information management ntu interprocess communication and middleware

Information Management NTU

Interprocess CommunicationandMiddleware


Sockets and Ports

Source: G. Coulouris et al., Distributed Systems: Concepts and Design

Node 1 Node 2


Crossing Hosts/Platforms



Purposes of Middleware

Higher-level abstractions (RPC, RMI, …) Location transparency Independent of communication protocols Independent of hardware/operating syste

ms Use of several programming languages


Local vs. Remote Modules

Variables Variables of a remote module cannot be directly accessed.

Parameter-passing Mechanisms Call by reference, for input parameters, is not feasible for a

remote procedure/method. Pointers

Pointers of a remote module are not very useful for the local module.

* For a module in some process, any other module in a different process, not necessarily a different computer, is a remote module.


The Middleware layer


* The operating system includes common network protocols (TCP/IP, …).


Marshalling (Serialization) in CORBA CDR



Request-Reply Communication



Request-Reply Communication Using HTTP

Source of figures: G. Coulouris et al., Distributed Systems: Concepts and Design

An HTTP request message:

An HTTP reply message:

HTTP is not only a request-reply protocol but also has been used as

the transport of other request-reply protocols.


Distributed Objects

Client/Server: two-tier, three-tier, … Interfaces and Interface Definition Languages (I

DLs) Object References (or Identifiers) and Remote M

ethod Invocation (RMI) Object Activations and Persistency Exceptions: timeouts, … Synchronization, Replication, Migration, Garbag

e Collection, …


Interactions among Distributed Objects



A Remote Object and Its Interface



A CORBA IDL Example



Client Proxy and Server Skeleton


A client proxy is also known as a client stub and server skeleton as server stub.

The remote reference module is mainly for translating between local and remote

object references.


The RMI Software

Proxy: the local representative of the remote object.

Dispatcher: relays a request to the appropriate skeleton method.

Skeleton: unmarshals the request and invokes the corresponding method in the remote object.

These RMI components are generated automatically by an interface compiler.


RMI/RPC Semantics and Transparency

Source of the table: G. Coulouris et al., Distributed Systems: Concepts and Design

Remote invocations may be made syntactically identical to local invocations,

but they have far more implications that both the client and the server designers

have to deal with.


Event-Based Systems

The publish-subscribe paradigmpublisher event and event typesubscribernotification

Heterogeneity Asynchrony


A Dealing Room System

Dealer’s computer

Informationprovider

Dealer

Externalsource

Externalsource

Informationprovider

Dealer

Dealer

Dealer

Notification

Notification

Notification

Notification

NotificationNotification

Notification

Notification

Dealer’s computer

Dealer’s computerDealer’s computer

NotificationNotification



An Architecture for Event Notification


subscriberobserverobject of interest

Event service

object of interest

object of interest observer

subscriber

subscriber

3.

1.

2. notification

notification

notification

notification


The Roles for Observers/Agents

Forwarding Filtering Patterns of Events Notification Mailboxes


Java Remote Interfaces



Parameter Passing in Java

By reference (passed as remote object references)For parameters whose type is defined as a remote i

nterface, one that extends Remote By value

For parameters of serializable types, including primitive types and classes that implement the serializable interface


A Java Server



A Java Server (cont.)



A Java Client



CORBA

Defined by OMG to facilitate the development of distributed object-oriented systems.

Language-independency is achieved through the use of a standard interface definition language---the CORBA IDL.

An ORB (Object Request Broker) receives invocations from a client and deliver them to a target object.

The main communication protocol is GIOP (General Inter-ORB Protocol), known as IIOP when implemented over the Internet.


The CORBA Architecture

Source of the figure: G. Coulouris et al., Distributed Systems: Concepts and Design

* The implementation repository allows server objects to be activated on demand.

** The interface repository gives run-time type information, mainly for dynamic

invocations.

***


The Object Adapter

Creates remote object references for CORBA objects

Maps the names of CORBA objects to their servants

Dispatches each remote invocation via a skeleton to the appropriate server object

Activate objects


CORBA Object Interfaces

Each object has an interface defined in IDL.

An interface defines the operations that can be called by the clients.

An interface can be implemented in one language and called from by another.

The CORBA IDL includes features such as inheritance of interfaces, exceptions, and compound data types.


CORBA Programming with Java

Define the interfaces using IDL and compile them into Java interfaces.

Implement the interfaces with Java classes. Write a server main function that creates

instances of these classes and then inform the underlying CORBA implementation.

Register the server. Write a client main function to connect to the

server and to use server’s objects.


Shape and Shapelist in CORBA IDL



Java Interface Generated from ShapeList



Java Implementation of Shapelist



Java Implementation of ShapeList (cont.)



Java Implementation of a ShapeList Client



CORBA Services

Naming Service locate objects by their names

Trading Service locate objects by their attributes

Event Service and Notification Service Security Service Transaction Service Persistent Object Service


The CORBA Naming Service

Allows (1) a name to be bound to an object and (2) that object to be found subsequently by resolving that name.

A name is a sequence of name components and is resolved within a given naming context.

The IDL interface NamingContext defines the core of the naming service.

A NamingContext object acts much like a directory in a filing system.


CORBA Naming Graph

initial naming context

ShapeList

CD E

B


P

R S T

V

Q U


XX



The NamingContext Interface (partial)

struct NameComponent { string id; string kind; };

typedef sequence <NameComponent> Name;

interface NamingContext {void bind (in Name n, in Object obj);

binds the given name and remote object reference in my context.void unbind (in Name n);

removes an existing binding with the given name.void bind_new_context(in Name n);

creates a new naming context and binds it to a given name in my context.Object resolve (in Name n);

looks up the name in my context and returns its remote object reference. void list (in unsigned long how_many, out BindingList bl, out BindingIterator bi);

returns the names in the bindings in my context.};



COM/DCOM

COM stands for Component Object Model. Its distributed version is referred to as DCOM.

It is a programming model for binary components reuse and a foundation of OLE (Object Linking and Embedding) and ActiveX controls.

COM interfaces are defined in the interface definition language IDL and compiled by MIDL.EXE.


COM Objects

All COM objects implement the IUnknown interface (defined in unknwn.idl) or one of its extended interfaces.

Methods of IUnknown: QueryInterface: checks if the named interface is supported

and, if so, returns the corresponding interface reference AddRef Release

A COM object may implement multiple interfaces.


GUIDS

To eliminate name collisions, all COM interfaces are assigned a unique binary name at design time that is the physical name of the interface.

These physical names are called Globally Unique Identifiers (GUIDs).

GUIDs are 128-bit extremely large numbers that are guaranteed to be unique in both time and space.

information management ntu interprocess communication and middleware

Documents

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remote proceduremethod

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http reply message

designan http request