Introduction to DCE

Chapter 3. DCE Technology Components

3.2 DCE Remote Procedure Call

3.2.1 What Is DCE RPC?

DCE RPC is a facility for calling a procedure on a remote machine as if it were a local procedure call. To the application programmer, a remote call looks (almost) like a local call, but there are several RPC components that work together to implement this facility, including the Interface Definition Language (IDL) and its compiler, a Universal Unique Identifier (UUID) generator, and the RPC Runtime, which supports two RPC protocol implementations. One RPC protocol operates over connection-oriented transports such as the Transmission Control Protocol/Internet Protocol (TCP/IP) and the other RPC protocol operates over connectionless transports such as the User Datagram Protocol/Internet Protocol (UDP/IP). The components that comprise the DCE RPC are as follows:

3.2.2 End User's Perspective

The end user does not come in direct contact with DCE RPC, but does see the end result, in the form of 3-8 December 14, 1993 DCE Technology Components * The availability of distributed applications built using RPC * The ability to use remote resources accessed via RPC An end user who is browsing through the namespace may also notice the names of RPC based servers, since these servers advertise themselves to their clients through the DCE Directory Service.

3.2.3 Programming with DCE RPC

This section provides a brief overview of the process a programmer goes through in using DCE RPC to write an application. For an example of how this process applies to a simple application, see Section 3.8 of this manual. For a more detailed description of the DCE RPC programming process, see the introductory chapters and the RPC chapters of the OSF DCE Application Development Guide. Figure 3-1 shows an overview of the DCE RPC application development process. The dashed boxes indicate application code written by the DCE programmer. The other boxes indicate compiled code or code generated automatically for the programmer by DCE RPC. December 14, 1993 3-9 Introduction to OSF DCE Figure 3-1. DCE RPC Programming Proce ss #typedef account IDL File idl Client Stub Header File Server Stub RPC Runtime Client Appl. Server Appl. RPC Runtime Link Link Bank Client Bank Server Install on Install on Define Interface in DCE IDL: Run IDL Compiler: credit() get_balance() debit() Client Server The IDL File

First, the application programmer defines the RPC interface, and associated data types, using the DCE Interface Definition Language (IDL). An interface is a group of operations that a server can perform. This grouping is similar to a module or library in a conventional programming language--a group of operations defined in a single file or unit. For example, a Bank Service might perform operations to debit, credit, or read the balance of an account. Each of those operations and their parameters must be defined in the IDL file. The collection of Bank Service operations--debit, credit, read balance -- together form the Bank Service interface. The process of defining RPC operations is similar to defining the input and output of a local procedure call, except in IDL only the calling interface is defined, not the 3-10 December 14, 1993 DCE Technology Components implementation of the procedure. (An IDL interface definition is similar to an ANSI C prototype definition.) Next, the programmer compiles the IDL file using the IDL compiler. The compiler produces output either in a conventional programming language, which is the C language in the DCE offering, or in object code. The output of the compilation consists of a client stub, a server stub, and a header file. The client and server stubs are routines that make the remoteness of the operation transparent to the caller or callee of the operation. The Client Side

For the client side of the application, the programmer writes application code that makes calls to the operations in the IDL file. The client stub code is linked with this application code, and (along with the RPC Runtime code) performs the tasks that turn what looks like a procedure call into network communications with the server side of the application. Usually the client side of the application contains a relatively small a mount of RPC code. The Server Side

For the server side, the programmer writes application routines that implement the operations defined in the IDL file. For example, in the banking application, a da tabase lookup might implement the operation to read an account balance. The server stub, generated by the IDL compiler, is linked with the server application code. The server stub unpacks the arguments and makes the call to the application routine as if the client program had called it directly. The server side of the application contains the bulk of the RPC code that needs to be written by the distributed application programmer. Binding

