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CS551 Object Oriented Middleware (I) (Chap. 3 of EDO)

CS551 Object Oriented Middleware (I) (Chap. 3 of EDO). Yugi Lee STB #555 (816) 235-5932 yugi@cstp.umkc.edu www.cstp.umkc.edu/~yugi. Outline. Computer Networks TCP UDP Types of Middleware Transaction-Oriented Middleware Message-Oriented Middleware Remote Procedure Calls

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CS551 Object Oriented Middleware (I) (Chap. 3 of EDO)

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  1. CS551 Object Oriented Middleware (I)(Chap. 3 of EDO) Yugi Lee STB #555 (816) 235-5932 yugi@cstp.umkc.edu www.cstp.umkc.edu/~yugi CS551 - Lecture 11

  2. Outline • Computer Networks • TCP • UDP • Types of Middleware • Transaction-Oriented Middleware • Message-Oriented Middleware • Remote Procedure Calls • Object-Oriented Middleware • Developing with Object-Oriented Middleware CS551 - Lecture 11

  3. Computer Networks • The communication between processes on different machines • At the high levels, a connection to another process on another machine to simply send each message as a high level logical unit. • logical messages each want to send to each other: simple strings or arrays of bytes (binary data). • At the underlying layers, responsible for sending the message may split it into smaller units for physical transfer called PDU's (protocol data units). • With each of these smaller packets will be a wrapper with additional header information: Address of sender/receiver, Error detection codes, Protocol control information CS551 - Lecture 11

  4. Transport Layer • Concerned with the transport of information through a network. • Two facets in UNIX/Windows networks: • TCP • UDP ISO/OSI Reference Model Application Presentation Session Transport Network Data link Physical CS551 - Lecture 11

  5. Protocols • An agreement between entities (programs) on the transmission of data. • Different layers of network software have different concerns. • At lower levels it allow for determining the amount of data to be sent, the size of packets and the speed of transmission. • Connection Control /Data transfer • Flow control/Error control • Synchronization/Addressing • At higher levels it may cover the syntax and sequencing of logical messages. CS551 - Lecture 11

  6. Transmission Control Protocol (TCP) • Provides bi-directional stream of bytes between two distributed components by establish a virtual connection • Steps: Connection establishment, Data transfer, Connection termination. • Reliable: send logical messages without having to explicitly deal with these other issues (sequencing and re-transmission of lost packets). • Slow: as more time is required to set up and terminate the link and buffering at both sides de-couples computation speeds. • UNIX rsh, rcp and rlogin are based on TCP. CS551 - Lecture 11

  7. TCP for Request Implementation Client Server Application Application Presentation Presentation Session Session Requests Transport Input Stream Transport Output Stream Results CS551 - Lecture 11

  8. Connection-oriented Communication CS551 - Lecture 11

  9. User Datagram Protocol (UDP) • Enables a component to pass a message containing a sequence of bytes (packet) to another component: • A program may send information to another at an indeterminate time with no explicit prior co-ordination • the packet may get lost or packets may arrive in a different order to that in which they were sent. • reorganize the packets and make requests for new packets when problems occur. • dynamically routed; split into a number of PDUs that take different routes to their destination depending on network congestion. • Unreliable but very fast protocol: restricted message length, queuing at receiver, e.g. UNIX rwho command. CS551 - Lecture 11

  10. UDP for Request Implementation Client Server Application Application Presentation Presentation Session Session Request Datagrams Transport Transport Result Datagrams CS551 - Lecture 11

  11. Datagram Communication CS551 - Lecture 11

  12. Direct Use of Network Protocols implies • Manual mapping of complex request parameters to byte streams • Manual resolution of data heterogeneity • Manual identification of components • Manual implementation of component activation • No guarantees for type safety • Manual synchronization of interaction between distributed components • No quality of service guarantees CS551 - Lecture 11

  13. Middleware • Layered between Application and OS/Network • Makes distribution transparent • Resolves heterogeneity of • Hardware • Operating Systems • Networks • Programming Languages • Provides development and run-time environment for distributed systems. CS551 - Lecture 11

  14. Forms of Middleware • Transaction-Oriented • IBM CICS • BEA Tuxedo • Encina • Message-Oriented • IBM MQSeries • DEC Message Queue • NCR TopEnd • RPC Systems • ANSA • Sun ONC • OSF/DCE • Object-Oriented • OMG/CORBA • DCOM • Java/RMI • First look at RPCs to understand origin of object-oriented middleware CS551 - Lecture 11

  15. Remote Procedure Calls • Enable procedure calls across host boundaries • Call interfaces are defined using an Interface Definition Language (IDL) • RPC compiler generates presentation and session layer implementation from IDL CS551 - Lecture 11

  16. IDL Example (Unix RPCs) const NL=64; struct Player { struct DoB {int day; int month; int year;} string name<NL>; }; program PLAYERPROG { version PLAYERVERSION { void PRINT(Player)=0; int STORE(Player)=1; Player LOAD(int)=2; }= 0; } = 105040; CS551 - Lecture 11

  17. ISO/OSI Presentation Layer Resolution of data heterogeneity Common data representation Transmission of data declaration Marshalling andUnmarshalling static dynamic CS551 - Lecture 11

  18. Marshalling and Unmarshalling char * marshal() { char * msg; msg=new char[4*(sizeof(int)+1) + strlen(name)+1]; sprintf(msg,"%d %d %d %d %s", dob.day,dob.month,dob.year, strlen(name),name); return(msg); }; void unmarshal(char * msg) { int name_len; sscanf(msg,"%d %d %d %d ", &dob.day,&dob.month, &dob.year,&name_len); name = new char[name_len+1]; sscanf(msg,"%d %d %d %d %s", &dob.day,&dob.month, &dob.year,&name_len,name); }; • Marshalling: Disassemble data structures into transmittable form • Unmarshalling: Reassemble the complex data structure. CS551 - Lecture 11

