Tonight. Communication. Middleware Protocols. Remote Procedure Call (RPC) Several common communication mechanisms RPC RMI Sockets Next week: HTTP,

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1 Tonight Communication Tanenbaum Ch. 4 Distributed Software Systems CS 707 Several common communication mechanisms RPC RMI Sockets Next week: HTTP, Middleware Protocols Remote Procedure Call (RPC) Figure 4-3. An adapted reference model for networked communication. 1

2 Conventional Procedure Call Client and Server Stubs Figure 4-5. (a) Parameter passing in a local procedure call: the stack before the call to read. (b) The stack while the called procedure is active. Figure 4-6. Principle of RPC between a client and server program. Remote Procedure Calls (1) A remote procedure call occurs in the following steps: 1. The client procedure calls the client stub in the normal way. 2. The client stub builds a message (marshalling the parameters) and calls the local operating system. 3. The client s OS sends the message across the network to the remote OS. 4. The remote OS gives the message to the server stub. 5. The server stub unpacks the parameters 6. Calls the server implementing the function. Continued RPC with value parameters Figure 4-7. The steps involved in a doing a remote computation through RPC. 2

3 Remote Procedure Calls (2) A remote procedure call occurs in the following steps (continued): 7. The server does the work and returns the result to the stub. 8. The server stub packs it in a message and calls its local OS. 9. The server s OS sends the message across the network to the client s OS. 10. The client s OS gives the message to the client stub. 11. The stub unpacks the result and returns to the client. Passing Value Parameters a) Original message on the Pentium b) The message after receipt on the SPARC c) The message after being inverted. The little numbers in boxes indicate the address of each byte Passing Reference Parameters A pointer is only meaningful in the address space of the process where it is used. One solution is to forbid pointers and reference parameters in RPC More typically, marshalling involves changing the mechanism to copy/restore Actual parameter is sent as a copy If the formal parameter is changed, the changed version is sent back to use as a replacement for the actual Optimizations include taking advantage of the direction of the changes (input only, output only) Straightforward for some parameter types (arrays for example) but not in general. Parameter Specification and Stub Generation Figure 4-9. (a) A procedure. (b) The corresponding message. array represented as a pointer is copied in the invocation 3

4 Writing a Client and a Server (1) User generates client, server and an interface definition file. The code file must #include the header file (to be generated). #include <stdio.h> #include <rpc/rpc.h> #include "RPCTest.h" Client Code main(int argc, char**argv) CLIENT *clienthandle; char *servername = "cs1.gmu.edu"; readargs a; Data *data; clienthandle= clnt_create(servername,filereadwrite,version,"udp"); /* creates a socket and a client handle */ if (clienthandle == NULL) clnt_pcreateerror(servername); /* unable to contact server */ exit(1); a.val1 = 10; a.val2 = 20; data = double_1(&a,clienthandle); /* call to remote procedure */ printf("%d**\n",*data); clnt_destroy(clienthandle); #include <stdio.h> #include <rpc/rpc.h> #include "RPCTest.h" Data *double_1(readargs *a) static Data result; /* must be static */ result = a->val1 + a->val2; return &result; Server Code Remote function double Writing a Client and a Server (2) Four files output by the Sun RPC IDL compiler from the interface definition file: typedef int Data; struct readargs int val1; int val2; ; /* pack multiple arguments into struct */ program FILEREADWRITE version VERSION Data DOUBLE(readargs)=1; =1; =9999; Interface Specification A header file (e.g., interface.h, in C terms). XDR file describes data details The client stub. The server stub. All must #include header file 4