In order for the client to send an RPC to the server, it must be able to find the server. This process is called binding. A client may have some direct way of knowing what server it needs to communicate with; for example, it may get this information from a file, a value hardcoded into its program, an environment variable, or a user. A more flexible way for a client to find a server is to take advantage of DCE RPC's use of the DCE Directory Service. A client can find a server by asking the Directory Service for the location of a server that handles the interface that the client is interested in (in our example, a Bank Server). In order for the Directory Service to be able to give the client this information, a server must first advertise itself in the Directory Service. The server adds itself to the namespace, along with information about what interfaces it implements, what protocols it uses to communicate with, and where it is located. This way, a server can move, or December 14, 1993 3-11 Introduction to OSF DCE there can be multiple servers implementing a given interface, without affecting the client. The client can still go to the Directory Service, a well-known central source of information, and find out where the server is located. The DCE programmer does not make calls directly to CDS; binding is supported by the Name Service Independent (NSI) API, an RPC-specific name service layer. Calls to this library are made by the client side of an application in order to look up binding information for a server based on various criteria, such as the type of service, the objects it manages, and the interfaces it supports. The server side of an application calls this library to advertise information about itself to the namespace where clients can find it. The RPC Daemon

There are two parts to a server's location: the address of the machine it resides on, and the address of the process--the network endpoint (for example, a UNIX port). The machine location is fairly stable, so its address can reasonably be entered into the Cell Directory Service. The network endpoint, however, can change each time the server process is started up. Instead of making frequent changes to CDS to update a server's endpoint address, DCE RPC uses a specialized type of directory service, the RPC daemon, or rpcd. When a server starts up, it registers its process address with rpcd. Every machine that runs an RPC server also runs an rpcd. The rpcd process always uses the same network endpoint, so its process address is well known. Therefore, once a client knows what machine a server is running on, it can find the rpcd process running on that same machine. It can then ask the rpcd process for the network endpoint of the server process. This process is shown in Figure 3-2 (read the messages, show in quotes, in clockwise order). Figure 3-2. Client Finds Server Using CDS and RPC Daemon @Port X Node A "Port X" "Node A" "Bank?" "Bank?" Server Client CDS rpcd 3-12 December 14, 1993 DCE Technology Components

3.2.4 DCE RPC Administration

A few a dministrative tasks must be performed when running a distributed application using RPC. The application server executes most of these tasks when it first starts up. As described in the previous section, the server registers its (dynamically assigned) listening endpoint with rpcd. The server also advertises information about itself and the interfaces it supports in the DCE Directory Service. Nonautomated RPC administration is minimal. The administrator must ensure that each DCE machine has an RPC daemon running on it. An administrator may be involved in registering servers in the namespace, but this can also be done from server code upon initialization as just described. A management program, rpccp, allows an administrator to control the rpcd and administer RPC information in the namespace. An administrator may be involved in installing a new RPC-based application. In particular, the server side of the application must be started up before it can begin receiving and servicing requests. The administrator may arrange for the server process to be run each time the machine is booted, for example.

3.2.5 How It Works

A short walk-through of what happens during an RPC call may help clarify the RPC concept and how it works. This section describes the RPC call shown in Figure 3-3. (This description is somewhat simplified. The use of rpcd is not shown.) December 14, 1993 3-13 Introduction to OSF DCE Figure 3-3. RPC Runtime Process 8. credit(acct) Arguments 5. Make RPC Arguments 4. Package 3. credit(acct) 2. Find Server 7. Unpack 6. Receive RPC 1. Advertise Init Client Client Appl Client Stub Client Runtime BANK CLIENT BANK SERVER Appl Server Stub Server Server Init Server Runtime CDS "Bank@Node B" "Bank?" "Node B" credit(acct) On the server side, the Bank Server process is started up. Before it begins its continuous cycle of receiving and servicing requests, the server process advertises its location in the Cell Directory Service (see Point 1 in Figure 3-3). In this way, when a client queries the Directory Service for a bank server, it will be able to find it. After initialization, the server listens for a request to come in from a client over the network. This call to wait for client requests is a call to the RPC Runtime, since the Runtime handles network communications. Eventually, a user on the Bank Client machine invokes the bank application. The Bank Client initialization code calls the RPC Runtime to find a server offering the Bank Service (see Point 2). The Bank Client application code makes a call to a re mote procedure; for example, a call to a procedure that credits a bank account (3). This results in a call to the client stub code. The stub transforms the arguments of the call into a network message (4). It then calls the client's RPC Runtime library, which sends the message to the server (5). Back on the server side, the RPC request is received by the RPC Runtime, which has been waiting for a client request (6). The Runtime passes control, and the received packet, to the server stub. The stub unpacks the arguments sent by the client (7) and passes them to the appropriate operation by making a procedure call to it. At this point, the server application code that implements the requested operation is called. The operation is performed--the account is credited (8). The RPC reply (not shown in the figure) returns in the reverse direction. The Bank Server application procedure returns the results of the credit operation to the stub. The 3-14 December 14, 1993 DCE Technology Components stub packs up the return parameters and passes the resulting message to the RPC Runtime to send off to the client over the network. The server then waits for the next client request to come in. The client's Runtime receives the server's reply. The client stub then unpacks the received network message, arranging returned parameters such that when the client application call to RPC returns, it looks like it has just performed a local procedure call. The mechanisms in both the client and server stubs and the Runtime library are transparent to the application programmer. The programmer writes the application calls to the RPC operations on the client side, and provides implementations for those operations on the server side, but the network communications code is generated automatic ally.

