分布对象 (1) distributed objects 2015-9-14 institute of computer software nanjing university 1

分布对象 (1)

Distributed Objects

23/4/19Institute of Computer Software

Nanjing University

1

摘要背景

Why, What, How 一般框架 CORBA Java RMI


Nanjing University

2

Acknowledgement

本节 ppt 有关 CORBA 部分内容从下列文献摘录或改编： Bamshad Mobasher

http://maya.cs.depaul.edu/~mobasher/classes/ds520/

Douglas C. Schmidt http://www.cs.wustl.edu/~schmidt/corba.html


Nanjing University

3



http://www.cs.wustl.edu/~schmidt/corba.html

摘要背景



Nanjing University

4

背景分布式计算（ Distributed computing ）

What: the process of running a single computational

task on more than one distinct computer -- from Wikipedia

Compare to “Parallel Computing” Why? How?

实践表明，分布式应用系统的开发比集中式应用系统开发困难的多。


Nanjing University

5

分布式计算 Why?

“世界是分布的”。有时将涉及的用户、应用、数据集中化不合适、不可行。

资源共享并行处理冗余容错

The main goal of a distributed computing system is to connect users and resources in a transparent, open, and scalable way. Ideally this arrangement is drastically more fault tolerant and more powerful than many combinations of stand-alone computer systems.


Nanjing University

6

Distributed Systems Frameworks 77

The Business Model Every application is part of your business model

must make them work together!

Shipping/ Receiving

Inventory

Engineering

Manufacturing

Accounting

Payables/ Receivables

Sales


The Problem Application Integration and Distributed Processing are the same

thing Constructing information-sharing distributed systems from

diverse sources: heterogeneous networked physically disparate multi-vendor


常见分布式应用模式

Client CodeClient Code

Stub

Server CodeServer Code

Skeleton

InfrastructureInfrastructure

Client: Thin or Fat?

• Client – Server Computing


常见分布式应用模式 Multi-Tier Systems

separates persistence from logic and logic from presentation all activity originates in user interface arrows represent client-server relation

Business LogicBusiness Logic

User Interface Persistent Store


常见分布式应用模式 Web Applications

interaction initiated by user UI based on HTML, Java, Javascript, etc. CGI/Servlet composes HTML, forwards interaction requests to business logic

BusinessLogic

BusinessLogic

User InterfaceData Store

FirewallFirewall HTTP ServersHTTP Servers

CGI / Servlets


常见分布式应用模式

Peer-to-Peer Systems

ApplicationApplication

User Interface(mixed initiative)

Persistent Store

MonitorMonitor

Enterprise Applications

Requests

NotificationsMessage

queue

• An architecture where there is no special machine or machines that provide a service or manage the network resources.

• Instead all responsibilities are uniformly divided among all machines, known as peers. Peers can serve both as clients and servers.


分布式应用的构成 3 layers of a distributed application:

network level (TCP/IP, network protocols) higher-level services (directory services, security protocols, etc.) application level

At the application level: processes threads objects: processes can be made up of one or more objects which can be

accessed by one or more threads within the process agents: an “independent” functional element (e.g., in a banking application

we may have a customer agent, a transaction agent, an information brokerage agent)


分布式应用的一般技术需求 Partitioning and Distributing Data and Functions

data-driven distribution functional distribution object-based distribution

Flexible, Extendible Communication Protocols allocation of tasks to agents has a direct influence on the

complexity of the communication protocol (type of data, amount of data, persistence of connections)

may also be dictated by legacy systems that need to be incorporated


分布式应用的一般技术需求 Multi-threading Requirements

server object may need to service multiple remote clients at once

effective way to optimize resources

Security Requirements authentication of client identities define resource access levels data encryption

分布计算 How ？

理想主义： Distributed Operating System 现实主义：

操作系统提供网络通信功能（中间件）分布应用程序


Nanjing University

16

分布计算 Socket programming in Java


Nanjing University

17

17

// listen to port 5000 on the local host for socket connection requestsServerSocket s = new ServerSocket(5000);while (true) { // wait for connection request from client Socket clientConn = s.accept(); InputStream in = clientConn.getInputStream(); OutputStream out = clientConn.getOutputStream(); // now IO streams are connected to client; do something with them...