5 Writing a Client and a Server (1) XDR code C compiler XDR obj file Figure The steps in writing a client and a server in Sun RPC. /* Please do not edit this file. It was generated using rpcgen. */ #ifndef _RPCTEST_H_RPCGEN #define _RPCTEST_H_RPCGEN #include <rpc/rpc.h> typedef int Data; struct readargs int val1; int val2; ; typedef struct readargs readargs; #define FILEREADWRITE 9999 #define VERSION 1 #define DOUBLE 1 extern Data * double_1(); extern int filereadwrite_1_freeresult(); /* the xdr functions */ extern bool_t xdr_data(); extern bool_t xdr_writeargs(); extern bool_t xdr_readargs(); #endif /*!_RPCTEST_H_RPCGEN */ Header File /* * Please do not edit this file. * It was generated using rpcgen. */ #include "RPCTest.h" #ifndef _KERNEL Client Stub #include <stdio.h> #include <stdlib.h> /* getenv, exit */ #endif /*!_KERNEL */ /* Default timeout can be changed using clnt_control() */ static struct timeval TIMEOUT = 25, 0 ; Data * double_1(argp, clnt) readargs *argp; CLIENT *clnt; static Data clnt_res; memset((char *)&clnt_res, 0, sizeof (clnt_res)); if (clnt_call(clnt, DOUBLE, (xdrproc_t) xdr_readargs, (caddr_t) argp, (xdrproc_t) xdr_data, (caddr_t) &clnt_res, TIMEOUT)!= RPC_SUCCESS) return (NULL); return (&clnt_res); /* * Please do not edit this file. * It was generated using rpcgen. */ #include "RPCTest.h" static void closedown(sig) int sig; static void filereadwrite_1(rqstp, transp) struct svc_req *rqstp; register SVCXPRT *transp; switch (rqstp->rq_proc) case DOUBLE: _xdr_argument = xdr_readargs; _xdr_result = xdr_data; local = (char *(*)()) double_1; break; main() Server Stub (lots of code omitted) 5

6 /* * Please do not edit this file. * It was generated using rpcgen. */ #include "RPCTest.h" bool_t xdr_data(xdrs, objp) register XDR *xdrs; Data *objp; #if defined(_lp64) defined(_kernel) register int *buf; #else register long *buf; #endif if (!xdr_int(xdrs, objp)) return (FALSE); return (TRUE); XDR file bool_t xdr_readargs(xdrs, objp) register XDR *xdrs; readargs *objp; #if defined(_lp64) defined(_kernel) register int *buf; #else register long *buf; #endif if (!xdr_int(xdrs, &objp->val1)) return (FALSE); if (!xdr_int(xdrs, &objp->val2)) return (FALSE); return (TRUE); Writing a Client and a Server (1) XDR code C compiler XDR obj file Figure The steps in writing a client and a server in Sun RPC. Binding a Client to a Server (1) Registration of a server makes it possible for a client to locate the server and bind to it. Binding a Client to a Server (2) Figure Client-to-server binding in DCE. SunRPC does not use directory server. Server location is done in two steps: 1. Connect to the server s machine on known port. 2. Locate the server s port on that machine. 6

7 Asynchronous RPC (1) Asynchronous RPC (2) Figure (a) The interaction between client and server in a traditional RPC. Figure (b) The interaction using asynchronous RPC. Asynchronous RPC (3) Remote Method Invocation (RMI) Figure A client and server interacting through two asynchronous RPCs. 7

8 RMI RMI = RPC + Object-orientation Java RMI CORBA Middleware that is language-independent Microsoft DCOM/COM+ SOAP RMI on top of HTTP Remote and local method invocations A remote invocation local C invocation local E invocation B local invocation D remote invocation F Distributed Objects Binding a Client to an Object 2-16 Distr_object* obj_ref; obj_ref = ; obj_ref-> do_something(); Distr_object obj_ref; Local_object* obj_ptr; obj_ref = ; obj_ptr = bind(obj_ref); obj_ptr -> do_something(); (a) (b) //Declare a systemwide object reference // Initialize the reference to a distributed object // Implicitly bind and invoke a method //Declare a systemwide object reference //Declare a pointer to local objects //Initialize the reference to a distributed object //Explicitly bind and obtain a pointer to the local proxy //Invoke a method on the local proxy Common organization of a remote object with client-side proxy (loaded when the client binds to a remote object). a) (a) Example with implicit binding using only global references b) (b) Example with explicit binding using global and local references 8