3.2.6 System Independence

There are several ways in which using DCE RPC frees a programmer from dependence on other parts of a system. It provides portability across programming languages, data transfer syntax mechanisms, transport and network protocols, and operating system and architecture platforms. * Language Independence DCE RPC is language independent in the sense that the stubs generated by the IDL compiler can be called by programs written in any traditional programming language, provided that the language follows the calling conventions that the stub expects. The DCE IDL compiler generates stubs that use the C language calling conventions. A client written in FORTRAN, for example, can call an IDL stub in the same way that it calls any local C procedure. It can then make a remote call to a server (possibly written in another language) that contains the server stub generated from the same IDL file as the client stub was generated from. * Data Representation Independence The default data representation format is the DCE Transfer Syntax, which is currently the Network Data Representation (NDR). It allows various representations for different types of data, including multiple encodings for characters, integers, and floating-point numbers. It is ``multicanonical;'' that is, there are several canonical formats that can be used. The sender chooses one of these formats (in most cases, it will be the sender's native data representation), with information about what representation it has chosen. The receiver transforms data into its own format, if it is different from the format the data was sent in. This model optimizes for the case when both sender and receiver use the same data representa tion, a frequent occurrence. (Note that this data transfer is handled by the RPC software, and is not visible to the application programmer.) The DCE RPC architecture allows the use of transfer syntaxes other than DCE Transfer Syntax (although the only transfer syntax currently provided in the OSF implementation is DCE Transfe r Syntax). For example, data could be formatted according to the ISO ASN.1/BER specification and sent over the wire in that way. December 14, 1993 3-15 Introduction to OSF DCE * Protocol Independence Independence of RPC, transport, and network protocols is achieved as follows. The DCE RPC offering includes two different RPC protocols. The first runs over connection-oriented transport protocols; the second runs over connec tionless (datagram) transport protocols. The programmer can specify the underlying RPC protocol, but the semantics of RPC calls are the same whether the RPC is running over a connectionless or connection-oriented transport. Another RPC protocol could be used in place of these two DCE RPC protocols; for example, when ISO defines an RPC standard, it could be used transparently as a third RPC protocol under the DCE RPC interfaces. Servers identify themselves to the Directory Service according to the interface they support and the protocols they use. In this way, a client can look up a server that uses network protocols that are compatible with those that the client supports. * Machine Independence Because DCE RPC uses the DCE Transfer Syntax, distributed applications are machine independent. DCE Transfer Syntax allows machines to transfer data even when their native data representations are not the same. * Operating System Independence Finally, DCE RPC offers independence from the local operating system. The application programmer is not directly using the networking system calls provided by the local operating system. By being one level of abstraction up from this layer, the programmer is insulated from networking system calls that are operating system specific.

3.2.7 Additional Information on DCE RPC

For additional information on DCE RPC, see the following: * The RPC chapters of the OSF DCE Application Development Guide and the OSF DCE Administration Guide * The (1rpc) and (3rpc) reference pages of the OSF DCE Application Development Reference * The (5rpc) and (8rpc) reference pages of the OSF DCE Administration Reference