分布计算


Nanjing University

18

18

// create the socketInetAddress addr =

InetAddress.getByName(“our.remote.host”);Socket s = new Socket(addr, 5000);InputStream in = s.getInputStream();OutputStream out = s.getOutputStream();// now we have IO streams to remote process; do something with them...

分布计算 Message Passing Libraries

PVM (Parallel Virtual Machine), MPI (Message Passing Interface)

Send/Recv, Multicast

Virtual Shared Memory OpenMP


Nanjing University

19

分布计算远程过程调用 (RPC ， Remote Procedure

Call) 让应用程序开发者可以

象调用本地过程一样调用分布在远程节点上的过程将结构化方法用于分布环境

实现上 Request – Response 消息传递模式 provides a function-oriented interface to

socket-level communications (sits on top of the TCP/IP transport layer)


Nanjing University

20

摘要背景



Nanjing University

21

Distributed Systems Frameworks 22Distributed Systems Frameworks 22

General Pattern for Remote Invocation

Call: marshal arguments convert to network format locate server transmit data

Client CodeClient Code

Stub

Server CodeServer Code

Skeleton

InfrastructureInfrastructure

Serve: receive data convert & unmarshal invoke method marshal return value transmit data

Distributed Systems Frameworks 23Distributed Systems Frameworks 23

Procedural API for Remote Invocation

DCE (Distributed Computing Environment)

OSF standard for RPC provides a common set of services

Directory Services--Store the names of resources that are available within the distributed environment. The Cell Directory Service (CDS) supports naming within a cell, and the Global Directory Service (GDS) supports naming across all cells within an enterprise. GDS implements the X.500 directory service standard.

Distributed File Service (DFS)--An optional DCE service that provides a seamless file system that operates across all computers contained within a cell.

Distributed Time Service (DTS)--Used for synchronization. Security Service--Used for authentication and control access. DCE Threads--Similar to Java threads. They are lightweight processes

that simplify the design of client/server applications.

分布对象计算（ DOC ）让应用程序开发者可以象使用本地对象一样使

用远程对象对象计算模型和分布计算模型的结合

自然 decentralized nature of dist. computing encapsulation required by distributed

components information hiding requirements (location

transparency, security, etc.) 优越 :


Nanjing University

24

分布对象技术 Location Transparency


Nanjing University

25

Machine 1 Machine 2

Caller

Proxy

Implementor

localcall remote

call

分布对象技术 Proxy Pattern


Nanjing University

26

分布对象技术 Providing the same enhancements to

procedural RPC toolkits that OO languages provide to conventional procedural languages 实现对象计算的全面支持

encapsulation, interface inheritance, object-based exception handling

良好的软件组织 promotes separation of interface from

implementation (crucial for scalable distributed applications)

甚至 DbC23/4/19

Institute of Computer SoftwareNanjing University

27

Key Advantages of DOC

Enabling interworking between applications at a higher level of abstraction developing distributed applications using familiar

techniques such as method calls on objects again, location transparency

Providing a foundation for building higher-level mechanisms that facilitate collaboration among services in distributed applications common object services (e.g., global naming,

transactional messaging, quality of service facilities, etc.)


Nanjing University

28

分布对象计算

实现分布对象计算的几个核心概念接口对象引用参数传递相关服务设施底层通信协议


Nanjing University

29

显式、抽象的对象接口定义 allow clients to access objects regardless of

implementation details allow object server flexibility in implementing objects

existing services may be incorporated via wrappers platform-independent specification languages

interface descriptions can be converted into server skeletons which can be compiled and implemented in any language

some object interfaces also generate client stub interfaces.

Examples: CORBA: Interface Definition Language (IDL) DCOM: Component Object Model language


Nanjing University

30

远程对象引用对象通过相互引用来通信

本地对象引用：仅在当前地址空间有效运行系统维护；

远程对象引用不同地址空间？异构？生命周期？

需要某种“对象管理器”来支持、维护之


Nanjing University

31

对象管理器（ Object Manager ） The core of a distributed object system Manages object skeletons and object

references on the server E.g., Object Request Broker (ORB) in CORBA or

Registry Service in RMI When a client requests a new object, the object

manager locates the skeleton for the class of the requested

object creates new instance based on skeleton stores new object in the object storage sends a reference to the new object back to the client


Nanjing University

32

对象管理器 Remote method calls by clients are routed by

object manager to the proper object on the server

Object manager may also destroy objects after clients are done

Some other possible features of OM dynamic object activation/deactivation persistent objects


