OOTI course Distributed Systemswsinmak/Education/OOTI/L4_Webservices.pdf• UDDI for Web services...

OOTI course

Distributed Systems

2013, Sep. 26 – Oct 01

Lecture 4: Web Services

Agenda

• Introduction

• Motivation

• Definition

• Architectural styles

• Naming: URIs

• Web services technologies

• Orchestration

• Replication

R.H. Mak PAGE 127-9-2013

Motivation

The desire for an open distributed system

• Interoperability and portability

• on a global scale, i.e., across the Internet

• on arbitrary hardware and software platforms

• at multiple quality levels

• Extendibility

• with new resources and services

• with contributions from various vendors/ developers

R.H. Mak PAGE 227-9-2013

Basic organization of the Web

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Many variations on this scheme, e.g.,

• Client-side caching• Clusters of servers• Multiple-tiers

No indication how the

client discovers theservice!

Layers and client / server functionality

• 3-layer organization of data access: cuts possible at many places

• cuts define tiers

• (a,b) thin client – changing control

− the user interface has small overhead; it typically runs on a machine that is a client to the application server

− however, after connection, the user interface becomes a server for the application

• (c,d) thick client

• (e) file server

PAGE 427-9-2013J.J. Lukkien

Combining layering and C/S in a distributed fashion

• Development (logical): layering (“presentation, business and data tier (layer)”)

• Deployment: Client-Server interactions between the components• also called: multi-tier client-server organization

• note: middle layer plays both client and server roles− that is actually what layering + request/reply is


Web service

“A Web service is nothing but a “traditional service” that is made available over the Internet.” (quote from TvS)

• Adheres to a collection of standards that allow it to be discoveredand accessed over the Internet by client applications that adhere to those standards

• Can be composed of more elementary services. The cooperation of elementary services that constitute a complex service need to be coordinated

Service oriented architectures organize the application’s services plus services needed for discovery, coordination (and many more) into a distributed system.

R.H. Mak PAGE 627-9-2013

Service

• a unit of functionality

• coarse-grained, e.g., stock market ticker, not a mathematical function like the sinus.

• accessed via a well-defined interface

• delivered at a well-defined quality level

• Agreed upon by server provider and consumer in a service level agreement (SLA)

A service is a logical concept. A service is realized by a component. Components that provide the same service can (will) differ in their non-functional properties depending on their vendor/developer.

R.H. Mak PAGE 727-9-2013

• Motivation:• Separate functionality (“service”), the

implementation, the deployment context and the application context

• Build applications by very late (dynamic) binding• Integration of enterprise information systems

• Vocabulary• SOA, service, interface, discovery, composition,

binding, orchestration, choreography

• Rules• Application is built ad-hoc out

of services that communicate in a standardized manner− via a network− see e.g. REST

• Service is a self-containedfunctionality. It does not depend on state of otherservices, or of the system (OS, language) it is running on.

• Services are discoverable

• Structure (conceptual):

• Typical behavior• Providers publish services; Applications

(“orchestrations”) discover services, and bind their interfaces− Service broker (registry) can exist to manage

discovery process

• Applications send data objects through a number of services as a work flow − Often XML based RPC (SOAP)− Services are typically kept simple and focused on

single task

Service oriented (SOA) style

Note: SOA is a huge research domain in business information systems


Enterprise application organization


RESTful Web service

A RESTful Web service is a Web service

• that is resource-oriented, i.e., organized as a set of resources

• Documents of various types are served to clients (similar to WWW)

• Not only static documents, but also dynamically generated upon request

• Resources may have multiple representations (e.g. in English and Chinese)

• Format for representations is free, although XML is popular

• in which all operations on resources are cast in the form of a CRUD-operation (create, read, update, delete)

• HTTP-messages (GET,PUT,POST,DELETE) are used to specify those

operations;

• URLs both specify the (location of) the resource and parameters for the

operation; often a HTTP message consist of a header only, thus creating a

light-weight protocol with a uniform interface

− e.g. GET http://www.example.com/MyBookmarks?topic=webservices to obtain a list of bookmarks, instead of a RPC getMyBookmarks(webservices)

R.H. Mak PAGE 1027-9-2013

RESTful Web service

• that is stateless;