9 Distributed Objects Remote object references An identifier that can be used throughout a distributed system to refer to a particular remote object Remote interfaces CORBA provides an interface definition language (IDL) for specifying a remote interface JAVA RMI: Java interface that extends Remote interface Actions: remote invocations Remote Exceptions may arise for reasons such as partial failure or message loss Distributed Garbage Collection: cooperation between local garbage collectors needed RMI Programming RMI software Generated by IDL compiler Proxy (client side) Behaves like remote object to clients (invoker) Marshals arguments, forwards message to remote object, unmarshals results, returns results to client Skeleton (server side) Server side stub; Unmarshals arguments, invokes method, marshals results and sends to sending proxy s method Dispatcher (server side) Receives the request message from communication module, passes on the message to the appropriate method in the skeleton RMI Programming Binder Client programs need a means of obtaining a remote object reference Binder is a service that maintains a mapping from textual names to remote object references Servers need to register the services they are exporting with the binder Java RMIregistry, CORBA Naming service Server threads Several choices: thread per object, thread per invocation Remote method invocations must allow for concurrent execution Java RMI Features Integrated with Java language + libraries Security, write once run anywhere, multithreaded Object orientation Can pass behavior Mobile code Not possible in CORBA, traditional RPC systems Distributed Garbage Collection Remoteness of objects intentionally not transparent 9

10 Remote Interfaces, Objects, and Methods Objects become remote by implementing a remote interface A remote interface extends the interface java.rmi.remote Each method of the interface declares java.rmi.remoteexception in its throws clause in addition to any application-specific clauses Creating distributed applications using RMI 1. Define the remote interfaces 2. Implement the remote objects 3. Implement the client (can be done anytime after remote interfaces have been defined) 4. Register the remote object in the name server registry 5. Generate the stub and client using rmic 6. Start the registry 7. Start the server 8. Run the client import java.rmi.*; import java.rmi.registry.*; import java.net.*; public class RmiClient Java Client static public void main(string args[]) ReceiveMessageInterface rmiserver; Registry registry; String serveraddress=args[0]; String serverport=args[1]; String text=args[2]; System.out.println("sending "+text+" to "+serveraddress+":"+serverport); try // get the registry registry=locateregistry.getregistry(serveraddress,(new Integer(serverPort)).intValue() ); // look up the remote object rmiserver= (ReceiveMessageInterface)(registry.lookup("rmiServer")); // call the remote method int i = rmiserver.receivemessage(text); System.out.println("received "+i); catch(remoteexception e) e.printstacktrace(); catch(notboundexception e) e.printstacktrace(); import java.rmi.*; import java.rmi.registry.*; import java.rmi.server.*; import java.net.*; Java Server (1) public class RmiServer extends java.rmi.server.unicastremoteobject implements ReceiveMessageInterface int thisport; String thisaddress; Registry registry; // rmi registry for lookup the remote objects. // This method is called from the remote client by the RMI. // This is the implementation of the ReceiveMessageInterface. public int receivemessage(string x) throws RemoteException System.out.println(x); return 42; 10

11 public RmiServer() throws RemoteException try // get the address of this host. thisaddress= (InetAddress.getLocalHost()).toString(); catch(exception e) throw new RemoteException("can't get inet address."); thisport=3232; // this port(registry s port) System.out.println("this address="+thisaddress+",port="+thisport); try // create the registry and bind the name and object. registry = LocateRegistry.createRegistry( thisport ); registry.rebind("rmiserver", this); catch(remoteexception e) throw e; Java Remote interface import java.rmi.*; public interface ReceiveMessageInterface extends Remote int receivemessage(string x) throws RemoteException; static public void main(string args[]) try RmiServer s=new RmiServer(); catch (Exception e) e.printstacktrace(); System.exit(1); Java Server (2) The Naming class of Java RMIregistry void rebind (String name, Remote obj) This method is used by a server to register the identifier of a remote object by name. void bind (String name, Remote obj) This method can alternatively be used by a server to register a remote object by name, but if the name is already bound to a remote object reference an exception is thrown. void unbind (String name, Remote obj) This method removes a binding. Remote lookup(string name) This method is used by clients to look up a remote object by name. A remote object reference is returned. String [] list() This method returns an array of Strings containing the names bound in the registry. Object References as Parameters 2-18 The situation when passing an object by reference or by value. There is no analogous activity in RPC. 11

12 Classes supporting Java RMI RemoteObject RemoteServer Readings Tannenbaum Chapters xxx WWW (see links on class web page) Java RMI tutorial on web Activatable UnicastRemoteObject <servant class> The programmer's conceptual view of a TCP/IP Internet Network Programming using Socket API Application TCP Application UDP IP 12

