outline concepts: services, soa, webservices services as reusable components service engineering...

Outline• Concepts: Services, SOA, WebServices

• Services as reusable components

• Service engineering

• Software development with servicesSoftware development with services– Simple case: a client consumes a serviceSimple case: a client consumes a service

• WS APIsWS APIs

– Common case: several services are composed with support of the Common case: several services are composed with support of the service platformservice platform• WorkflowsWorkflows

• BPELBPEL

– Service Oriented Software Engineering particularitiesService Oriented Software Engineering particularities

Review: 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

Software development with services

• Simplest case: – a client uses (“consumes”) a service

• Common case: – several services are composed

How Clients Use Services

• In order to use a service, a Client program needs only its WSDL (contains abstract interface description and URI of service endpoint)

Types

Messages

Operations

Port Type

Bindings

Port

Service

WHAT

HOW

WHERE

Interoperability

.NET<Vendor B>

J2EE<Vendor A>

<Vendor D>

<Vendor C>

WSDL descriptionClient1

Client2

Service1

Service2WSDL

description

How can a client bind to a service ?

• Static binding– Service at fixed URL

• Dynamic binding by reference– Service URL given at runtime

• Dynamic binding by lookup– Look up service URL in registry (need lookup API)

• Dynamic operation selection– Service type/operation name given at runtime

• => API’s for Web Services: Java, .NET

Java APIs for Web Services

• SOAP messages as Java objects– SAAJ ( SOAP with Attachments API for Java)

• Programming Model– JAX-RPC (Java API for XML-based RPC) => JAX-WS (Java

API for XML Web Services)

• Accessing WSDL descriptions– JWSDL

• Accessing Web Services Registries– JAXR (Java API for XML Registries)

JAX-WS (JAX-RPC)

• WSDL/XML to Java Mapping (wsimport)

• Java to WSDL/XML Mapping (wsgen)

• Client API– Classes generated from WSDL

– Dynamic Proxy

– DII call Interface

Web Service Example

<wsdl:message name="addIntResponse"> <wsdl:part name="addIntReturn" type="xsd:int" /> </wsdl:message><wsdl:message name="addIntRequest"> <wsdl:part name="a" type="xsd:int" /> <wsdl:part name="b" type="xsd:int" /> </wsdl:message><wsdl:portType name="AddFunction"> <wsdl:operation name="addInt" parameterOrder="a b"> <wsdl:input message="impl:addIntRequest" name="addIntRequest" /> <wsdl:output message="impl:addIntResponse" name="addIntResponse" /> </wsdl:operation> </wsdl:portType> // possible implementation of WS:

// AddFunction.jwspublic class AddFunction { int addInt(int a, int b){ return(a+b); }}

A Web service AddFunction with operation addInt is known through its WSDL:

Writing the Client Program

• There are many ways to write a Client program that uses the AddFunction Service (invoking its addInt operation)– Using Dynamic Invocation Interface ( DII)

– Using generated Stubs from Service WSDL description

– Using Dynamic Proxy

Client – using DII

• Using Dynamic Invocation Interface ( DII):– Service type (WSDL) can be discovered at runtime (WSDL

description is actually not even needed !)

– Service URL is given at runtime (could be extracted from a WSDL)

– Operation name can also be given at runtime

– Invocation is done by constructing and sending a call message

– Most flexible way; but client code looks “ugly”

Client - using DII - Exampleimport javax.xml.rpc.Call;

import javax.xml.rpc.Service;

import javax.xml.namespace.QName;

String endpoint = "http://localhost:8080/axis/AddFunction.jws";

Service service = new Service();

Call call = (Call) service.createCall();

call.setOperationName(new QName(endpoint, "addInt"));

call.setTargetEndpointAddress( new java.net.URL(endpoint) );

Integer ret = (Integer)call.invoke(new Object[]{

new Integer(5), new Integer(6)});

System.out.println("addInt(5, 6) = " + ret);

• Using generated Stubs from Service WSDL description– Service to be used is known from the beginning and the

WSDL is available at client development time

– Service Endpoint Interface (SEI): the (Java) programming language representation of a WSDL port type. Can be generated automatically by tools from a WSDL

– Stubs (proxies) are classes that implement the SEI. They are generated from the WSDL description (similar with RMI or CORBA middleware for distributed object computing)

Client – using generated stubs

Client – using Generated Stubs

import localhost.*;

AddFunctionService afs = new AddFunctionServiceLocator();

AddFunction af = afs.getAddFunction();

System.out.println("addInt(5, 3) = " + af.addInt(5, 3));

Generate the stubs:

java org.apache.axis.wsdl.WSDL2Java \

http://localhost:8080/axis/AddFunction.jws?wsdl

• Using Dynamic Proxy– you need to know the abstract WSDL (port type) at

development-time

– you need to run your WSDL mapping tool against the WSDL document before runtime in order to get the Service Endpoint Interface

– The proxy (a class implementing the SEI) is obtained at runtime (here is the difference with generated stubs: these are obtained at development time)

Client – using Dynamic Proxy

Client – using Dynamic Proxy

import javax.xml.namespace.QName;

import javax.xml.rpc.*;