• servers do not maintain application state, i.e., information from previous interactions with a client, hence the client must supply this information in

successive operations

• of course, servers maintain resource state, in the form of representations

• that supports client-side caching

• Through HTTP cache control mechanisms

• that support code on demand

• Servers may send code necessary to execute other representations they

have sent

• Usually JAVAscript

R.H. Mak PAGE 1127-9-2013

REST: REpresentational State Transfer

• Motivation:• special usage of C&S aiming at

− portability− independent development & deployment− reduction of interaction complexity− reliability

− scalability

• Vocabulary• user agent, origin server, gateway, proxy• cache, layer• state• code-on-demand• resource, resource identifier (e.g. URL),

representation, metadata

• Rules• stateless communication (no server state)• response is labeled as cache-able, and can then

be cached• uniform interface between components

− decouples structure from functionality

• layering• client functionality can be adapted by code-on-

demand

• Structure

(model in the process view from R. Fielding, Architectural Styles and the

Design of Network-based Software Architectures)

• Typical behavior

• as (layered) C&S, but only a single service (interface)

• Prime example: WWW

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Recap: naming on the Web

R.H. Mak PAGE 1327-9-2013

• Often-used structures for URLs.

a) Using only a DNS name.

b) Combining a DNS name with a port number.

c) combining an IP address with a port number.

• By means of Uniform Resource Identifiers (URIs)• either a URL(ocator) or a URN(ame);on the Web mostly URLs

Recap question:

How are these Names resolved?

Agenda

• Introduction


• SOAP

• WSDL,

• WADL

• UPnP

• Orchestration

• Replication

R.H. Mak PAGE 1427-9-2013

Messages use carrier protocols

R.H. Mak PAGE 1527-9-2013

EXAMPLE ?

Messages use carrier protocols

R.H. Mak PAGE 1627-9-2013

On the Web, HTTP serves as the bottle.

SOAP message in an envelope

envelope

header

body

header element

body element

header element

body element

PAGE 1727-9-2013R.H. Mak

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012

Example: 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

PAGE 1827-9-2013R.H. Mak


Example: reply corresponding to the request

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

PAGE 1927-9-2013R.H. Mak


Example: Soap request in xml format

• POST /InStock HTTP/1.1Host: www.example.orgContent-Type: application/soap+xml; charset=utf-8Content-Length: nnn

<?xml version="1.0"?><soap:Envelope

xmlns:soap="http://www.w3.org/2001/12/soap-envelope"soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

<soap:Body xmlns:m="http://www.example.org/stock"><m:GetStockPrice>

<m:StockName>IBM</m:StockName></m:GetStockPrice>

</soap:Body>

</soap:Envelope>

R.H. Mak PAGE 2027-9-2013

©Taken from http://www.w3schools.com/soap/soap_example.asp

SOAP

=

HTTP

+

XML

Notice:No header

Example: SOAP response

• HTTP/1.1 200 OKContent-Type: application/soap+xml; charset=utf-8Content-Length: nnn

<?xml version="1.0"?><soap:Envelope

xmlns:soap="http://www.w3.org/2001/12/soap-envelope"soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

<soap:Body xmlns:m="http://www.example.org/stock"><m:GetStockPriceResponse>

<m:Price>34.5</m:Price></m:GetStockPriceResponse>

</soap:Body>

</soap:Envelope>

R.H. Mak PAGE 2127-9-2013

Example: SOAP response

HTTP/1.1 200 OKContent-Type: application/soap+xml; charset=utf-8Content-Length: nnn

<?xml version="1.0"?><soap:Envelopexmlns:soap="http://www.w3.org/2001/12/soap-envelope"soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

<soap:Body xmlns:m="http://www.example.org/stock"><m:GetStockPriceResponse><m:Price>34.5</m:Price>

</m:GetStockPriceResponse></soap:Body>

</soap:Envelope>

R.H. Mak PAGE 2227-9-2013

©Taken from http://www.w3schools.com/soap/soap_example.asp

Web service description

• Defines the service

• the interface (functionality)

• the location and the communication protocols used

− URI, HTTP, SOAP,,XML, …

• machine readable format

− WSDL

• Is stored in a repository and made available through a directory service

• UDDI for Web services

• Facilitates the development of clients and servers

• Through automatic generation of stubs stubs for RPC

R.H. Mak PAGE 2327-9-2013

Web-service model

R.H. Mak PAGE 2427-9-2013

WSDL: document structure

R.H. Mak PAGE 2527-9-2013

<definitions>

<types>definition of types....