  19. Called Caller Caller Called Stub Method Call vs. Object Request Caller Stub Transport Layer (e.g. TCP or UDP) CS551 - Lecture 11

  20. Stubs • Creating code for marshalling and unmarshalling is tedious and error-prone. • Code can be generated fully automatically from interface definition. • Code is embedded in stubs for client and server. • Client stub represents server for client, Server stub represents client for server. • Stubs achieve type safety. • Stubs also perform synchronization. CS551 - Lecture 11

  21. Synchronization • Goal: achieve similar synchronization to local method invocation • Achieved by stubs: • Client stub sends request and waits until server finishes • Server stub waits for requests and calls server when request arrives CS551 - Lecture 11

  22. Type Safety • How can we make sure that • servers are able to perform operations requested by clients? • actual parameters provided by clients match the expected parameters of the server? • results provided by the server match the expectations of client? • Middleware acts as mediator between client and server to ensure type safety. • Achieved by interface definition in an agreed language. CS551 - Lecture 11

  23. Facilitating Type Safety Interface Definition Server Request Client Reply CS551 - Lecture 11

  24. Session Layer • Implements • identification of RPC servers • activation of RPC servers • dispatch of operations Application Presentation Session Transport Network Data link Physical CS551 - Lecture 11

  25. Example: RPC Server Identification print_person(char * host, Player * pers) { CLIENT *clnt; clnt = clnt_create(host, 105040, 0, "udp"); if (clnt == (CLIENT *) NULL) exit(1); if (print_0(pers, clnt)==NULL) clnt_perror(clnt, "call failed"); clnt_destroy(clnt); } CS551 - Lecture 11

  26. Interface Definition Language • Every object-oriented middleware has an interface definition language (IDL) • Beyond RPCs, object-oriented IDLs support object types as parameters, failure handling and inheritance • Object-oriented middleware provide IDL compilers that create client and server stubs to implement session and presentation layer CS551 - Lecture 11

  27. Why do we use an IDL? PL PL PL PL 1 2 1 2 PL PL PL IDL PL 6 3 6 3 PL PL PL PL 5 4 5 4 CS551 - Lecture 11

  28. Smalltalk C++ Ada-95 IDL Common Object Model Java C Cobol CORBA Programming Language Bindings CS551 - Lecture 11

  29. IDL Example (OO Middleware) interface Player : Object { typedef struct _Date { short day; short month; short year; } Date; attribute string name; readonly attribute Date DoB; }; interface PlayerStore : Object { exception IDNotFound{}; short save (in Player p); Player load(in short id) raises(IDNotFound); void print(in Player p); }; CS551 - Lecture 11

  30. Presentation Layer Implementation • In addition to RPC presentation layer implementation, object-oriented middleware needs to • define a transport representation for object references • deal with exceptions • need to marshal inherited attributes CS551 - Lecture 11

  31. Session Layer Implementation Object References Hosts Processes Objects CS551 - Lecture 11

  32. Server Stub Generation Client Stub Generation Server Coding Client Coding Development Steps Design Interface Definition Server Registration CS551 - Lecture 11

  33. Facilitating Access Transparency • Client stubs have the same operations as server objects • Hence, clients can • make local call to client stub • or local call to server object without changing the call. • Middleware can accelerate communication if objects are local by not using the stub. CS551 - Lecture 11

  34. Facilitating Location Transparency • Object identity • Object references • Client requests operation from server object identified by object reference • No information about physical location of server necessary • How to obtain object references? CS551 - Lecture 11

  35. Stub Generation The CORBA IDL is translated into stubs and skeletons (server stubs): IDL-compiler generates four files (two for client stubs and two for server stubs) included in generates reads Team.idl IDL-Compiler Teamcl.hh Teamsv.hh Teamcl.cc Teamsv.cc CS551 - Lecture 11

  36. Team.idl Client.cc Server.cc IDL-Compiler Teamcl.hh Teamsv.hh Teamcl.cc Teamsv.cc C++ Compiler, Linker C++ Compiler, Linker included in generates reads Server Client Client and Server Implementation The client implementation (Client.cc) includes Teamcl.hh so as to enable the compiler to check consistency between the declaration and use of client stub implementation. When translated to an executable, Client.cc is linked with the object code of the client stub (Teamcl.cc). Similar to Server. CS551 - Lecture 11

  37. Type Safe Server Object Implement. Inheritance is used in classes Play_Dispatch and Player_Impl, which are contained in the server stub for object type Player. An Interface-based implementation with the relationships of interfaces and classes involved on the server side <<uses>> <<uses>> Player_Impl <<interface>> Player_Dispatch Player_Dispatch Player_Impl <<implements>> Player_Server Player_Server CS551 - Lecture 11

  38. Server Registration • Object adapters need to be able to locate and start servers • Server objects are registered in some form of implementation repository • Registration processes is middleware and product-specific • Object adapter performs implementation repository lookup prior to activation CS551 - Lecture 11

  39. Key Points • Middleware builds on the transport layer • There are several forms of middleware • Object-oriented middleware provides IDLs • Object-oriented middleware implements session and presentation layer • Presentation layer implementation in client/server stubs is derived from IDL • Session layer is implemented in object adapters CS551 - Lecture 11

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