II. Middleware for Distributed Systems
 Outline
 Principles of Object-Oriented Middleware
 CORBA, COM and Java/RMI
 ORB Runtime
 Resolving Heterogeneity
 Dynamic Object Requests
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Principles of Object-Oriented Middleware
 Outline
 Computer Networks
 Types of Middleware
 Object-oriented Middleware
 Developing with Object-Oriented Middleware
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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
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Computer Networks
 ISO/OSI Reference Model (7-layer model)
Application
Presentation
Session
Transport
Network
Data Link
Physical
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Computer Networks
 ISO/OSI Reference Model (7-layer model)
Application
Presentation
Middleware
Session
Transport
Network
Data Link
Physical
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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
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Available Middleware
 RPC
 Call across host boundaries
 Origin of OO middleware
 Interface Definition Language
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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);
};
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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);
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General Pattern for Remote Invocation
Client Code
Server Code
Stub
Skeleton
Infrastructure
 Serve:
 Call:
 marshal arguments
 convert to network format
 locate server
 transmit data
 receive data
 convert & unmarshal
 invoke method
 marshal return value
 transmit data
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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);
};
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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
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Object-Oriented Middleware
 Session Layer
Object Request Broker
 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 Reference
Hosts
Principles of Object-Oriented Middleware
Processes
Objects
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Developing with Object-Oriented
Middleware
Design
Interface
Definition
Server Stub
Generation
Server
Coding
Client Stub
Generation
Client
Coding
Server
Registration
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General Architecture for a DOC System
Registration
Service
Object
Skeleton
Object
Storage
Server
Implementation
Object
Interface
Specification
IDL Compilers
Object Manager
Naming Service
Client Stub
Interface
Client Application
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CORBA
 OMG standard
 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
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CORBA
 CORBA
 Message-passing
 Object-oriented programming
 ORB
 Automate network functions
 Sit on the host between the data and the application layer
 Handles request messages from clients to servers in transparent manner
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CORBA Objects
 A CORBA Object has an interface and an implementation
 Interface
 Interface is not bound to a specific implementation PL,
 Interface Definition Language
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Interface Definition
 Purpose of Defining Interface
 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
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Stub Generation
 Difference between Method Calls and Object Requests
Caller
Caller
Stub
Called
Called
Stub
Transport Layer (e.g. TCP or UDP)
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IDL Compiler
 For each .idl file, idl compiler generates 4 files
Test.idl
IDL-Compiler
Testcl.hh
Testsv.hh
Testcl.cc
Testsv.cc
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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
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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>>
Player_Imp
Player_Dispatch
<<uses>>
Player_Dispatch
<<Interface>>
Player_Imp
<<Implements>>
Player_Server
Player_Server
Inheritance
Interface
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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
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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, messageoriented, 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
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