</types>

<message>definition of a message....

</message>

<portType><operation>

definition of a operation.... </operation>

</portType>

…..

The service interfacefor each operation its messages and their types (in, out) are listed

The target namespace:

complex data structures specified using XML

Messages associated with operationsContaining parameter listor result list

The abstract part of the service description

WSDL: document structure (cntd)

R.H. Mak PAGE 2627-9-2013

…..<binding>

definition of a binding.... </binding>

<service>definition of a service....

</service>

</definitions>

The concrete part of the service description

How the service is exposedEndpoint

Binding

The protocol usedMessage format

Data representation

WSDL example: message and portType element

<message name="getTermRequest"><part name="term" type="xs:string"/>

</message>

<message name="getTermResponse"><part name="value" type="xs:string"/>

</message>

<portType name="glossaryTerms"><operation name="getTerm"><input message="getTermRequest"/><output message="getTermResponse"/>

</operation></portType>

R.H. Mak PAGE 2727-9-2013

WSDL example: binding element

<binding name="Hello_Binding" type="tns:Hello_PortType">

<soap:binding style="rpc“

transport="http://schemas.xmlsoap.org/soap/http"/>

<operation name="sayHello">

<soap:operation soapAction="sayHello"/>

<input>

<soap:body

encodingStyle=http://schemas.xmlsoap.org/soap/encoding/

namespace="urn:examples:helloservice" use="encoded“/>

</input>

<output>

<soap:body … />

</output>

</operation>

</binding>

R.H. Mak PAGE 2827-9-2013

WSDL example: service element

<service name="Hello_Service">

<documentation>WSDL File for HelloService </documentation>

<port binding="tns:Hello_Binding" name="Hello_Port">

<soap:address

location="http://www.examples.com/SayHello/">

</port>

</service>

R.H. Mak PAGE 2927-9-2013

WSDL message exchange patterns

R.H. Mak PAGE 3027-9-2013

Name

In-Out

In-Only

Robust In-Only

Out-In

Out-Only

Robust Out-Only

Client Server Delivery Fault message

Request Reply may replace Reply

Request no fault message

Request guaranteed may be sent

Reply Request may replace Reply

Request no fault message

Request guaranteed may send fault

Messages sent by


WADL

Web Application Description language

• Suitable to describe ReSTful Web services

• Definition and examples on following slides taken from

• http://www.w3.org/Submission/wadl/

• Not a formal W3C standard

• This document does not mention ReST, but refers to”

“HTTP-based applications that provide programmatic access to their internal data”

• A similar description language is the Atom Publishing Protocol

• more dedicated, however, to a specific application domain

R.H. Mak PAGE 3127-9-2013

WADL: document structure

<application>

<grammars>

definitions of data formats … [optional]

</grammars>

<resources base=“…”>

specifies the base URI

defines a list of resources

<resource>

definition of an individual resource

</resource>

</resources>

</application>

R.H. Mak PAGE 3227-9-2013

WADL example: resources element

<resources base="http://example.com/">

<resource path="widgets">

<resource path="reports/stock">

<param name="instockonly" style="matrix" 5 type="xsd:boolean"/>

...

</resource>

<resource path="{widgetId}">

</resource>

</resource>

<resource path="accounts/{accountId}">

...

</resource>

</resources>

R.H. Mak PAGE 3327-9-2013

WADL example: resources element

The <resources> element on the previous slide describes

• A resource identified by a static URI:

• http://example.com/widgets

• A resource identified by a static URI:

• http://example.com/widgets/reports/stock

• A resource identified by a matrix URI:

• http://example.com/widgets/reports/stock;instockonly

• Multiple resources identified by generative URIs:

• http://example.com/widgets/widgetId,

• where widgetId is replaced at runtime with the value of a runtime parameter called widgetId.

• Multiple resources identified by generative URIs:

• http://example.com/accounts/accountId, where …

R.H. Mak PAGE 3427-9-2013

<resources base="http://example.com/widgets">

<resource path="{widgetId}">

<param name="customerId" style="query"/>

<method name="GET">

<request>

<param name="verbose" style="query" type="xsd:boolean"/>

</request>

<response>

...

