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Distributes objects and Rmi


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Distributes objects and Rmi

  1. 1. Distributed Objects and Remote Invocation Unit-3 Prof. Mayank Jain CSS434 RMI 1
  2. 2. CSS434 RMI 2 Outline  RPC/RMI Model  Implementation Issues  Parameter-passing semantics  Invocation semantics  Stateless/stateful server  Server creation semantics  SunRPC  Java RMI
  3. 3. Why RPC/RMI Calling a function/method at a remote server:  Advanced form of communication  Sockets and MPI: data transfer  RPC/RMI: control transfer  Transparency in function calls  No distinguish between local and remote calls (in theoretical)  Also applied to IPC on the same machine CSS434 RMI 3  Ease of use  Compiled-time argument type checking  Automatic stub generation
  4. 4. Remote and Local Method/Function Invocation invocation invocation remote invocation CSS434 RMI 4 remote invocation local local local invocation A B C D E F
  5. 5. Service Interface and Remote Interface RPC: an service interface defined with an XDR file struct doubles { double a; double b; }; program EXAMPLE_PROG { remoteobject CSS434 RMI 5 remote interface m 1m2 m3 m4 m5 m6 Data implementation { of methods RMI: a remote interface defined by extending the “Remote” interface Interface MyRmoteInterface extends Remote { ReturnClass1 m1( ArgClass1 arg ) throws RemoteException; ReturnClass2 m2( ArgClass2 arg ) throws RemoteException; ReturnClass3 m3( ArgClass3 arg ) throws RemoteException; } version EXAMPLE_VERS { int FACT( int ) = 1; /* procedure number = 1 */ double POWER( doubles ) = 2; /* procedure number = 2 */ } = 1; /* version number = 1 */ } = 0x31234567; /* program number = 0x31234567 */
  6. 6. RPC/RMI Model Caller (Client) Callee (Server) RPC and wait Request message including arguments Reply message Including a return value CSS434 RMI 6 Suspended Resume execution Request message accepted Execution environment created Execution of function body Send reply and wait for the next request
  7. 7. Implementation Issues CSS434 RMI 7  Transparency property  Syntactic transparency  Semantic transparency  Analogy in semantics b/w local and remote procedure calls  Caller capable of passing arguments (Automatic marshalling)  Caller suspended till a return from a function  Callee capable of returning a value to caller  Difference in semantics b/w local and remote procedure calls  No call by reference and no pointer-involved arguments  Error handling required for communication (Ex. RemoteException in Java)  Performance much slower than local calls.
  8. 8. Parameter-Passing Semantics  Call by Value  Most PRCs take this semantics.  Voluminous data incurs copying overhead.  Call by Reference  Passing pointers and references are meaningless.  Then, how about object-based systems?  The value of a variable is a reference to an object  Call by object reference  Additional remote object invocations  Call by visit: all related objects moved to a server every RPC.  Call by move: all related objects moved and left to a server upon the first RPC. CSS434 RMI 8 Client Server class objA { objB b; objC c; } class objB { } class objC { }
  9. 9. Invocation Semantics Fault tolerance measures Invocation semantics Maybe At-least-once CSS434 RMI 9 Retransmit request message Duplicate filtering Re-execute procedure or retransmit reply No Yes Yes Not applicable No Yes Not applicable Re-execute procedure Retransmit reply At-most-once
  10. 10. Stateful/Stateless Servers CSS434 RMI 10  Stateful servers:  Servers keep track of client information.  RPC/RMI reply depends on that client information.  Pros: Simplify client design  Cons: A server crash loses client information. A client crash leaves old client information at server.  At-most-once invocation semantics (Java RMI takes this design.)  Stateless servers:  Clients must maintain Inter-call information.  RPC/RMI reply is always the same.  At-least-once invocation semantics (Some RPC implementations take this design.)
  11. 11. Server Creation Semantics CSS434 RMI 11  Instance-per-Call Servers  A new server process launched every call  Statelss servers only  OS resource allocation/de-allocation involved every call  Instance-per-Session Servers  A server process maintained for the entire session with a client  Stateful servers: inter-call state maintained for a single client  OS resource allocation/de-allocation involved every session  Persistent Servers  A server process remains in existence indefinitely.  Stateful and shared servers: concurrency control required  OS resource allocation/de-allocation involved only once.  SunRPC/Java RMI take instance-per-call servers, while a thread but not a process is created per a call. (The server process must remain active to accept a new call and thus can still be stateful.)