String wsdlUrl = "http://localhost:8080/axis/AddFunction.jws?wsdl";

String nameSpaceUri = "http://localhost:8080/axis/AddFunction.jws";

String serviceName = "AddFunctionService";

String portName = "AddFunction";

ServiceFactory serviceFactory = ServiceFactory.newInstance();

Service afs = serviceFactory.createService(new java.net.URL(wsdlUrl),

new QName(nameSpaceUri, serviceName));

AddFunctionServiceIntf afsIntf = (AddFunctionServiceIntf)afs.getPort(

new QName(nameSpaceUri, portName), AddFunctionServiceIntf.class);

System.out.println("addInt(5, 3) = " + afsIntf.addInt(5, 3));

Where and How to find Services ?

• Service registies:– UDDI

• Standard for representing and organizing registry informations• Standard API’s for:

– publishing services on UDDI registry– Lookup services from registry

• Private UDDI registries: inside one enterprise • Public UDDI registries:

– Existed maintained by major companies– Not anymore (since 2008)

• Problems of UDDI: (why public UDDI registries died):– Complex standard and API– No semantic information– No certification, no trust– Info published in UDDI registry accessible only via UDDI lookup API’s, not accessible via usual

search engines

• Using usual search engines • Using Web service search engines

– http://webservices.seekda.com/




• Software development with services– Simple case: a client consumes a service

• WS APIs

– Common case: several services are composed with support of the Common case: several services are composed with support of the service platformservice platform• WorkflowsWorkflows

• BPELBPEL

– Service Oriented Software Engineering particularities

Software development with services

• Common case: a client uses multiple services• Existing services are composed and configured to create new composite

services and applications• Solution 1: implement root of composite service in usual programming

language.– The internal application implements the composition logic, by invoking services

as needed– The service platform is not involved in the composition

• Solution 2: use specific facilities of the service platform to support composition– A composition model describes the business logic – The service platform provides runtime support for the execution of the

composition model by invoking other services – Fundamental concepts:

• Workflows• Orchestration• Choreography

implement root of composite service in usual programming language

use specific facilities of the service platform to support composition

Workflow

• Workflows represent a set of activities to be executed, their interdependencies relations, inputs and outputs.

• Activities can be executed in parallel or in sequence

activity1

activity2

activity3input data1

input data2

input data3

activity4

output data

What are workflows?

The automation of a business process, in whole or part, during which documents, information or tasks are passed from one participant to another for action, according to a set of procedural rules.

workflow management consortium

Participants perform the work represented by a workflow activity instance and can be human or machine resources

Workflow Management Systems

• History:– Workflow management systems date back to the late 1980.

They are used to coordinate the work of multiple people in a project that has a fixed process.

• What a WMS supports:– defining process in terms of finer-grained tasks or activities– scheduling these activities and– dispatching (invoking) the activities– passing of information between activities

Two-level Programming Model

• Programming in the large– Non-programmers implementing flows

• Flow logic deals with combining functions in order to solve a more complex problem (such as processing an order)

• Programming in the small– Programmers implementing functions

• Function logic deals with a discrete fine-grained task (such as retrieving an order document or updating a customer record)

Workflow patterns• Basic Control Flow

– Sequence

– Parallel Split

– Synchronization

– Exclusive Choice

• Advanced Branching and Synchronization– Multi-Choice

– Structured Synchronizing Merge

– Multi-Merge

– Structured Discriminator

– Blocking Discriminator

• Multiple Instance Patterns– Multiple Instances without Synchronization

– Multiple Instances with a Priori Design-Time Knowledge

– Multiple Instances with a Priori Run-Time Knowledge

• State-Based Patterns– Deferred Choice

– Interleaved Parallel Routing

– Milestone

• Cancellation and Force Completion Patterns– Cancel Task – Cancel Case

• Interation Patterns– Arbitrary Cycles

– Structured Loop

– Recursion

• Termination Patterns– Implicit Termination

– Explicit Termination

• Trigger Patterns– Transient Trigger

– Persistent Trigger

www.workflowpatterns.com

Some Challenges for WfMS

• Process Representation (control flow & data flow)

• Process Specification

• Process Definition Interoperability

• Process Enactment (automated sequencing)

• Process Monitoring & Control/Config.

• Process Participant Modelling, Monitoring & Control

Workflows and services

• Workflow technology is the predecessor of service composition

• Disadvantages of workflows:– high license costs– complex software– heterogeneity

• But service composition is different (or, the business and IT environment is different): – standardization (components and composers)– maturity– reduced costs (small layer on top of other middleware)

Types of WFMS

• Centralized (Orchestration)

• Decentralized (Choreography)

Orchestration vs Choreography

• Orchestration:

• A centralized mechanism that describes how diverse services can interact. This interaction includes message exchange, business logic and order of execution.

• interacting components are not be aware of each other

• Choreography:• Choreography focuses on

enabling the description of how to interact with services at a larger scale than the individual message exchange pattern

• Interacting components are aware of each other.

