©Ian Sommerville 2006MSc module: Advanced Software Engineering Slide 1 Service-centric Software...

©Ian Sommerville 2006 MSc module: Advanced Software Engineering Slide 1

Service-centric Software Engineering


Objectives

To introduce the idea of service-oriented architectures To explain the notion of a reusable service, based on web

service standards, that provides a mechanism for inter-organisational computing;

To describe the service engineering process that is intended to produce reusable web services;

To introduce service composition as a means of application development;

To show how business process models may be used as a basis for the design of service-oriented systems.


Topics covered

Service-oriented architectures Services as reusable components Service engineering Software development with services


Service-oriented architectures

Based around the notion of externally provided services (web services).

A web service is a standard approach to making a reusable component available and accessible across the web• A tax filing service could provide support for users to fill

in their tax forms and submit these to the tax authorities.

Services may execute on different computers from different service providers.


A generic service

An act or performance offered by one party to another. Although the process may be tied to a physical product, the performance is essentially intangible and does not normally result in ownership of any of the factors of production.

Service provision is therefore independent of the application using the service.


Web services

Serviceregistry

Servicerequestor

Serviceprovider

Publish

Bind

Find

service


Service roles

Service registry• Provides details of the range of services that are

available (possibly from different providers) Service provider

• Hosts a service that can be accessed and used by external clients

Service requestor• Seeks a service to be incorporated into an application

or a business process


Services and distributed objects

Provider independence. Public advertising of service availability. Potentially, run-time service binding. Opportunistic construction of new services through

composition. Pay for use of services. Smaller, more compact applications. Reactive and adaptive applications.


Services scenario

An in-car information system provides drivers with information on weather, road traffic conditions, local information etc. This is linked to car radio so that information is delivered as a signal on a specific radio channel.

The car is equipped with GPS receiver to discover its position and, based on that position, the system accesses a range of information services. Information may be delivered in the driver’s specified language.


Automotive system

User interface

Locator

Discovers carposition

Weatherinfo

Receives requestfrom user

Receiver

Receivesinformation stream

from services

Transmitter

Sends position andinformation request

to services

Radio

Translates digitalinfo stream toradio signal

In-car software system

Mobile Info Service

Facilitiesinfo

Translator

Roadlocator

Trafficinfo

Collates information

Road traffic info

commandgps coord

gpscoord gps coordgps coord

Languageinfo

Infostream

Service discovery

Finds availableservices


Hybrid system

The in-car system is an example of a hybrid system where local computation does not use a web service based approach (Too much computational and communication overhead)

However, features that rely on external information or which require more processing than can be provided on-board (e,g, translation) are accessed as services.

Mobility means binding to different services depending on location.


Benefits of SOA

Services can be provided locally or outsourced to external providers

Services are language-independent Investment in legacy systems can be preserved Applications can be adaptive, binding to different

services as availability and capability changes Inter-organisational computing is facilitated through

simplified information exchange


Web service standards

Transport (HTTP, HTTPS, SMTP, ...)

Messaging (SOAP)

Service definition (UDDI, WSDL)

Process (WS-BPEL)

Support (WS-Security , WS-Addressing, ...)

XML technologies (XML, XSD, XSL T, ....)


Key standards

SOAP• A message exchange standard that supports service

communication WSDL (Web Service Definition Language)

• This standard allows a service interface and its bindings to be defined

UDDI• Defines the components of a service specification that may

be used to discover the existence of a service WS-BPEL

• A standard for workflow languages used to define service composition


Other standards

A standards body exists (OASIS) to ratify service standards but it is not completely effective

There are a huge number of proposed standards to support other aspects of SOA (e.g. security, dependability, QoS, etc.) but few of these standards have been agreed or accepted by the major vendors.


Service-oriented software engineering

Existing approaches to software engineering have to evolve to reflect the service-oriented approach to software development• Service engineering. The development of dependable,

reusable services• Software development for reuse

• Software development with services. The development of dependable software where services are the fundamental components• Software development with reuse


Services as reusable components

A service can be defined as:• A loosely-coupled, reusable software component that encapsulates

discrete functionality which may be distributed and programmatically accessed. A web service is a service that is accessed using standard Internet and XML-based protocols

A critical distinction between a service and a component as defined in CBSE is that services are independent• Services rely on message-based communication with messages

expressed in XML• Services are stateless ie they do not maintain information from one

invocation to another


RPC vs message based interaction

Component-based system generally use a style of interaction which is based on a remote procedure or method call i.e. the remote component is called as if it was local.