  12. 12. RPC Mechanism Interface Definition Language File Client Program Server Program (5) Exception? (4) Invalid arguments? (3) Invalid procedure? RPC Runtime (Dispatcher) (2) Unauthorized client? CSS434 RMI 12 Return Call Receive Send Define arguments Register remote functions Client Stub IDL Compiler Message Decoding Encoding marshaling RPC Runtime Retransmission acknowledgments Routing encryption Return Call Server Stub Message Decoding Encoding Receive Send (1) Intelligible messages? msg id type (call) args PRC id client id type (reply) reply status fesults failur msg id
  13. 13. Client-Server Binding (SunRPC) Client Machine Server Machine (2) Locating server (3) Server port (xxx) write(), read(), Sendto(), recvfrom() TCP or UDP Transport level or below TCP or UDP  (1) pmap_set(prognum, versnum, protocol, port)  (2) and (3) pmap_getport(addr, prognum, versnum, protocol)  To check the status of portmap, rpcinfo CSS434 RMI 13 Client Portmap Daemon Server Network (LAN/WAN) write(), read(), Sendto(), recvfrom() (1) register (4) RPC with port xxx port: 111 port: xxx
  14. 14. example_client.o Modify by yourself CSS434 RMI 14 SunRPC example_client.c rpcgen –a example.x Your client example_clnt.c Client stub example_h.c example_xdr.c example_svc.c example_server.c example_clnt.o example_xdr.o example_svc.o example_server.o example.x example_client example_server Marshalling Server stub Your server gcc –c -o Header ld -o Modify by yourself Interface descriptions Client program Server program
  15. 15. Sun RPC (Interface definition) /* * example.x - Speicification of some arithmetic/string service. * Define 2 procedures: * fact( int n ) returns n!. * power( double x, double y ) returns x^^y. * strconc( struct strings ) concatenates src to dest. */ CSS434 RMI 15 const BUFFER_SIZE = 1024; struct doubles { double a; double b; }; struct strings { char src[BUFFER_SIZE]; char dst[BUFFER_SIZE]; }; program EXAMPLE_PROG { version EXAMPLE_VERS { int FACT( int ) = 1; /* procedure number = 1 */ double POWER( doubles ) = 2; /* procedure number = 2 */ string STRCONC( strings ) = 3; /* procedure number = 3 */ } = 1; /* version number = 1 */ } = 0x31234567; /* program number = 0x31234567 */
  16. 16. Sun RPC (Client) CSS434 RMI 16 #include "example.h" void example_prog_1(char *host) { CLIENT *clnt; int *result_1; int fact_1_arg; double *result_2; doubles power_1_arg; char * *result_3; strings strconc_1_arg; clnt = clnt_create (host, EXAMPLE_PROG, EXAMPLE_VERS, "udp"); if (clnt == NULL) { clnt_pcreateerror (host); exit (1); } fact_1_arg = 10; result_1 = fact_1(&fact_1_arg, clnt); if (result_1 == (int *) NULL) { clnt_perror (clnt, "call failed"); } printf( "fact( 10 ) = %dn", *result_1 ); power_1_arg.a = 2.0; power_1_arg.b = 6.0; result_2 = power_1(&power_1_arg, clnt); if (result_2 == (double *) NULL) { clnt_perror (clnt, "call failed"); } printf( "power( 2.0, 6.0 ) = %fn", *result_2 ); strncpy( strconc_1_arg.dst, "xyz0", BUFFER_SIZE ); strncpy( strconc_1_arg.src, "abc0", BUFFER_SIZE ); result_3 = strconc_1(&strconc_1_arg, clnt); if (result_3 == (char **) NULL) { clnt_perror (clnt, "call failed"); } printf( "strconc( "xyz", "abc" ) = %sn", *result_3 ); clnt_destroy (clnt); } int main (int argc, char *argv[]) { char *host; if (argc < 2) { exit (1); host = argv[1]; example_prog_1 (host); exit (0); }
  17. 17. Sun RPC (Server) CSS434 RMI 17 #include "example.h" #include <math.h> #include <string.h> int * fact_1_svc(int *argp, struct svc_req *rqstp) { static int result; int i; result = 1; for ( i = *argp; i > 0; i-- ) result *= i; return &result; } double * power_1_svc(doubles *argp, struct svc_req *rqstp) { static double result; result = pow( argp->a, argp->b ); return &result; } char ** strconc_1_svc(strings *argp, struct svc_req *rqstp) { static char * result; result = strcat( argp->dst, argp->src ); return &result; }
  18. 18. SunRPC v.s. Java RMI SunRPC Java RMI Advantages Template generation Dynamic port assignment High performance Automatic serialization Manual IP port handling Good security Disadvantages Manual serialization CSS434 RMI 18 Weak to hackers Low performance
  19. 19. RMI Mechanism skeleton Convert object/method/arguments in a TCP message. object A object B Request & dispatcher for B’s class CSS434 RMI 19 proxy for B Reply Remote Communication Communication Remote reference reference module module module module remote client server Exchange a request and a replay in TCP. Maintain a table of local objects and their remote object references (i.e., object proxies). Find the corresponding remote object, unmarshal arguments, and invoke the object.
  20. 20. 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, as shown in Figure 15.13, line 3. CSS434 RMI 20 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, as shown in Figure 15.15 line 1. A remote object reference is returned. String [] list() This method returns an array of Strings containing the names bound in the registry.
