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Java RMI and WS

B. Ramamurthy

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Inside RMI• http://java.sun.com/j2se/1.5.0/docs/index.html• Basic RMI classes: /usr/java1.1/src/java/rmi

– java.rmi.registry.*– java.rmi.Naming class (static/class methods)– java.rmi.Remote interface (marker interface)– java.rmi.server.*– Default RMI port 1099

– Both lookup from local and remote are acceptable.

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Implementation of RMI (5.2.5)

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The role of proxy and skeleton in remote method invocation

object A object BskeletonRequestproxy for B

Reply

CommunicationRemote Remote referenceCommunication module modulereference module module

for B’s class& dispatcher

remoteclient server

servant

Object A invokes a remote object in Object B for which it holds a remote object reference.“System Model”

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RMI Internals: Communication Module

• Carries out request-reply protocol; • On the client side {message type, message

id, remote reference to object} are gathered and sent out. At most once invocation semantics;

• On the server side, it gets local reference for remote reference from remote reference module, invokes a dispatcher with this reference.

• See UnicastRemote (implements UnicastRemote)

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RMI Internals: Remote Reference module

• Responsible for translating between local and remote object references and for creating remote object references.

• A remote object table has a mapping between local and remote references. A table at server (entry for object ref for B) and a table at client (entry for object ref for proxy B).

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RMI Internals: Remote References

• Action of remote reference module: See RemoteRef.java interface– When a remote object is to be passed as

argument or result for the first time, the remote ref is asked to create a remote ref object which is added to the table.

– When a remote object reference arrives in a request or reply, the remote ref module is asked for corresponding local object ref, which may either a proxy or remote object. If it is not in the table RMI runtime creates it and asks remote ref module to add it to the table.

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RMI Internals: RMI software

• Layer of software between application level objects and communication and remote reference modules: “Middleware”

• Proxy: provides remote access transparency. One proxy for every remote object in the client.

• Dispatcher: A server has one dispatcher and skeleton for each class representing a remote object. – It receives request message from comm. Module– It used MessageId to select appropriate method in skeleton.– Proxy and dispatcher use same MessageId.

• Skeleton: A class of remote object has a skeleton that implements of the remote interface. All the access dependencies are hidden in this class. A remote object has a servant that directly implements the methods. Java 5 creates this dynamically.

• Proxies, dispatcher and skeleton are automatically generated by interface compiler.

• Binder: binds textual names to remote object references. RMiRegistry is a binder; Naming class; see fig.5.13

• Server Threads: one thread per invocation• Distributed garbage collection: See Andrew Birell’s paper [1995].

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RMI Internals: Distributed Garbage Collection

• Based on reference counts.• Local garbage collectors and a distributed support.• Each server holds the list of processes that hold remote object

references: for example, B.Holders• When a client C first receives a remote reference to a particular

remote object, say B, it makes a addRef(B) invocation to server of that remote object and then creates proxy; server adds C to B.Holders.

• When client C’s garbage collector finds that proxy is no longer reachable (ref count), it makes a removeRef(B) invocation to server and then deletes proxy; the server removes C from B.Holders.

• When B.Holders is empty, server’s local garbage collector will reclaim the space occupied B unless there are any local holders.

• These extra calls for updates occur during proxy creation and deletion and do not affect normal opertion.

• Tolerates communication failures: addRef() and removeRef() are idempotent: effects of N > 0 identical requests is the same as for a single request.

• If addRef() fails with an exception, proxy is not created, removeRef() is transmitted; removeRef() failures are dealt with by “leases” (Jini kind).

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RMI Internals: Use of Reflection

• What is reflection? (See Reflection package)• Reflection enables Java code to discover

information about the fields, methods and constructors of loaded classes, and

• To use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.

http://java.sun.com/docs/books/tutorial/reflect/class/index.html• Reflection feature allowed for dynamic creation of skeleton

and proxy in Java 2 version onwards.• Skeleton has been deprecated since JDk1.4.x• Read more about reflection model of computing.

