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Let's get a quick reminder on some different aspects of these listeners.

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The person we're gonna focus on is the cougar name.

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Let's take a look at a quick diagram OK.

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So in this diagram we've got some publisher up here on the top left hand side.

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That's going to be eventually published an event up ticket created on the right hand side is our net

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streaming server and we're listening inside of our two instances of the order service to some channel

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called Ticket created when these two order services connected to this channel and created a subscription.

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They joined a Q group called orders service by being a member of this Q group that ensures that any

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time an event comes into this channel this event is only going to be sent to one of the members inside

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this Q Group.

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So the entire idea behind this supposed to make sure that we do not have our different services all

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independently processing the same copy of this event at the same time.

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So instead the Q Group makes sure that this event is only going to be sent to either order service a

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or order service.

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The only requirement of the Q Group identifier right here is that it is unique for all the different

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services that are going to create subscriptions inside this channel number one and number two it must

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stay consistent over time so we should not try to randomly generate a Q Group name or anything like

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that.

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Now the Q Group Name is one of those little variables or properties right here that's really easy to

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make a typo on.

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So at some point time we're gonna create another listener inside of our app let's imagine that it's

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some other listener inside of some other file and when we did so we would have to define this Q Group

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name another time or second time would be really easy for us to actually make a little typo here.

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Maybe we call it order service or maybe we call it order service without a dash or maybe do a slash

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instead or a colon or whatever else.

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And so usually rather than defining the Q Group name as a string directly inside the listener we might

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be a little bit better off if we define the string somewhere else instead of a project just one time

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and then import that value into our project or something into this listener file so let's try that right

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now.

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Let's declare a Q Group name and a separate file and then imported into this file again just so we avoid

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making any typos inside of my listeners directory that we just created.

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I'm gonna make a new file of Q Group name dot t s then inside there I'm going to export a concept of

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Q Group name and I'll hardcoded value to orders.

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Service does save this file.

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Pull it back over to our listener at the very top.

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I'm going to import you.

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Group Name

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from Q Group name and now I'll reference that variable right there instead of this hardcoded string

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though.

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Q Group name.

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So again just avoiding little typos.

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OK so let's get a quick reminder now about this data property coming too on message.

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So remember we had said that we're going to create an interface to describe all the different events

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that are going to be flowing throughout our application.

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In this case that interface we created was the ticket created event interface.

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We could do a command click on ticket created even anywhere inside this file or a control click through

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on Windows.

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And it should take us to the definition of that interface so all of our event interfaces had the subject

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property and the data property and data was describing the different pieces of data that was going to

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be contained inside this event.

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So to make sure that we applied some type checking on the data flowing into this on message function

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we reference the event type interface and then we're saying off that interface.

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Take a look at the data property and that's going to be the type of this data argument right here then

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finally last little reminder is this message argument as a second argument.

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So what is the message.

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Well remember this is an object coming from the node not streaming library.

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The message thing is something that tells us about the underlying data coming from the known out streaming

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server.

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But most importantly for you and I absolute bar none.

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Most important thing about it we do a command click on message and go to the type definition you might

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recall.

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This thing has a method of EQ and that was what you and I really really cared about.

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We call EQ when we have successfully processed a message or an event and that is a signal to note that

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streaming server that we have process this event and it does not have to worry about trying to send

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it off to another copy of our service again.

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So after we successfully process any event inside of our different listeners we're always going to finish

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up by calling back.

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Now the keyword here is successfully process.

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So I guess the keywords successfully process only when we successfully process an event do we call ach.

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Otherwise if there's some kind of error or something like that we usually it will not be calling EC

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because an error will usually indicate that we want to try to re process this event again and we would

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do that by just not calling Ach that would cause our event to time out after a couple of seconds and

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that string server is going to try to take that event and send it off to another copy of our service

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for reprocessing.

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All right.

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So hopefully that gives you a really good reminder on what's going on inside of our listeners.

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Again we're going to be writing on the same exact code again and again and again moving forward for

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all these different listeners.

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Just really important that you keep the definitions of all these different things in mind.

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All right sounds good.

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So let's now take a quick pause.

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We're going to start to implement the body of our on message function in the next video.