II. Middleware for Distributed Systems Outline Principles of Object-Oriented Middleware CORBA, COM and Java/RMI ORB Runtime Resolving Heterogeneity Dynamic Object Requests Principles of Object-Oriented Middleware

Principles of Object-Oriented Middleware Outline Computer Networks Types of Middleware Object-oriented Middleware Developing with Object-Oriented Middleware Principles of Object-Oriented Middleware

Principles of Object-Oriented Middleware Network Operating System Facilitate physical interchange of electrical or optical signals as packets of information Detect and correct transmission errors Implement routing of packets between hosts Compose packets into messages Application Requesting operations from remote objects Middleware Shield lower level details from applications Component-1 … Component-n Middleware Network OS Hardware Host-1 Principles of Object-Oriented Middleware

Computer Networks ISO/OSI Reference Model (7-layer model) Application Presentation Session Transport Network physical layer is concerned with the transmission of bits over a physical circuit data link layer implements a single connection between two computers network layer implements the interconnection of multiple nodes in a network. transport layer adds the ability to route longer messages between two hosts. session layer establishes and maintains connections between distributed system components. presentation layer resolves differences in data representation and provides the ability to transport complex data. application layer provides application functionality (in this book, the application layer will be implemented using distributed objects.) Data Link Physical Principles of Object-Oriented Middleware

Computer Networks ISO/OSI Reference Model (7-layer model) Application Presentation Middleware Session Transport Network physical layer is concerned with the transmission of bits over a physical circuit data link layer implements a single connection between two computers network layer implements the interconnection of multiple nodes in a network. transport layer adds the ability to route longer messages between two hosts. session layer establishes and maintains connections between distributed system components. presentation layer resolves differences in data representation and provides the ability to transport complex data. application layer provides application functionality (in this book, the application layer will be implemented using distributed objects.) Data Link Physical Principles of Object-Oriented Middleware

Available Middleware Transaction-Oriented Middleware Support transactions across different distributed db systems Two-phase commit protocol to implement distributed transaction Products: IBM’s CICS, etc. Message-Oriented Middleware Supports communication between distributed system components by facilitating message exchange Support asynchronous message delivery naturally Support multi-casting Fault tolerance (message queue on temporarily persistent storage) De-coupling of client and server Products: IBM’s MQSeries Object Oriented Middleware Has transaction-oriented middleware capabilities Support synchronous communication with at-most-once semantics Trend: to be integrated with message-oriented middleware Principles of Object-Oriented Middleware

Available Middleware RPC Call across host boundaries Origin of OO middleware Interface Definition Language See notes Principles of Object-Oriented Middleware

Remote Procedure Calls Presentation Layer Resolution of data heterogeneity Common data representation Transmission of data declaration Marshalling and Unmarshalling static Dynamic Client and server stubs Static implementations of 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); }; Principles of Object-Oriented Middleware

Remote Procedure Calls Session Layer Enable client to locate an RPC server Static binding: simple, lost location transparency Dynamic binding: depending on a deamon Support location transparency 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); Principles of Object-Oriented Middleware

General Pattern for Remote Invocation Client Code Server Code Stub Skeleton Infrastructure Call: marshal arguments convert to network format locate server transmit data Serve: receive data convert & unmarshal invoke method marshal return value transmit data Principles of Object-Oriented Middleware

Object-Oriented Middleware IDL Object types as parameters; Failure handling; inheritance Interface Player: Object { typedef struct Date { short day; short onth; short year; }; 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); }; Principles of Object-Oriented Middleware

Object-Oriented Middleware Presentation Layer Similar to RPC Support client and server stubs Perform marshalling and unmarshalling Resolve heterogeneity of data representation Different from RPC Define representation of object references Marshalling/unmarshalling object references Principles of Object-Oriented Middleware

Object-Oriented Middleware Session Layer Map object references to hosts Implements object activation policies in the object adapter Object adapters need to be able to start up severs, which register in an implementation repository or registry Implement operation dispatch Implement synchronization Object Request Broker Processes Object Reference Hosts Objects Principles of Object-Oriented Middleware

Developing with Object-Oriented Middleware Design Interface Definition Server Stub Generation Client Stub Generation Server Coding Client Coding Server Registration Principles of Object-Oriented Middleware

General Architecture for a DOC System Registration Service Object Skeleton Object Storage Server Implementation Object Manager Object Interface Specification IDL Compilers Naming Service Client Stub Interface Client Application Principles of Object-Oriented Middleware

CORBA OMG standard Common architecture framework Enable interoperability between applications in heterogeneous distributed environment Common architecture framework Common framework across heterogeneous hardware platforms and operating systems Common framework for inter-communication of application objects Open distributed object computing infrastructure Automate many common network programming tasks, such as object registration, location, and activation; Request demultiplexing Framing and error-handling Parameter marshalling and un-marshalling Operation dispatching Principles of Object-Oriented Middleware

CORBA CORBA ORB Message-passing Object-oriented programming Automate network functions Sit on the host between the data and the application layer Handles request messages from clients to servers in transparent manner Principles of Object-Oriented Middleware

CORBA Objects A CORBA Object has an interface and an implementation Interface is not bound to a specific implementation PL, Interface Definition Language Principles of Object-Oriented Middleware

Interface Definition Purpose of Defining Interface Reading Material Instantiate meta-models Provide details to class diagrams Govern interaction between client and server Provide basis for distributing type information Provide basis for automatically generate client and server stub Reading Material Brose’s book chapter 2 Principles of Object-Oriented Middleware

Stub Generation Difference between Method Calls and Object Requests Caller Caller Called Stub Stub Called Transport Layer (e.g. TCP or UDP) Principles of Object-Oriented Middleware

IDL Compiler For each .idl file, idl compiler generates 4 files Test.idl IDL-Compiler Testcl.hh Testsv.hh Testcl.cc Testsv.cc Principles of Object-Oriented Middleware

Implementation of Client Objects A static object request is made by a client calling a local method of a client stub Stubs are typed Stubs can achieve access transparency Middleware may shortcut a stub if server and client reside on the same host Principles of Object-Oriented Middleware

Implementation of Server Objects The generated server stub has to call the server implementation that an application builder designed. Interfaces and inheritance make server object implementations type safe. <<uses>> <<uses>> Player_Dispatch Player_Imp Player_Dispatch <<Interface>> Player_Imp <<Implements>> Player_Server Player_Server Inheritance Interface Principles of Object-Oriented Middleware

Server Registration Server object must be registered Registry Implementation repository System administrator maintain the repository Middleware provide tools for maintain the repository Register new server objects Startup server objects Stop/delete existing server objects Principles of Object-Oriented Middleware

Summary OO middleware built on top of the transport layer OO middleware implements the session and presentation layers Other than OO middleware, there are transactional, message-oriented, remote procedure call middleware Session layer implements an object adaptor activating/deactivating objects, synchronize client and server objects Presentation layer resolve data heterogeneity Development process for distributed objects Idl, stub generation, coding, registration Type safety Implementation repository Principles of Object-Oriented Middleware