</response>

</method>

</resource>

</resources>

RESTful query using HTTP GET

R.H. Mak PAGE 3527-9-2013

http://example.com/widgets/123456?customerId=cust1234&verbose=true

correspondence

parameter value

widgetId 123456

customerId cust134

Verbose true

global for all methods

of this resource

local to the GET method

Example: query to determine the response mediaType

<method name="GET">

<request>

<param name="format" style="query">

<option value="xml" mediaType="application/xml"/>

<option value="json" mediaType="application/json"/>

</param>

...

</request>

<response>

<representation mediaType="application/xml"/>

<representation mediaType="application/json"/>

</response>

</method>

R.H. Mak PAGE 3627-9-2013

Agenda

• Introduction


• Orchestration

• Replication

R.H. Mak PAGE 3727-9-2013

Orchestration and choreography

Orchestration

• Pertains to lifecycle management of a (service-oriented) application

• Requires active party (the orchestrator) to control that lifecycle

− Instantiation, binding, activation, monitoring, (re)-configuration, …

− Implement policies

Choreography

• Pattern of service interaction resulting from a set of rules (or the set of rules itself

• Occurs spontaneously, between services themselves

− only the rules are provided

R.H. Mak PAGE 3827-9-2013

For a discussion see: http://www.infoq.com/news/2008/09/Orchestration

Example: FLUENT framework

Orchestrator

1

Orchestrator

2

Resource

manager

Component

Repository

Device

managerDevice

manager

Component

1

Component

3

Component

4

Component

2

Host 1

Host 2

Component

5

Device

manager

Host 3

R.H. Mak PAGE 3927-9-2013

Example taken from: R.H. Mak, Resource-aware Life Cycle Models for Service-oriented Application managed by a component Framework

Exampe: FLUENT Life Cycle

R.H. Mak PAGE 4027-9-2013

[deploy]

allocate hosts

assign modes

reserve resources

[success]

[non-feasible]

[redesign]

failure

instantiate

components

bind

interfaces

operateredesign

[redeploy]

[stop]

(1) (2)

(4)

install

components

[all installed]

[otherwise]

(2)

(3)

(Re-)Design (Re-)Deployment Operation

compose

application

[new design]

allocate

component

types

[partial]

[total]

define

location

constraints

define

resource

constraints

[stop]

[recompose]

[reallocate]

failure deploy

[finish]

[redesign]

define

reconfiguration

strategies

(1)

(2)

(3)

[recompose]

activate

components

execute

recover

error

deactivate

affected

components

Adapt

QoS

deallocate

affected

components

[operate]

[control cmd]

[all activated]

[otherwise] [RM 3 applies]

[non fatal]

[otherwise]

[error]

[other error] [resource conflict]

[fatal]

redesign redeployfailure

determine

Recovery

Mechanism

renegotiate

resource

reservations

deactivate

affected

components

[otherwise]

[RM 1 applies]

[RM 1or 2 applies]

(1)

Example: Service –oriented virtual community

R.H. Mak PAGE 4127-9-2013

Example: Service –oriented virtual community

R.H. Mak PAGE 4227-9-2013

Example taken from: Shudong Chen, J.J. Lukkien, A service-oriented virtual community overlay network for secure external service orchestration

Agenda

• Introduction


• Orchestration

• Replication

R.H. Mak PAGE 4327-9-2013

Server clusters

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Server clusters (TCP-hand-offf)

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• Web server uses IP-spoofing

• Selection of webserver by FE based on server load, not on

content of the request

Server clusters

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Client-side caching / proxy servers

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Origin server

Squirrel webcache

• Local caches in web browsers of clients in a single LAN are used to implement a distributed LAN-based web-server proxy.

• Can handle corporate networks from 100-100,000 nodes or more.

• Alternative for (a cluster of) web-proxy servers

• Uses a peer-to-peer architecture (overlay) on the LAN

• Built on top of Pastry, but any P2P routing overlay will do.

• Inserts a squirrel proxy between the browser and the local web cache.

• Uses HTTP conditional GET

• Scales horizontally and automatically

• As the number of browsers increases so does the cache capacity, whereas cache response time stays almost constant.

• No administrative effort required.PAGE 4827-9-2013R.H. Mak

Home-store scheme

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Sitaram lyer, Antony Rowstron, and Peter Druschel, Squirrel: A decentralized peer-to-peer web cache

Directory scheme

PAGE 5027-9-2013R.H. Mak

OOTI course Distributed Systemswsinmak/Education/OOTI/L4_Webservices.pdf• UDDI for Web services...

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