[SOUND] Okay. Now we're going to look at how remote procedure calls actually are implemented. We'll step through it. It's the same for RMI, it's the same for SOAP. This will give you an idea of what mechanisms are required, and then we'll talk about particular ways that they're done in the web. So looking at the communication, remember we have a client that's trying to talk to a process. We're trying to implement a procedure effectively with the remote method implication. And we have to actually convert it into a bunch of messages, and an implementation on the server side. So split up the computation into two pieces, that which occurs on the client, and that which occurs on the server. And there's going to be reply request between the two. The client is generally known as a proxy. It's going to make the requests. There is a skeleton and a dispatcher on the server side that responds to the requests, executes the method, procedure, whatever. And returns the values back having done the computation. And this can change the state of the server as well as providing different types of return values. So in this diagram, you will see that there's different objects that can be around in the client. And they can talk to different objects in the server. We're just going to look at one of those. So, if we make a request, then what's going to occur? And in the first we'll pick a object pay. It's going to make a request through its proxy. And that proxy will bundle up the parameters into a message, send that across the request to the server. At the server there's a skeleton and dispatcher that receives the request. What it will do is to unbundle parameters and then it will find the method that needs to be executed. It'll have to look at perhaps the object to determine which method it is, and then it's going to make that request, execute the remote object get the return parameters. All of the operations together need to work in order for the procedure to act correctly. And as we mentioned in the previous set of slides, if something goes wrong we can have a different semantics than a normal procedure call. And that's why we have those exactly once or at least once types of semantics. The proxy it's responsible for making RMI transparent to the clients. It's the interface, it's like an API. It really separates out what the computation is on the client side from all the mechanism that's needed. The proxy is going to forward that request. That request will be formatted, it'll have a particular way of putting it into a message so that it can be easily decoded. That process of putting it into a message is called marshaling, and we'll cover that in a second. When it gets to the other side, it'll have to be unmarshalled and the actual method invoked. Marshalling is done mainly through compiling a special marshalling compiler. It can be done by hand if necessary. It takes the parameter and transforms it into an external data representation, which is generally agreed everywhere as a sort of standard. And then it will package that and transmit it using TCP/IP across the network. The marshalling, as I said, was the action of taking those data values, converting them into the external data representation. Usually that external data representation contains all the information you need in order to be able to decode what are the parameters. So you'll not only give, say, an integer, but it'll say it's of type integer in the actual message. The unmarshalling again is done by a very, very simple script that takes the contents of the message. And using the information in the message, it is allowed to actually rebuild the parameter space again and the method invocation that's needed to go ahead. The remote reference module, when you're talking about remote methods, typically they'll be an object. And that object will be the end of the URI that's like a fancy URL. And that method will be in a table associated with the object. And what happens in the message is to say, well we want method five performed on that particular object. So, it finds the object through the URL, and then it will index down to five. Find that particular method, unmarshal, and make the invocation. And when it's finished then pretty much the reverse happens. You take the results of executing the method and you bundle about into a message using the same sort of protocol and the same way of incurring the data and you transmit it back. What happens when requests occur? Well, you have to make sure that all of the communication occurs, that you wait for your reply. That the reply, if it doesn't occur, causes some sort of semantics in the client to actually take account of the fact that something broke. On the other side, on the server side, you're going to receive a message, and you're going to decode it. But you also have to take care of the fact that when you send the message back to the client, it may not actually get back to the client. And so there might be exceptions and other things. All of that is usually done by middleware because it's the same for every single object call, every single method call. And so we get a great deal of simplicity because everything is operating the same way. So in summary, remote method core. What we're doing is provide the procedure call between machines between pieces of code on different machines. We have a stylized approach to do that. We have a client process. We have objects in that client process that want to talk to other remote objects, object B here. And what's going to happen is that it would talk to a proxy object instead on the local client. That bundles everything up, thus goes through a communication module, communicates to a remote service. And the communication module on the remote service unbundles it, and then calls a dispatcher. The parameters are taken care of by a skeleton. And there's indications on the object. Now you're going to see different instances of this as we talk. And you're going to see this happen in Java. You're also going to see this happen in just plain HTTP using soap. But it's the same style of approach. Some of it's automated. Some of it's done by hand, but it's doing exactly this type of operation. And in each case when you get errors you have to handle the errors and it's exactly the sort of approach to handling those errors. But it may be coded as a SOAP, or it may be coded as a Java. [MUSIC]