13 Socket programming Goal: learn how to build client/server application that communicate using sockets Socket API introduced in BSD4.1 UNIX explicitly created, used, released by apps client/server paradigm two types of transport service via socket API: unreliable datagram (UDP) reliable, byte streamoriented (TCP) socket a host-local, applicationcreated/owned, OS-controlled interface (a door ) into which application process can both send and receive messages to/from another (remote or local) application process Sockets and ports agreed port socket any port socket message client server other ports Internet address = Internet address = Socket programming with TCP Client must contact server server process must first be running server must have created socket (door) that welcomes client s contact Client contacts server by: creating client-local TCP socket specifying IP address, port number of server process When client creates socket: client TCP establishes connection to server TCP When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk with multiple clients application viewpoint TCP provides reliable, in-order transfer of bytes ( pipe ) between client and server Client/server socket interaction: TCP Server (running on hostid) sockfd = socket(af_inet, SOCK_STREAM, 0); bind(sockfd, (struct sockaddr *) &serv_addr,sizeof(serv_addr)) listen(sockfd,5); newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen); n = read(newsockfd, ); n = write(newsockfd, ); close(newsockfd); Client sockfd = socket(af_inet, SOCK_STREAM, 0); TCP connect(sockfd,&serv_addr,sizeof(serv_addr)) Connection setup n = write(sockfd,buffer,strlen(buffer)); n = read(sockfd,buffer,255); close(sockfd); 13

14 Berkeley Sockets API (1) Socket primitives for TCP/IP. Primitive Socket Bind Listen Accept Connect Send Receive Close Meaning Create a new communication endpoint Attach a local address to a socket Announce willingness to accept connections Block caller until a connection request arrives Actively attempt to establish a connection Send some data over the connection Receive some data over the connection Release the connection /* A simple server in the internet domain using TCP The port number is passed as an argument */ #include <stdio.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> void error(char *msg) perror(msg); exit(1); int main(int argc, char *argv[]) int sockfd, newsockfd, portno, clilen, n; char buffer[256]; struct sockaddr_in serv_addr, cli_addr; if (argc < 2) fprintf(stderr,"error, no port provided\n"); exit(1); sockfd = socket(af_inet, SOCK_STREAM, 0); if (sockfd < 0) error("error opening socket"); bzero((char *) &serv_addr, sizeof(serv_addr)); portno = atoi(argv[1]); serv_addr.sin_family = AF_INET; serv_addr.sin_addr.s_addr = INADDR_ANY; serv_addr.sin_port = htons(portno); if (bind(sockfd, (struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) error("error on binding"); listen(sockfd,5); clilen = sizeof(cli_addr); newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen); if (newsockfd < 0) error("error on accept"); bzero(buffer,256); n = read(newsockfd,buffer,255); if (n < 0) error("error reading from socket"); printf("here is the message: %s\n",buffer); n = write(newsockfd,"i got your message",18); if (n < 0) error("error writing to socket"); return 0; C Server #include <stdio.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <netdb.h> void error(char *msg) perror(msg); exit(0); int main(int argc, char *argv[]) int sockfd, portno, n; struct sockaddr_in serv_addr; struct hostent *server; char buffer[256]; if (argc < 3) fprintf(stderr,"usage %s hostname port\n", argv[0]); exit(0); portno = atoi(argv[2]); sockfd = socket(af_inet, SOCK_STREAM, 0); if (sockfd < 0) error("error opening socket"); server = gethostbyname(argv[1]); if (server == NULL) fprintf(stderr,"error, no such host\n"); exit(0); bzero((char *) &serv_addr, sizeof(serv_addr)); serv_addr.sin_family = AF_INET; bcopy((char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr,server->h_length); serv_addr.sin_port = htons(portno); if (connect(sockfd,&serv_addr,sizeof(serv_addr)) < 0) error("error connecting"); printf("please enter the message: "); bzero(buffer,256); fgets(buffer,255,stdin); n = write(sockfd,buffer,strlen(buffer)); if (n < 0) error("error writing to socket"); bzero(buffer,256); n = read(sockfd,buffer,255); if (n < 0) error("error reading from socket"); printf("%s\n",buffer); return 0; C Client Berkeley Sockets (2) Connection-oriented communication pattern using sockets. 14