Middleware is used to manage this interaction. Services use a different approach based on

message passing where all information about the service required is bundled into a single message


Synchronous interaction

Tomato soup please

Waiter Diner

What would you like?

And to follow?

Fillet steak

How would you like it cooked?

Rare please

With salad or french fries?

Salad please

ETC.


An order as an XML message

<starter> <dish name = “soup” type = “tomato” /> <dish name = “soup” type = “fish” /> <dish name = “pigeon salad” /> </starter> <main course> <dish name = “steak” type = “sirloin” cooking = “medium” /> <dish name = “steak” type = “fillet” cooking = “rare” /> <dish name = “sea bass”> </main> <accompaniment> <dish name = “french fries” portions = “2” /> <dish name = “salad” portions = “1” /> </accompaniment>


Web service description language

The service interface is defined in a service description expressed in WSDL. This has been agreed as a standard and is generally adopted.

The WSDL specification defines• What operations the service supports and the format of

the messages that are sent and received by the service

• How the service is accessed - that is, the binding maps the abstract interface ontoa concrete set of protocols

• Where the service is located. This is usually expressed as a URI (Universal Resource Identifier)


Structure of a WSDL specification

Intro

Abstract interface

Concreteimplementation

WSDL service definition

XML namespace declarations

Type declarationsInterface declarationsMessage declarations

Binding declarationsEndpoint declarations


A WSDL description fragment

Define some of the types used. Assume that the namespace prefixes ‘ws’ refers to the namespace URI for XML schemas and the namespace prefix associated with this definition is weathns. <types> <xs: schema targetNameSpace = “http://.../weathns” xmlns: weathns = “http://…/weathns” > <xs:element name = “PlaceAndDate” type = “pdrec” /> <xs:element name = “MaxMinTemp” type = “mmtrec” /> <xs: element name = “InDataFault” type = “errmess” /> <xs: complexType name = “pdrec” <xs: sequence> <xs:element name = “town” type = “xs:string”/> <xs:element name = “country” type = “xs:string”/> <xs:element name = “day” type = “xs:date” /> </xs:complexType> Definitions of MaxMinType and InDataFault here </schema> </types>


A WSDL description fragment 2


Service engineering

The process of developing services for reuse in service-oriented applications

The service has to be designed as a reusable abstraction that can be used in different systems

Involves• Service candidate identification

• Service design

• Service implementation


The service engineering process

Service designService

candidateidentification

Serviceimplementationand deployment

Servicerequirements

Service interfacespecification

Validated anddeployed service


Service candidate identification

Three fundamental types of service• Utility services that implement general functionality

used by different business processes

• Business services that are associated with a specific business function e.g., in a university, student registration

• Coordination services that support composite processes such as ordering


Service classification

Utility Business Co ordination Task Currency

convertor Employee locator

Validate claim form Check credit rating

Process expense claim Pay external supplier

Entity Document style checker Web form to XML converter

Expenses form Student application form


Service identification

Is the service associated with a single logical entity used in different business processes?

Is the task one that is carried out by different people in the organisation?

Is the service independent? Does the service have to maintain state? Is a database

required? Could the service be used by clients outside the

organisation? Are different users of the service likely to have different non-

functional requirements?


Catalogue services

Created by a supplier to show which goods can be ordered from them by other companies

Service requirements• Specific version of catalogue should be created for each client• Catalogue shall be downloadable• The specification and prices of up to 6 items may be compared• Browsing and searching facilities shall be provided• A function shall be provided that allows the delivery date for

ordered items to be predicted• Virtual orders shall be supported which reserve the goods for 48

hours to allow a company order to be placed


Catalogue - Non-functional requirements

Access shall be restricted to employees of accredited organisations

Prices and configurations offered to each organisation shall be confidential• Because different prices may be agreed for different

customers The catalogue shall be available from 0700 to 1100 The catalogue shall be able to process up to 10

requests per second


Catalogue service operations

Operation Description

MakeCatalogue Creates a version of the cat alogue tailored for a specific customer. Inc ludes an o ptional parameter to create a downloadable PDF version of the catalogue.