Nanjing University

33

Registration / Naming Service Acts as an intermediary between the object

client and the object manager Once the interface to an object is defined, an

implementation of the interface must be registered with the service so that it can be addressed by clients

Through the naming service the client can specify the type of the object it needs or the name of a particular object, if it already exists


Nanjing University

34

对象通信协议

General protocol for handling remote object requests

Must support a means of transmitting and receiving object and method references, and data in the form of objects or basic data types

Ideally should be transparent to clients should interact with local object proxies (stubs) and let

the object distribution scheme handle communication behind the scenes


Nanjing University

35

分布计算的一般框架

23/4/19

36

IDL

Com

pil

ers

Registration Service

Object Skeleton

Object Storage

Object Manager

Naming Service

Client Application

ServerImplementation

Client StubInterface

Object Interface

Specification

运行时刻远程对象交互


Nanjing University

37

ObjectStubs

Server ObjectImplementation Object

Skeleton

Object Manager Naming Service

Client Application

2. Resolve Object

4. Object Interactions

1. Request Object

3. Object Handle

Comparing DOC Frameworks

Cross Cross Complexity Client Server SecurityPlatform Lang. Portability Portability

CORBA yes yes medium high medium many options

DCOM limited yes high low low NT security

RMI yes no low high high add-on


Nanjing University

38

Distributed Object Computing Frameworks CORBA - Common Object Request Broker Architecture, an industry

standard developed by OMG DCOM - Microsoft’s Distributed Component Object Model, a

descendant of DCE RPC RMI - Java’s Remote Method Invocation

摘要背景



Nanjing University

39

OMG Reference Model Architecture


Nanjing University

40

Object Management Architecture

Object Services Domain-independent foundational services for

use by developers of implementation objects some published services:

Naming: allows clients to find objects based on names

Trading: allows clients to find objects based on their properties

Lifecycle managementEvent notification TransactionsSecurity…


Nanjing University

41


Common Facilities object services provide functionality for use

by objects, CORBA facilities provide standards for services used by applications

generic functionality needed by many applications (e.g., printing, document management, email, etc.)


Nanjing University

42


Domain Interfaces provide domain-specific objects for vertical

application domains Examples: Finance, Healthcare, Manufacturing,

Telecom, Electronic Commerce, Transportation

Application Interfaces Thus not standardized


Nanjing University

43

Common Object Request Broker Architecture

一个典型的分布对象计算架构 The architecture

对象 Objects and Clients 接口 OMG IDL “对象管理器” ORB 底层通信协议 IIOP 其他


Nanjing University

44


Nanjing University45

Distributed Systems Frameworks 46Distributed Systems Frameworks

Overview of CORBA Objects CORBA Object

<identity, interface, implementation>

CORBA objects differ from typical programming language objects: CORBA objects can be located anywhere on a network. CORBA objects (like Java objects) can run on any platform. CORBA objects can be written in any of several languages.

CORBA object developers need know nothing of where their clients will be, what hardware or OS they will run on, or what language they will be written in. CORBA objects approach universal accessibility.


Overview of CORBA Objects An object implementation provides the

semantics of the object, usually by defining data for the object instance and code for the object's methods. (called a “servant”)

A client of an object has access to an object reference for the object, and invokes operations on the object. A client knows only the logical structure of the object according to its

interface and experiences the behavior of the object through invocations.

Client code has no knowledge of the implementation of the object or which ORB is used to access the implementation.


Role of OMG IDL

Distributed Systems Frameworks

ORB

C

C++

Ada

I D L

I D L

I D L

I D L

I D L

I D L

Client Side Object Implementation

Side

COBOL

C

Ada

C++

Smalltalk

JAVA

I D L

I D L

I D L

I D L

I D L

I D L

ORB

C++

COBOL

Smalltalk

JAVA

Internet InterORB

Protocol (IIOP)


OMG IDL OMG Interface Definition Language (IDL):

mappings for many languages/compilers; independent of any particular language/compiler; multiple-inheritance, public interface-structured

specification language; not for implementation. primary support for interoperability between static and

dynamic requests mechanisms.