15 Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket Example: Java server (TCP) import java.io.*; import java.net.*; class TCPServer public static void main(string argv[]) throws Exception String clientsentence; String capitalizedsentence; ServerSocket welcomesocket = new ServerSocket(6789); while(true) Socket connectionsocket = welcomesocket.accept(); BufferedReader infromclient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); Example: Java server (TCP), cont Create output stream, attached to socket Read in line from socket DataOutputStream outtoclient = new DataOutputStream(connectionSocket.getOutputStream()); clientsentence = infromclient.readline(); capitalizedsentence = clientsentence.touppercase() + '\n'; Write out line outtoclient.writebytes(capitalizedsentence); to socket End of while loop, loop back and wait for another client connection Example: Java client (TCP) Example: Java client (TCP), cont. Create input stream Create client socket, connect to server Create output stream attached to socket import java.io.*; import java.net.*; class TCPClient public static void main(string argv[]) throws Exception String sentence; String modifiedsentence; BufferedReader infromuser = new BufferedReader(new InputStreamReader(System.in)); Socket clientsocket = new Socket("hostname", 6789); DataOutputStream outtoserver = new DataOutputStream(clientSocket.getOutputStream()); Create input stream attached to socket Send line to server Read line from server BufferedReader infromserver = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = infromuser.readline(); outtoserver.writebytes(sentence + '\n'); modifiedsentence = infromserver.readline(); System.out.println("FROM SERVER: " + modifiedsentence); clientsocket.close(); 15

16 Socket programming with UDP Client/server socket interaction: UDP UDP: no connection between client and server no handshaking sender explicitly attaches IP address and port of destination server must extract IP address, port of sender from received datagram UDP: transmitted data may be received out of order, or lost application viewpoint UDP provides unreliable transfer of groups of bytes ( datagrams ) between client and server Server (running on hostid) create socket, port=x, for incoming request: serversocket = DatagramSocket() read request from serversocket write reply to serversocket specifying client host address, port umber Client create socket, clientsocket = DatagramSocket() Create, address (hostid, port=x, send datagram request using clientsocket read reply from clientsocket close clientsocket Sockets used for datagrams Example: Java server (UDP) Sending a message s = socket(af_inet, SOCK_DGRAM, 0) bind(s, ClientAddress) sendto(s, "message", ServerAddress) Receiving a message s = socket(af_inet, SOCK_DGRAM, 0) bind(s, ServerAddress) amount = recvfrom(s, buffer, from) Create datagram socket at port 9876 import java.io.*; import java.net.*; class UDPServer public static void main(string args[]) throws Exception DatagramSocket serversocket = new DatagramSocket(9876); byte[] receivedata = new byte[1024]; byte[] senddata = new byte[1024]; ServerAddress and ClientAddress are socket addresses Create space for received datagram Receive datagram while(true) DatagramPacket receivepacket = new DatagramPacket(receiveData, receivedata.length); serversocket.receive(receivepacket); 16

17 Example: Java server (UDP), cont Example: Java client (UDP) Get IP addr port #, of sender Create datagram to send to client Write out datagram to socket String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivepacket.getaddress(); int port = receivepacket.getport(); String capitalizedsentence = sentence.touppercase(); senddata = capitalizedsentence.getbytes(); DatagramPacket sendpacket = new DatagramPacket(sendData, senddata.length, IPAddress, port); serversocket.send(sendpacket); End of while loop, loop back and wait for another datagram Create input stream Create client socket Translate hostname to IP address using DNS import java.io.*; import java.net.*; class UDPClient public static void main(string args[]) throws Exception BufferedReader infromuser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientsocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] senddata = new byte[1024]; byte[] receivedata = new byte[1024]; String sentence = infromuser.readline(); senddata = sentence.getbytes(); Example: Java client (UDP), cont. Create datagram with data-to-send, length, IP addr, port Send datagram to server Read datagram from server DatagramPacket sendpacket = new DatagramPacket(sendData, senddata.length, IPAddress, 9876); clientsocket.send(sendpacket); DatagramPacket receivepacket = new DatagramPacket(receiveData, receivedata.length); clientsocket.receive(receivepacket); String modifiedsentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedsentence); clientsocket.close(); 17