Compare Provides a comparison of up to 6 characteristics (e.g. price, dimensions, p rocessor speed, etc .) of up to 4 catalogue it ems for comparison.

Lookup Displays all of the data assoc iated with a specified catalogue item.

Search This operation takes a logical expression and searche s the catalogue according to that expression. It displays a list of all items that match the search expression.

CheckDelivery Returns the predicted delivery date for an item if it is ordered today.

MakeVirtualOrder Reserves the number of items to be ordered by a customer and provides item information for the customer’s own procurement system.


Service interface design

Involves thinking about the operations associated with the service and the messages exchanged

The number of messages exchanged to complete a service request should normally be minimised to reduce the communication overhead.

Service state information may have to be included in messages as services should not maintain state.


Interface design stages

Logical interface design• Starts with the service requirements and defines the

operation names and parameters associated with the service. Exceptions should also be defined

Message design• Design the structure and organisation of the input and

output messages. Notations such as the UML are a more abstract representation than XML

WSDL description• The logical specification is converted to a WSDL

description


Catalogue interface design

Operation Inputs Outputs Exceptions

MakeCatalogue mcIn Company id PDF-flag

mcOut URL of the catalogue for that company

mcFault Invalid company id

Compare compIn Company id Entry attribute (up to 6) Catalogue number (up to 4)

compOut URL of page showing comparison table

compFault Invalid company id Invalid catalogue number Unknown attr ibute

Lookup lookIn Company id Catalogue number

lookOut URL of page with the item information

lookFault Invalid company id Invalid catalogue number

Search searchIn Company id Search string

searchOut URL of web page with search results

searchFault Invalid company id Badly-formed search string

CheckDelivery gdIn Company id Catalogue number Number of items required

gdOut Catalogue number Expected delivery date

gdFault Invalid company id Invalid catalogue number No availability Zero items requested

PlaceOrder poIn Company id Number of items required Catalogue number

poOut Catalogue number Number of items required Predicted delivery date Unit price estimate Total price estimate

poFault Invalid company id Invalid catalogue number Zero items requested


Message structure definition

This has to be (ultimately) expressed in XML but I think it is easier to think about and express this graphically (using a notation such as the UML) and then translate this to XML.

Rules about the information in the message may be added using UML comments and OCL (the object constraint language)


Input and output message structure

gdIn

cID: stringcatNum: stringnumItems: integer

size (cID) = 6size (catNum) = 10numItems > 0

gdOut

catNum: stringdelivDate: date

size (catNum) = 10delivDate > Today

gdFault

errCode: integer

Invalid company iderrCode=1

Invalid catalogue numbererrCode = 2

No availabilityerrCode = 3

Zero items requestederrCode = 4


Service implementation and deployment

Services are programmed using a standard programming language or a workflow language

Services then have to be tested by creating input messages and checking that the output messages produced are as expected

Deployment involves publicising the service using UDDI or in some other way and installing it on a web server.

Current servers provide support for service installation


A UDDI description

Details of the business providing the service An informal description of the functionality provided by the

service Information where to find the service’s WSDL specification Subscription information that allows users to register for

service updates UDDI is very limited in the information that it can maintain

about a service. Service functionality is generally expressed in one or two sentences of natural language.


Legacy system services

An important application of services is to provide access to functionality embedded in legacy systems

Legacy systems offer extensive functionality and this can reduce the cost of service implementation

External applications can access this functionality through the service interfaces


Maintenance support system

A large company maintains an inventory of equipment and an associated maintenance database. This keeps track of maintenance requests for equipment, when maintenance was carried out, etc. and generates daily job lists, costs of maintainance, etc.