OMG IDL IDL Structure

Module a namespace

Interface abstract type multiple inheritance

Struct structured data

Module auction { exception NotAllowed {};

struct Sale { int price; string item; }

interface Auction { void bid (in long price) raises NotAllowed; }}

Module auction { exception NotAllowed {};

struct Sale { int price; string item; }

interface Auction { void bid (in long price) raises NotAllowed; }}


ClientClient Object ImplementationObject Implementation

ORB

RequestRequest

A Request A request consists of:

Target object (target object identified by a unique object reference) Operation Parameters (the input, output and in-out parameters defined for the

operation; may be specified individually or as a list) Optional request context

Skeletoncode

Skeletoncode

Client Proxy(stub code)

Client Proxy(stub code)


CORBA Framework Elements

Object Request Broker (ORB) This is the object manager in CORBA

Mechanisms for specifying interfaces Interface Definition Language (IDL) - for static interface definitions Dynamic Invocation Interface (DII) - lets clients access interfaces as

first-class objects at run-time from an Interface Repository.

Internet Inter-Orb Protocol (IIOP) A binary protocol for communication between ORBs. Was added in CORBA 2.0


Object Request Broker (ORB) The Object Manager in CORBA Both on the client side and the server side (allows user programs

to act as both clients and servers of remote objects) On client side the ORB is responsible for

accepting requests for a remote object finding implementation of the object accepting client-side reference to the remote object(converted to a language

specific form, e.g., a Java stub object) routing client method calls through the object reference to the object

implementation

On server side the ORB lets object servers register new objects receives requests from the client ORB uses object’s skeleton interface to invoke object’s activation method creates reference for new object and sends it back to client


Internet Inter-Orb Protocol (IIOP) CORBA specification is neutral with respect to network protocols

the CORBA standard specifies what is known as the General Inter-ORB Protocol (GIOP) GIOP is a high-level standard protocol for communication between ORBs not used directly; instead, it is specialized by a particular protocol that would then be

used directly

Internet Inter-ORB Protocol (IIOP) IIOP is the GIOP-based protocol for TCP/IP networks As of the 2.0 version of the CORBA specification, vendors are required to implement the

IIOP protocol

CORBA Networking Model CORBA applications are built on top of GIOP-derived protocols such as IIOP these protocols, in turn, rest on top of TCP/IP, DCE, or other underlying transport

protocol the network uses an application architecture can be designed to use a bridge that would interconnect, for

instance, DCE-based application components with IIOP-based ones.


Passing Objects by Reference In a distributed application, there are two possible methods for

an application component to obtain access to an object in another process: When an object is passed by reference, the object itself remains "in place"

while an object reference for that object is passed. Operations on the object through the object reference are actually processed by the object itself.

When an object is passed by value, the object's state is copied and passed to its destination (via object serialization), where a new copy of the object is instantiated. Operations on that object's copy are processed by the copy, not by the original object.

Note: in CORBA, objects are only passed by reference (however, the new CORBA specifications include facilities for passing objects by value).


Object References An Object Reference is the information needed to specify an object

within an ORB. The representation of an object reference handed to a client is only valid for the lifetime

of that client. The language mapping also provides additional ways to access object references in a

typed way for the convenience of the programmer. There is a distinguished object reference, the null reference, guaranteed to be different

from all object references, that denotes no object. In Java, this is a Java null.

To invoke a CORBA object, you need a reference for the object. There are two ways to get a reference for a CORBA object: from another object, such as a factory or a name service

from a string that was specially created from an object reference

Interoperable Object References CORBA uses IOR as a “pointer” to a specific instance of a class in a distributed

environment encodes host, port, object identity may be externalized (using object_to_string)


CORBA Components Client stub

Each stub represents (it is a proxy) an object operation (a possible request) which a client invokes in a language-dependent manner (e.g., by calling a subroutine which represents the operation).

The stubs make calls on the rest of the ORB using interfaces that are private to Java IDL. Alternatively, a client may dynamically construct and invoke request objects which can

represent any object operation.

Implementation Skeleton Each skeleton provides the interface through which a method receives a request (dynamic

and static skeletons)

Object Adapter Purpose is to interface an object's implementation with its ORB Each object adapter provides access to those services of an ORB (such as activation,

deactivation, object creation, object reference management) used by a particular type of object implementation.

ORB Interface The interface to the small set of ORB operations common to all objects, e.g., the

operation which returns an object's interface type.


