, that message visibility timeout

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is 30 seconds.

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That means that during these 30 seconds,

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the message has to be processed, okay?

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And if you do so,

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that means that if the same or other consumers

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do a message request API call,

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then the message will not be returned.

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And again another one during the timeout window,

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the message will not be returned.

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So effectively, during the visibility timeout,

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that message is invisible to other consumers.

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But after the visibility timeout is elapsed

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and if the message has not been deleted,

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then the message will be

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quote unquote put back into the queue

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and therefore, another consumer or the same consumer

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doing a receive message API call

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will receive the message again, the same message as before.

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So, this is something that's really important to understand.

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As you can see, while we receive a message,

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it becomes invisible during the visibility timeout period.

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Now, if we take a look at the same diagram,

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what we notice is that

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if we don't process a message

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within the visibility timeout window,

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then it will be processed twice potentially, right?

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Because it will be received by two different consumers,

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or twice by the same consumer.

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And so, if a consumer is actively processing a message

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but knows that it needs a bit more time

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to process the message because otherwise,

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it will go out of the visibility timeout window,

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there is an API called ChangeMessageVisibility.

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And so if a consumer knows

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that a message needs a bit more time to be processed

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and you don't want to process that message twice,

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then the consumer should call

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the ChangeMessageVisibility API to tell SQS,

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hey, do not make that message visible for now, okay?

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I just need a bit more time to process this message.

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And so, how do you set this message visibility timeout?

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Well, if you set it to

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a really, really high value by default, say, hours,

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and the consumer crashes,

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then it will take hours until this message reappears,

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re-becomes visible in your SQS queue,

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and that will take a lot of time.

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If you set it to something really, really low,

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like a few seconds,

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what happens that if the consumer

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doesn't end up having enough time to process that message

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for whatever reason,

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then it will be read many times by different consumers

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and you may get duplicate processing.

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So the idea is that the visibility timeout

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should be set to something reasonable for your application

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and your consumer should be programmed

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that if they know they need a bit more time,

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then they should call the ChangeMessageVisibility API

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to get more time and increase the timeout

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of that visibility window.

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But understanding this concept

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is very important from an exam perspective

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because there will be scenarios on that.

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So, let's go in the console

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to see how that works in practice.

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So I'm going to open

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two windows of send and receive messages

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just to show you how that works, so.

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In the first window, I'm going to enter a hello world

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and it's going to be sent into the queue.

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And if you remember,

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the queue has a default timeout of 30 seconds.

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So what's going to happen is that I have two consumers,

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I have the first window and the second window consumer,

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and so I'm going to read the messages from the first window.

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So let's poll for messages.

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Now my message has appeared right here,

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it has been received.

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And if I go into my second consumer and poll for messages,

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as you can see, the message does not appear in here.

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And the reason it doesn't appear is that we are still within

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the visibility window timeout of that message

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and therefore, during these 30 seconds

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this message is trying to be processed by this consumer,

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this consumer right here will not see it.

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But let's assume that we stop polling, okay?

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So we don't delete the message

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and we know this message will timeout at some point.

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What will happen is that yes, I'm already here,

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in the second window, in the second consumer,

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this is the second consumer,

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then the message has been received

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because it was being put back into the queue.

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And now let's assume we do something correctly,

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we delete that message,

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then we have fully processed that message.

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But remember, that message was received twice.

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It said received count was two.

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So it's important for you to understand

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how this visibility window works,

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and this was a good demonstration of it.

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Now, if you wanted to change this setting by default,

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what you could be doing is go into Edit,

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and then for visibility timeout,

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you could set the default value

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between zero seconds, which is definitely not recommended,

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up to 12 hours.

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I think 30 seconds is fine, but again, remember,

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if a consumer needs more time to process a message,

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you should just call the ChangeMessageVisibility API

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to edit the visibility of that one message

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and increase the value

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so that another consumer will not see that message

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and the first consumer will get enough time

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to process that message accordingly.

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So, that's it.

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I hope you liked it, and I will see you in the next lecture.