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A Little bit of Reflection

• Method class, invoke method• Invoke method requires two parameters:

first the object to receive invocation, second an array of Object parameters.

• Invoke executes the method on the object and returns result as Object.

• Method m;• Object result = m.invoke(String, Args);

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Using Reflection in RMI

• Proxy has to marshal info. about a method and its arguments into a request message.

• For a method it marshals an object of class Method into the request. It then adds an array of objects for the method’s arguments.

• The dispatcher unmarshals the Method object and its arguments from request message.

• The remote object reference is obtained from remote ref. table.

• The dispatcher then calls the “invoke” method on the object reference and array of arguments values.

• After the method execution the dispatcher marshals the result or any exceptions into the reply message.

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Putting it all together

Server side: Write a an interface and implement it.

Implements Remote, Inside code publishes the object by

(exporting to runtime) and by registering it.Client Side: Code: look up server object name from the

registry {host, port}; Invoke operations.

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Lifecycle of a remote call

1. First time, an operation is invoked, remote object reference is obtained from remote registry, addRef() is sent to remote server, an entry made in the local ref table and proxy is created.

2. Proxy has message ids while the client’s ref table has remote object reference.

3. Remote ref, method id and arguments are marshaled into a message and sent across via the communication module.

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Lifecycle of a remote call (contd.)

• On the server side RMI runtime maps the remote reference to a local object.

• Unmarshalls the operation and parameters and uses reflection to “invoke” the method on the object reference.

• The result is marshaled back into the response and sent back to the caller.

• “Skeleton” that includes the “dispatch” is subsumed into the RMI runtime in the latest versions of Java.

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Critique of RMI (Sun Java’s) /RPC (Microsoft’s)

• Performs very well for single-platform limited distributed system.

• Platform dependent• Tightly coupled• Inherently synchronous (No chance for

eventing or notification)• Object-oriented: Objects not deployable units• Non-standard• Not scalable, location dependent, no global

registry/discovery• Object reference passed as parameter and/or

returned as result.

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Interface Semantics

Process1 Process2

getCustomer()

retrieveCustomerData()

returnResult()

Semantics of the activity is explicitly stated in the message/method call

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Payload Semantics

Process 1 Process 2

EnvelopWith

message

Requested transaction/activity is embedded in the messageDetails of the activity not explicit; the semantics are embedded in the message

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Payload SemanticsQueueClient Server

createMessage()

sendMessage()

receiveMessage()

receiveMessage()

onMessage()

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Payload semantics is generic

String transferMoney (amt: decimal, accTo: String)

{ …}

String executeService (message: String)

{ …}

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XML

• XML is a markup language, developed by W3C (World Wide Web Consortium), mainly to overcome the limitations of HTML.

• But it took a life of its own and has become a very popular part of distributed systems.

• We will examine its definition, associated specifications (DTD, XSLT etc.), Java APIs available to process XML, protocols and services based on XML, and the role XML plays in a distributed computing environment.

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Memo.html vs memo.xml<!DOCTYPE html PUBLIC "-//W3C//DTD

HTML 4.01 Transitional//EN"><html><head> <meta http-equiv="content-type" content="text/html; charset=ISO-8859-

1"> <title>memo.html</title></head><body><h3>Hello World</h3>Bina<br>CSE4/586 Students <br>Good Morning everyone<br>

BR<br><br></body></html>

<?xml version="1.0" ?>   <!DOCTYPE memo (View Source for

full doctype...)> - <memo>  <header>Hello World</header>   <from>bina</from>   <to>CSE4/586 Students</to>   <body> Good Morning

everyone</body>   <sign>br</sign>   </memo>

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XML to SOAP

• Simple xml can facilitate sending message to receive information.

• The message could be operations to be performed on objects.

• Simple Object Access Protocol (SOAP)

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Web Services

• Web Services is a technology that allows for applications to communicate with each other in a standard format.

• A Web Service exposes an interface that can be accessed through messaging.

