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In this lecture, we're going to take a look at a service called the Messaging Service.

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This is a very simple service that allows you to send messages between servers.

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And it only has two functions.

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One of these functions is to listen to a particular topic and receive messages from it.

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It's called subscribe async and it says this function registers a callback to begin listening to the

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given topic.

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The callback is invoked when a topic receives a message.

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It can be called multiple times for the same topic, and it says the callback is invoked with a single

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argument.

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A table with the following two entries.

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It has a key called data and it says developer supplied payload and then sent and it says unit time

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in seconds at which the message was sent.

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And then the other function we have is the ability to send messages to a particular topic.

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So this one's called publish async and it says this function sends the provided message to all subscribers

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to the topic triggering their register callbacks to be invoked.

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So the messaging service allows servers of the same experience to communicate with each other in real

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time, less than one second using topics.

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Topics are developer defined strings 1 to 80 characters that serve as used to send and receive messages.

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Delivery is best effort and not guaranteed.

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Make sure to architect your experience so delivery failures are not critical.

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They also give us some information about the different limitations with messages.

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For example, what the maximum size of a message can be, how many messages you can send per minute.

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How many messages can be received per minute per topic, how many messages can be received for an entire

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game, the subscriptions allowed per game server and subscribe requests per game server.

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There are also some other limitations with the messaging service that we're going to take a look at

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in a moment, but basically, the service, as it says, allows you to send information between different

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servers.

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This is how you communicate or have cross server messaging.

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So inside of studio I've made a new place.

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This is just called my messaging service demonstration, and I've published it to Roblox and I've made

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the server count only one.

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So that way I can have two separate accounts hopping into two separate servers.

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So that way we can see the messages being sent between them in action.

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I have a script and server script service.

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I'll just call it messaging service.

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And let's go ahead and grab the messaging service.

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So it's going to be game git service messaging service.

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And we can go ahead and take a look at the two functions that it has.

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Now the first function we're going to take a look at is subscribe.

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Async begins listening to a given topic.

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So we need to define whatever topic we would like.

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For this I'm just going to call it my player message topic because I want it to receive information.

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When a player my game sends a chat message.

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So we're going to call it player message, but you can call it whatever you'd like.

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And then we have to give it a callback function.

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So we'll pass a lambda function here.

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And if you remember the documentation this function is given an argument.

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That's a table that contains two keys.

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One is called data and one is called sent.

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So I will just call this my um I guess I'll just call this my message and I'll type annotate it as a

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table that has a key of data.

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And this could be anything.

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And then it also has a key called sent which is a Unix timestamp.

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So that's going to be a number.

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And now what I want to do is I want to be able to print what the message is inside of the console.

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So I'm going to print whenever this callback is invoked I'm going to print that we received a message.

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And that message is going to be inside of our message table.

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And we're going to print data as well as when this message was sent.

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And now to send messages to different servers.

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What I'm going to do is I'm also going to grab the player service.

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And I want to go ahead and listen for when a player is added into my game, because I want to listen

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for when this player chats.

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So we're going to do player dot chatted.

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We're going to connect a function.

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We're going to get that message.

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And to demonstrate some of the limitations with the messaging service, we're going to have some predefined

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messages here.

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So for example if let's say our player messages, let's say they say something like, um, I kind of

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want this to be like command.

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So we could do exclamation mark.

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And the first example we'll do here is mixed.

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So what I'm going to do when a player chats this is I'm going to try and attempt to send a mixed table

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through the messaging service.

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So we're going to do messaging service publish async.

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We need to refer to what topic we want to publish this message to.

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In this case that's our player message topic.

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So we'll copy that there.

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And the message that I want to send is I want to send a mixed table or a table that has both indexes

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and keys inside of it.

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So inside of this table I'll have an index of one.

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We could set that to something like a string of hello.

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But then I also want to have keys.

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So we'll have key one.

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We'll have this one say howdy and then we'll have key number two.

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And this one will be called hi.

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Now if a player sends a different message let's say they say, um, let's try to pass a function through

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the messaging service.

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So we could do messaging service.

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And actually let's just copy what we've done here.

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Paste this here.

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And then we're going to try to pass a function through the messaging service.

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This will just be some kind of simple, uh, lambda function will pass through here.

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We'll see if that works.

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Um, what else do we want to do?

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Let's go ahead and try to see if we can pass a meta table through the messaging service.

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So we'll do we'll call this one meta table.

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And again we'll just copy this paste this here.

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And we'll call the set meta table function to return a new table.

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So we'll just have an empty table.

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Actually let's put some keys in and we'll call this key one is equal to.

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We'll just say meta.

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And then this one key two we'll just say hahaha whatever.

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And then we want to attach a meta table to this.

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And this meta table is just going to have some kind of key in it.

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We'll just call it uh key two.

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And we'll just say table something like that.

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So let's go ahead and try to pass a table with a meta table attached to it through the messaging service.

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What else should we try.

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Um, let's do it for the other data types.

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So let's do it for like strings.

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We should be able to send strings through the messaging service.

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So again I'm just going to copy this paste this here.

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And let's go ahead and send a message of like hello there.

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Let's check to see if the player says uh they want to send a number.

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So we'll copy this, paste this here.

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We'll just pass a number like one two, three.

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And then finally let's go ahead and try to send a boolean to the messaging service.

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So we'll just copy this paste this there and we'll send the value of true.

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Now of course, the subscribe async and the publish async are asynchronous functions, meaning they

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have the chance to error.

