25-08-2017, 09:32 PM
Remote Procedure Calls (RPC)
[attachment=49431]
What Is RPC
RPC is a powerful technique for constructing distributed, client-server based applications. It is based on extending the notion of conventional, or local procedure calling, so that the called procedure need not exist in the same address space as the calling procedure. The two processes may be on the same system, or they may be on different systems with a network connecting them. By using RPC, programmers of distributed applications avoid the details of the interface with the network. The transport independence of RPC isolates the application from the physical and logical elements of the data communications mechanism and allows the application to use a variety of transports.
RPC makes the client/server model of computing more powerful and easier to program. When combined with the ONC RPCGEN protocol compiler clients transparently make remote calls through a local procedure interface.
How RPC Works
An RPC is analogous to a function call. Like a function call, when an RPC is made, the calling arguments are passed to the remote procedure and the caller waits for a response to be returned from the remote procedure. The figure shows the flow of activity that takes place during an RPC call between two networked systems. The client makes a procedure call that sends a request to the server and waits. The thread is blocked from processing until either a reply is received, or it times out. When the request arrives, the server calls a dispatch routine that performs the requested service, and sends the reply to the client. After the RPC call is completed, the client program continues. RPC specifically supports network applications.
Remote Procedure Calling Mechanism
A remote procedure is uniquely identified by the triple: (program number, version number, procedure number) The program number identifies a group of related remote procedures, each of which has a unique procedure number. A program may consist of one or more versions. Each version consists of a collection of procedures which are available to be called remotely. Version numbers enable multiple versions of an RPC protocol to be available simultaneously. Each version contains a a number of procedures that can be called remotely. Each procedure has a procedure number.
Defining Client and Server Application Code
We must now write the the client and application code. They must communicate via procedures and data types specified in the Protocol.
The service side will have to register the procedures that may be called by the client and receive and return any data required for processing.
The client application call the remote procedure pass any required data and will receive the retruned data.
There are several levels of application interfaces that may be used to develop RPC applications. We will briefly disuss these below before exapnading thhe most common of these in later chapters.
Overview of Interface Routines
RPC has multiple levels of application interface to its services. These levels provide different degrees of control balanced with different amounts of interface code to implement. In order of increasing control and complexity. This section gives a summary of the routines available at each level. Simplified Interface Routines
The simplified interfaces are used to make remote procedure calls to routines on other machines, and specify only the type of transport to use. The routines at this level are used for most applications. Descriptions and code samples can be found in the section, Simplified Interface @ 3-2.
Simplified Interface
The simplified interface is the easiest level to use because it does not require the use of any other RPC routines. It also limits control of the underlying communications mechanisms. Program development at this level can be rapid, and is directly supported by the rpcgen compiler. For most applications, rpcgen and its facilities are sufficient. Some RPC services are not available as C functions, but they are available as RPC programs. The simplified interface library routines provide direct access to the RPC facilities for programs that do not require fine levels of control.
Routines such as rusers are in the RPC services library librpcsvc. rusers.c, below, is a program that displays the number of users on a remote host. It calls the RPC library routine,rusers.
Passing Arbitrary Data Types
Data types passed to and received from remote procedures can be any of a set of predefined types, or can be programmer-defined types. RPC handles arbitrary data structures, regardless of different machines' byte orders or structure layout conventions, by always converting them to a standard transfer format called external data representation (XDR) before sending them over the transport. The conversion from a machine representation to XDR is called serializing, and the reverse process is called deserializing.
[attachment=49431]
What Is RPC
RPC is a powerful technique for constructing distributed, client-server based applications. It is based on extending the notion of conventional, or local procedure calling, so that the called procedure need not exist in the same address space as the calling procedure. The two processes may be on the same system, or they may be on different systems with a network connecting them. By using RPC, programmers of distributed applications avoid the details of the interface with the network. The transport independence of RPC isolates the application from the physical and logical elements of the data communications mechanism and allows the application to use a variety of transports.
RPC makes the client/server model of computing more powerful and easier to program. When combined with the ONC RPCGEN protocol compiler clients transparently make remote calls through a local procedure interface.
How RPC Works
An RPC is analogous to a function call. Like a function call, when an RPC is made, the calling arguments are passed to the remote procedure and the caller waits for a response to be returned from the remote procedure. The figure shows the flow of activity that takes place during an RPC call between two networked systems. The client makes a procedure call that sends a request to the server and waits. The thread is blocked from processing until either a reply is received, or it times out. When the request arrives, the server calls a dispatch routine that performs the requested service, and sends the reply to the client. After the RPC call is completed, the client program continues. RPC specifically supports network applications.
Remote Procedure Calling Mechanism
A remote procedure is uniquely identified by the triple: (program number, version number, procedure number) The program number identifies a group of related remote procedures, each of which has a unique procedure number. A program may consist of one or more versions. Each version consists of a collection of procedures which are available to be called remotely. Version numbers enable multiple versions of an RPC protocol to be available simultaneously. Each version contains a a number of procedures that can be called remotely. Each procedure has a procedure number.
Defining Client and Server Application Code
We must now write the the client and application code. They must communicate via procedures and data types specified in the Protocol.
The service side will have to register the procedures that may be called by the client and receive and return any data required for processing.
The client application call the remote procedure pass any required data and will receive the retruned data.
There are several levels of application interfaces that may be used to develop RPC applications. We will briefly disuss these below before exapnading thhe most common of these in later chapters.
Overview of Interface Routines
RPC has multiple levels of application interface to its services. These levels provide different degrees of control balanced with different amounts of interface code to implement. In order of increasing control and complexity. This section gives a summary of the routines available at each level. Simplified Interface Routines
The simplified interfaces are used to make remote procedure calls to routines on other machines, and specify only the type of transport to use. The routines at this level are used for most applications. Descriptions and code samples can be found in the section, Simplified Interface @ 3-2.
Simplified Interface
The simplified interface is the easiest level to use because it does not require the use of any other RPC routines. It also limits control of the underlying communications mechanisms. Program development at this level can be rapid, and is directly supported by the rpcgen compiler. For most applications, rpcgen and its facilities are sufficient. Some RPC services are not available as C functions, but they are available as RPC programs. The simplified interface library routines provide direct access to the RPC facilities for programs that do not require fine levels of control.
Routines such as rusers are in the RPC services library librpcsvc. rusers.c, below, is a program that displays the number of users on a remote host. It calls the RPC library routine,rusers.
Passing Arbitrary Data Types
Data types passed to and received from remote procedures can be any of a set of predefined types, or can be programmer-defined types. RPC handles arbitrary data structures, regardless of different machines' byte orders or structure layout conventions, by always converting them to a standard transfer format called external data representation (XDR) before sending them over the transport. The conversion from a machine representation to XDR is called serializing, and the reverse process is called deserializing.