• Deployable unit. • A Web service uses protocol to describe an operation

and the data exchange with another web service. Ex: SOAP

• Platform independent, say, through WSDL.• Publishable, discoverable, searchable, queryable• Scalability issues: A group of web services collaborating

accomplish the tasks of a large-scale application. The architecture of such an application is called Service-Oriented Architecture (SOA).

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Web Services and SOA

• Web Services is a technology that allows for applications to communicate with each other in a standard format.

• A Web Service exposes an interface that can be accessed through XML messaging.

• A Web service uses XML based protocol to describe an operation or the data exchange with another web service. Ex: SOAP

• A group of web services collaborating accomplish the tasks of an application. The architecture of such an application is called Service-Oriented Architecture (SOA).

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A Little bit of History: XML to SOAP

• Simple xml can facilitate sending message to receive information.

• The message could be operations to be performed on objects.

• Standardize the tags for object access.

• Simple Object Access Protocol (SOAP).

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SOAP Request (Not WS request)

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"> <soap:Body> <getProductDetails xmlns="http://warehouse.example.com/ws"> <productId>827635</productId> </getProductDetails> </soap:Body> </soap:Envelope>

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SOAP Reply<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"> <soap:Body> <getProductDetailsResponse xmlns="http://warehouse.example.com/ws"> <getProductDetailsResult> <productName>Toptimate 3-Piece Set</productName> <productId>827635</productId> <description>3-Piece luggage set. Black Polyester.</description> <price>96.50</price> <inStock>true</inStock> </getProductDetailsResult> </getProductDetailsResponse> </soap:Body> </soap:Envelope>

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SOAPWeb Services (WS)

Take a look at

1. http://www.w3.org/DesignIssues/WebServices.html2. O’Reilly book on Web Services: Kim Topley’s

Webservices in a Nutshell: http://www.oreilly.com/catalog/javawsian/index.html

This link has a sample chapter (SAAJ) and zip of all examples in the book.

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Web Services (Colouris)• A web service provides a service interface

enabling clients to interact with servers in a more general way than web browsers do.

• Clients access operations in the interface usually by XML messages over http.

• However other architectural models such as REST and CORBA could access WS.

• WSDL provides additional details than for standard operation: for encoding, security, communication and location.

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Web services infrastructure and components

Security

Service descriptions (in WSDL)

ApplicationsDirectory service

Web Services

XML

Choreography

SOAP

URIs (URLs or URNs) HTTP, SMTP or other transport

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SOAP message in an envelope

envelope

header

body

header element

body element

header element

body element

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Example of a simple request without

headers

m:exchange

env:envelope xmlns:env =namespace URI for SOAP envelopes

m:arg1

env:body

xmlns:m = namespace URI of the service description

Hellom:arg2

World

In this figure and the next, each XML element is represented by a shaded box with its name in italic followed by any attributes and its content

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Example of a reply corresponding to

the request in

env:envelope xmlns:env = namespace URI for SOAP envelope

m:res1

env:body

xmlns:m = namespace URI for the service description

m:res2World

m:exchangeResponse

Hello

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Use of HTTP POST Request in SOAP client-server communication

endpoint address

action

POST /examples/stringerHost: www.cdk4.net

Content-Type: application/soap+xmlAction: http://www.cdk4.net/examples/stringer#exchange

<env:envelope xmlns:env= namespace URI for SOAP envelope><env:header> </env:header><env:body> </env:body></env:Envelope>

So

ap

me

ssa

ge

HT

TP

h

ead

er

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Services, ports and bindings

• Service endpoint interface (SEI) or service endpoint that defines one or more operations that the web service offers.

• Access to an endpoint is provided by binding it to a protocol stack through a port.– A port has an address that the client can use to

communicate with the service and invoke its operations.

• An endpoint can be bound to different ports each offering a different suite of protocols for interaction.

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Endpoint, Port and binding

SOAP1.1Over http

SOAP 1.1 overhttps

Other. Ex:ebXML overSMTP

Port1 port2 port3

endpoint

Web services Client

Web service

https 1.1 transportsoap1.1 messages

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WS Interoperability Infrastructure

Network

XML Messaging

Service DescriptionWSDL

SOAP

HTTP