ObjectAdapter

ImplementationSkeletons

ORB Core

Client Object Implementation

ClientStubs

ORBInterface

DynamicInvocation

standard interface

One interface per object operation

One interface per object adaptor

Proprietary ORB interface

Normal call interfaceUp call interface

CORBA Components


Client

ClientStubs

ORBInterface

DynamicInvocation

Clients perform requests using object references.

Clients may issue requests through object interface stubs (static) or dynamic invocation interface.

Clients may access general ORB services:

• Interface Repository.• Context Management.• List Management.• Request Management.

Client Side


Object Implementation

ORBInterface

ObjectAdapter

Implementations receive requests through skeletons (without knowledge of invocation approach).

The Object Adapter provides for:• management of references;• method invocation;• authentication;• implementation registration;• activation/deactivation.

ImplementationSkeletons

Implementation Side


Static v. Dynamic Invocation Static Invocation

Static interfaces are generated in form of client stubs by the IDL (pre-) compiler.

This means that the structure of the object has to be known before hand (at compile time).

Allows for better type checking; less runtime overhead; self-documentation.

Dynamic Invocation Dynamic Invocation Interface (DII) allows clients to invoke operations on

remote objects without having access to object stubs (another way to do this without dynamic invocation is to download static client stubs via a Java applet).

Clients must discover interface-related information at runtime (e.g., using the interface repository)

Servers can offer new services anytime without the need for recompilation on the client side.


Dynamic Requests The Dynamic Invocation Interface (DII) allows clients to

dynamically: discover objects; discover objects’ interfaces; create requests; invoke requests; receive responses.

Major features of Dynamic Invocation Interface: requests appear as objects themselves; requests are reusable; invocation may be synchronous or asynchronous; requests may be generated dynamically, statically or in combination approach.


CORBA Interface Repository The Interface Repository is a service that provides persistent

objects that represent the IDL information in a form available at runtime. Note: The Java IDL runtime does not include an implementation of an

Interface Repository and one is not generally required by clients at runtime. Using the IR, it is possible for a program to encounter an object whose

interface was not known at compile time, yet be able to determine what operations are valid on the object and make invocation on it.

Interface Repository provides: Dynamic client access to interface definitions to construct a request. Dynamic type-checking of request signatures. Traversal of inheritance graphs. ORB-to-ORB interoperability.


CORBA Implementation Repository The Implementation Repository contains information that allows the

ORB to locate and activate implementations of objects. Ordinarily, installation of implementations and control of policies

related to the activation and execution of object implementations is done through operations on the Implementation Repository.

In addition to its role in the functioning of the ORB, the Implementation Repository is a common place to store additional information associated with implementations of ORB objects. (e.g., debugging information, administrative control, resource allocation, security, etc)

The Implementation Repository supports the implementation of object servers. It is not needed by clients in order to access servers.


IDL InterfaceDefinitions

ImplementationInstallation

ClientStubs

InterfaceRepository

ImplementationRepositoryImplementation

Skeletons

Client Object Implementation

Accesses Includes DescribesIncludes

Summary of CORBA Interfaces

All objects are defined in IDL by specifying their interfaces. Object definitions (interfaces) are manifested as objects in the Interface

Repository, as client stubs, and as implementation skeletons. Descriptions of object implementations are maintained as objects in the

Implementation Repository.


Summary: CORBA Remote Method Invocation

Clients use “object interfaces” through language mapping Java clients should work on any ORB that supports the Java language bindings. Clients can call any object instance remotely, so long as the object instance

implements the interface.

Clients can call remote objects statically or dynamically The server cannot tell whether the client is using static or dynamic invocation.

Objects are identified using a unique id: Interoperable Object Reference (IOR) CORBA normally passes objects by reference IOR was Introduced in CORBA 2.0 Object references can be converted to strings and back to “live” objects via

ORB interface functions.

摘要背景



Nanjing University

67

RMI: Java distributed object model

Remote Method Invocation Java 语言之内，充分利用这一点！

Stub 可下载！可以传“对象”！ Garbage Collection!

传“引用” java.rmi.Remote (RemoteException)


Nanjing University

68


Nanjing University69

作业与 DOC （分布对象计算）相比，传统的

Socket 编程有什么优势和劣势？为什么需要DOC ？

在分布式应用中，在不同进程中传递对象有哪两种方法？ COBRA 采用的是哪种？


Nanjing University

70

分布对象 (1) distributed objects 2015-9-14 institute of computer software nanjing university 1

Documents