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So you're more than likely going to want to wrap these function calls inside of a p call or a protected

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call.

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I'm not going to do it here just because I'm not going to waste my time with that.

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This is just a demonstration, but let's go ahead and publish these changes.

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We'll publish this to Roblox, and then I'm going to hop into this game with two different accounts,

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so we can see this messaging service in action.

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And actually before I do that, um, I need to account for the fact that when we print messages out,

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like, for example, this is a table, it's only going to print the location of the table in memory

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inside of the console, because when you're doing it in studio, studio gives it some nice extra formatting

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so you can actually see what's inside of the table.

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But unfortunately, when we're actually in the Roblox player using the developer console, it's not

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going to show up that way.

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So if the type of the value of this message we receive.

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So we could do if message dot data, if the type of that is a table, then let's just go ahead and loop

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through every single key value pair inside of the table.

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So in pairs message dot data.

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This will work for both key value pairs and index value pairs.

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It'll just print everything.

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And then we can just go ahead and print out the key and the value.

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Now I don't think that we'll be able to send a mixed tables through the messaging service.

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And just to confirm this, let's go ahead and print out inside of this table to see if it has a key

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one.

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And also let's check to see if this table has a key number two.

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And then to check to see if this table has a meta table.

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We can just print something like um get meta table.

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We'll get the meta table for this particular table.

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And we're only going to do that if this table has that key one inside of it of meta.

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So this way we can tell if we actually had the table passed to us that contained the meta table, if

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it has uh, this value stored in key one.

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So if message dot data key one, if that's equal to meta, then we know this is supposed to be the table

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that has the meta table attached to it.

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So we'll print to see if it actually has a meta table after being passed through the messaging service.

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Okay.

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Let's publish those changes.

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And now let's go ahead and hop into the Roblox player and test it out.

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Okay, so here I am in my game on two separate accounts.

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And what I'm going to do is I'm going to open up the F9 menu, and we're going to go to the server side

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of the console, and we're going to do the same thing for my other account.

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And let's go ahead and see if we are able to receive messages on this other server.

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So the first command we could go ahead and try out is let's go ahead and try out the string command.

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So we're going to chat string.

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And as you can see, both servers receive the message of hello there and it gives us the timestamp of

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when that message was sent on both of the servers.

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So you can see that both of them, since they were both listening to that exact same topic, which was

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the player message topic, they both received the message of hello there!

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Very cool.

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Now let's go ahead and try it out with a number.

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We'll do exclamation mark number.

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And there you go.

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Both servers receive that message and the data was indeed one, two, three for both servers.

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Okay.

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Let's go ahead and try out sending a boolean.

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So we'll do a boolean.

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Hey look at that.

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We received the message on both servers as well which was the value of true.

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Okay let's get a little bit more fancy.

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Let's go ahead and try sending a mixed table.

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So we're going to do exclamation mark mixed.

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And what are you going to see.

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Well it did receive a table.

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But if you take a look at that it only got that first index value pair in the table which was the index

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of one and the value of hello with it, meaning those other two keys in the table, key one and key

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two did not exist.

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So this is one of the limitations with the messaging service.

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You cannot send mixed tables through the publish async function.

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Let's go ahead and try using let's say um let's try to send a meta table through the messaging service.

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So if we do exclamation mark meta table you're going to see that we do get the keys for the table printed

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out.

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But if you notice when we try to print out the meta table itself we got nil.

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So that's another limitation with the messaging service.

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Any tables that have meta tables attached to them those meta tables will be lost.

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So this is basically the same thing as losing, um, meta tables through remote events and losing mixed

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table data through remote events.

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You can't send them through remote events and you can't send them through the messaging service either.

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Let's go ahead and try a function.

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Let's send a function through the messaging service.

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And we're going to see is we get an error cannot publish function, can only accept valid UTF eight

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characters.

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So as you can see we cannot send functions through the messaging service either.

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This one actually caused an error.

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And that's basically all there is to the messaging service.

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It's a very simple service that only has two functions.

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One of the functions listens to a particular topic and is given a table that contains the data for that

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particular topic.

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When we use the publish async function and when that message was sent.

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And then of course we use the publish async message to send a message to a particular topic, whatever

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values we want, as long as they aren't functions, we'll lose meta tables through the publishing service,

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and we can't send mixed tables through the messaging service either.

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And of course, you can't send other stuff like custom Roblox data types or different things like for

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example, you can't send instances through the messaging service.

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And actually, just to have a little fun, let's go ahead and try to send some kind of Roblox data type

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through the messaging service.

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Like, let's try to send, uh, a color, a color three data type, and let's go ahead and see what

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happens.

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We're going to publish it to player message and we'll just pass a new color.

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Three let's just do a white color.

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And let's go ahead and see what happens when we try to publish this through the messaging service.

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So I'll publish changes.

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All right, we'll do exclamation mark color, and we should.

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There we go.

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We get the same error.

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Cannot publish color three can only accept valid UTF eight characters.

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So you are not able to send custom Roblox data types through the messaging service either.

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Hopefully this lecture has given you a better understanding of the messaging service and how to use

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its two functions.

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This should open up many possibilities in your games.

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For example, maybe you could create global announcements across multiple servers in your game.

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You could create some kind of real time server browser, or even some kind of cross server chat, where

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other players can chat with other players that are in completely separate servers.

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Thank you for watching and I'll see you in the next lecture.