New features are to add real-time access to the system from portable terminals

This cannot be done by simply enhancing the existing system


Legacy system access

Maintenance supportlegacy application

«service»Maintenance

getJobsuspendJobcompleteJob

«service»Facilities

addEquipmentdeleteEquipmenteditEquipment

«service»Logging

addRequestdeleteRequestqueryRequests


Maintenance services

Maintenance service• Manages maintainance jobs

Facilities service• Manages the addition and deletion of equipment from

the database Logging service

• Manages maintenance requests New system is built on top of these services


Software development with services

Existing services are composed and configured to create new composite services and applications

The basis for service composition is often a workflow• Workflows are logical sequences of activities that,

together, model a coherent business process

• For example, provide a travel reservation services which allows flights, car hire and hotel bookings to be coordinated


Vacation package workflow

Bookflights

Bookhotel

Arrangecar or taxi

Browseattractions

Bookattractions

Arrival/departuredates/times Hotel location

Dates/preferences


Construction by composition

Formulateoutline

workflow

Discoverservices

Workflowdesign

Service list Servicespecifications

Workflowdesign

Selectservices

Refineworkflow

Createworkflowprogram

Executableworkflow

Testservice

Deployableservice


Hotel booking workflow

Hotels.GetRequirements

Customer

Hotels.CheckAvailability

Hotels.NoAvailability

Hotels.ReserveRooms

Hotels.ConfirmReservation

Retry

Cancel

Rooms OK

No rooms


Workflow design and implementation

WS-BPEL is an XML-standard for workflow specification. However, WS-BPEL descriptions are long and unreadable.

Graphical workflow notations, such as BPMN, are more readable and WS-BPEL can be generated from them.

In inter-organisational systems, separate workflows are created for each organisation and linked through message exchange.


Interacting workflows

Requestprocessor

Setup jobparameters

Downloaddata

Startcomputation

Storeresults

Reportcompletion

Restart

Fail

CheckAvailability

Allocateresources

Initialise Compute

Returnresults

OK

No processor

OK


Service testing

Testing is intended to find defects and demonstrate that a system meets its functional and non-functional requirements.

Service testing is difficult as (external) services are ‘black-boxes’. Testing techniques that rely on the program source code cannot be used.


Service testing problems

External services may be modified by the service provider thus invalidating tests which have been completed.

Dynamic binding means that the service used in an application may vary - the application tests are not, therefore, reliable.

The non-functional behaviour of the service is unpredictable because it depends on load.

If services have to be paid for as used, testing a service may be expensive.

It may be difficult to invoke compensating actions in external services as these may rely on the failure of other services which cannot be simulated.


Service modification

A service is under the control of the service provider who may change this as they wish

While you would expect the interface to be maintained, the non-functional characteristics of a service may change thus changing the non-functional characteristics of an application using the service

If the service behaviour changes, how can a service user get to the previous version of the service.


Dynamic binding

Different versions of the ‘same’ service which an application may bind to may:• Have different non-functional characteristics

• Have different bugs Binding to different services may therefore have

different results A ‘successful’ test may not be replicatable as the

service used may disappear


Behaviour under load

If a service is tested at some time A, it may perform satisfactorily because few other users are using it at that time

At some later time, when it is more heavily loaded, its behaviour may not be acceptable

The load behaviour of the service is outside the control of the service client


Service payment

If a service provider expects to be paid for a service then:• There may be a significant cost in testing a service and

that cost may not be justifiable if the tests suggest that the service does not meet the requirements

• If the service provider allows free access for testing, they may limit this so that the load on the service does not degrade the performance for other users


Compensating actions

Compensating actions occur when there is a need to reverse some previous action e.g. cancel a flight booking because a hotel is unavailable

However, the conditions where these actions occur is not under the control of the application - therefore, it may be difficult to simulate failure in other to ensure that the compensating actions are invoked.


Key points

Service-oriented software engineering is based on the notion that programs can be constructed by composing independent services which encapsulate reusable functionality.

Service interfaces are defined in WSDL. A WSDL specification includes a definition of the interface types and operations, the binding protocol used by the service and the service location.

Services may be classified as utility services, business services or coordination services.

The service engineering process involves identifying candidate services for implementation, defining the service interface and implementing, testing and deploying the service.


Key points

Service interfaces may be defined for legacy software systems which may then be reused in other applications.

Software development using services involves creating programs by composing and configuring services to create new composite services.

Business process models define the activities and information exchange in business processes. Activities in the business process may be implemented by services so the business process model represents a service composition.

Techniques of software testing based on source-code analysis cannot be used in service-oriented systems that rely on externally provided services.

©Ian Sommerville 2006MSc module: Advanced Software Engineering Slide 1 Service-centric Software...

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