  21. 21. RMI Programming Procedure (2) Program a class (4) Program a class (5) javac (7) Run Server with java Server (8) Run Client with java Client Application Layer: Client .java (implements remote interface) Stub/Skeleton: Stub Skeleton (1) Define a remote interface (3) javac rmic Server Server_Stub.class Server_Skel.class (6) Invoke a rmiregistry Remote Reference: rmiregistry [port#] (object manager/name service) request and result Transport Layer: TCP/IP CSS434 RMI 21
  22. 22. RMI Remote Interface & Return Object // Remote interface: A server must implements this interface in its class define import java.rmi.* public interface ServerInterface extends Remote { public ReturnObj get( ) throws RemoteException; // A server returns a ReturnObj to a client CSS434 RMI 22 } // Define the class of a object returned from a server to a client (ReturnObj) import* import java.util.* public class ReturnObj implements Serializable { // A return object must be serializable. private id; SubObject subObj; public ReturnObj( int i, SubObject s ) { id = i; subObj = s; } public void print( ) throws IOException { System.out.println( “ReturnObj: id=“ + id ); } }
  23. 23. RMI Server Implementation CSS434 RMI 23 import*; import java.util.*; import java.rmi.*; import java.rmi.server.*; public class Server extends UnicastRemoteObject implements ServerInterface { static private int i = 0; public Server( ) throws RemoteException{ } public static void main( String args[] ) { try { Server server = new Server( ); Naming.rebind( “server”, server ); } catch ( Exception e ) { System.err.println( “Exception: “ + e ); System.exit( 1 ); } } public ReturnObject get( ) throws RemoteException { ReturnObject f = new ReturnObject( i++, new SubObject( i ) ); return f; } } (1) Implement a remote interface (2) Inherit a RemoteObject (3) Define a constructor (4) Instantiate a RemoteObject (5) Implement all methods declared in a remote interface
  24. 24. RMI Client Implementation CSS434 RMI 24 Import java.rmi.*; Import*; public class Client { public static void main( String[] args ) { try { ServerInterface si = (ServerInterface)Naming.lookup( “rmi://adonis1/server” ); ReturnObject f = si.get( ); f.print( ); f.subObj.print( ); } catch ( RemoteException re ) { System.out.println( “Exception : “ + re ); System.exit( 1 ); } catch ( IOException ie ) { System.out.println( “Exception : “ + ie ); System.exit( 1 ); } catch ( NotBoundException nbe ) { System.out.println( “Exception : “ + nbe ); System.exit( 1 ); } } } (1) RMI registry returns a reference to RemoteObject (2) Call a method of this RemoteObject (3) Exception handling for RMI (4) Exception handling for I/O (5) Exception handling for Naming
  25. 25. RMI Compilation and Execution % javac // Compile a return class % javac // Compile a server class % rmic Server // Create a stub and a skelton % javac // Compile a client class % ls ReturnObject.class Server.class Server_Stub.class Server_Skel.class % rmiregistry& // Invoke RMI registory % java Server& // Invoke a server % java Client // Invoke a client ReturnObject id = 0 SubObject id = 1 % java Client // Invoke a client again ReturnObject id = 2 SubObject id = 3 % CSS434 RMI 25
  26. 26. Exercises (No turn-in) 1. The caller process of an RPC/RMI must wait for a reply from the callee process after making a call. Explain how this can actually be done. 2. Which types of server did you implement for the programming assignments 3 and 4, a stateful or a stateless server? Then, why did you implement such a type of server? 3. Discuss the similarities and differences among the following parameter passing: CSS434 RMI 26 1. Call-by-object-reference 2. Call-by-move 3. Call-by-visit 4. Discuss about the pros and the cons of dynamic binding in RPC. 5. Textbook p227, Q5.12: Discuss the invocation semantics that can be achieved when the request-reply protocol is implemented over a TCP connection, which guarantees that data is delivered in the order sent, without loss or duplication. Take into account all of the conditions causing a connection to be broken. 6. Textbook p227, Q5.14: The Election service must ensure that a vote is recorded whenever any user thinks they have cast a vote. Discuss the effect of maybe call semantics on the Election service. Would at-least-once call semantics be acceptable for the Election service or would you recommend at-most-once call semantics?
  27. 27. Exercises (No turn-in) 7. Textbook p227, Q5.15: A request-reply protocol is implemented over a communication service with omission failures to provide at-least-once RMI invocation semantics. In the first case the implementor assumes an asynchronous distributed system. In the second case the implementor assumes that the maximum time for the communication and the execution of a remote method is T. In what way does the latter assumption simplify the implementation? 8. Textbook p228, Q5.22: A client makes remote procedure calls to a server. The client takes 5 milliseconds to compute the arguments for each request, and the server takes 10 milliseconds to process each request. The local operating system processing time for each send or receive operation is 0.5 milliseconds, and the network time to transmit each request or reply message is 3 milliseconds. Marshalling or unmarshalling takes 0.5 milliseconds per message. Calculate the time take by the client to generate and return from two requests: (i) if it is single-threaded, and (ii) if it has two threads that can make requests concurrently on a single processor. You can ignore context-switching times. CSS434 RMI 27