• “A choreography defines re-usable common rules that govern the ordering of exchanged messages, and the provisioning patterns of collaborative behavior, as agreed upon between two or more interacting participants “ [W3C]

Orchestration vs Choreography

Business Process Modelling

• Formal model based on Petri-Nets– Formal mathematical model = directed graph– Basic concepts:

• Places• Transitions• Arcs• Markers

• Languages for modelling busines processes– Petri net based– UML activity diagrams– YAWL (Yet Another Workflow Language)– BPMN (Business Process Modelling Notation)

BPMN

• Developed by Sun, BEA and Intalio• Modeling of activities that are performed at

certain points in time• Basic elements:– Flow elements– Connection Objects– Swimlanes– Artefacts

• translation of business process model to executable model required: direct mapping from BPMN to BPEL

BPEL• Business Process Execution Language• Language for Business Process Description & Composition of Web

Services• Facilitates automated process integration• Presented in 2002 as BPEL4WS by Microsoft, IBM and BEA• Standardised by OASIS as WS-BPEL – 2.0 latest version (2007)• Based on XML and Web Services• Syntax defined by XML schema• No standardised graphical notation• Successor of 2 earlier work flow languages:

– WSFL: Web Services Flow Language (by IBM)– XLANG: Web Services for Business Process Design (by Microsoft)

WS-BPEL in the WS-* Stack

WS-BPEL

XML, Encoding

Other protocols

Other services

Transportand

Encoding

BusinessProcesses

WSDL, Policy, UDDI, Inspection Description

SecurityReliable

Messaging

Transactions

Coordination

SOAP (Logical Messaging)

QualityOf

Service

BPEL supported types of business processes

• Executable Process– Orchestration of specific activities and particular services which must be

executed– Executable via an execution engine– => Definition of an Orchestration

• Abstract Process– Specification of the message exchanges between multiple participants– No definition of internal process details– Interfaces defined through set of all receive and reply– => Definition of a Choreography– Still BPEL is not used for choreography– WS-CDL (Choreography Description Language) – A choreography is not an executable process -> usually it is translated

into a concrete orchestration description and executed

WS-BPEL Language StructureProcess

Partner Links

Variables

Correlation Sets

Fault Handlers

Compensation Handlers

Event Handlers

Activities

Receive Reply

Invoke

WS-BPEL Process

• WS-BPEL Process:• Made up of “Activities” – the

individual steps of a process• Activities are implemented

by Web Services• Specifies order in which

participating Web Services should be invoked

• WS-BPEL process is itself represented as Web Services

Process

Partner Links

Variables

Correlation Sets

Fault Handlers


Event Handlers

Activities

Receive Reply

Invoke

BPEL and WSDL

• BPEL processes are exposed as WSDL services– Message exchanges map to WSDL operations

– WSDL can be derived from partner definitions and the role played by the process in interactions with partners

WebService

Loan Approval Process

receive

reply

WSDLLoan Approval

PortType

Recursive Composition

• BPEL processes interact with WSDL services exposed by business partners

WebService

Loan Approval Process

WebService

WSDLLoan Approval

PortType

Financial Institution‘sWeb Service

(Loan Approver)invoke

Interfaces exposed

by the BPEL process

Interfaces consumed

by the BPEL process

receive

reply

WS-BPEL PartnerLinks

• partnerLinks:

• Defined in WSDL files

• Represent the interaction between a BPEL process and the participants (the different web services)

• Possess roles with portTypes e.g. invocation role: computePrice and callback role: provideInvoice

Process

Partner Links

Variables

Correlation Sets

Fault Handlers


Event Handlers

Activities

Receive Reply

Invoke

Composition of Web Services

Partner Link Type Partner Link Type

Service PService A Service B

A’s WSDL P’s WSDL B’s WSDL

invoke

receive

receive

invoke

invoke

WS-BPEL variables

• Variables– Save data which is

exchanged within processes

– Activities access data stored in variables

Process

Partner Links

Variables

Correlation Sets

Fault Handlers


Event Handlers

Activities

Receive Reply

Invoke

BPEL- WSDL- XML Schema

WS-BPEL primitive activities• Primitive Activities – for common tasks• • Invoke: Calling other Web Service

Activities (port types)• • Receive: Waiting in blocked state for

arrival of messages• • Reply: Generation of reply to received

message• • Wait: Wait for a certain amount of time• • Assign: Copying of data from one place to

another• • Throw: Error handling, creation of error

message• • Terminate: End complete process

instance• • Empty: Inserting of NOOP (no operations)

Process

Partner Links

Variables

Correlation Sets

Fault Handlers


Event Handlers

Activities

Receive Reply

Invoke

WS-BPEL structured activities

• Structured Activities – to combine primitive activities

• Sequence - set of activities that will be invoked in ordered sequence

• Flow - set of activities that will be invoked in parallel

• Switch - Case-switch construct for implementing branches

• While - for defining loops• Pick - select one of several

alternative paths

Process

Partner Links

Variables

Correlation Sets

Fault Handlers


Event Handlers

Activities

Receive Reply

Invoke

WS-BPEL correlation sets

• Correlation Sets

• Declarative description of a group of properties which together describe a conversation

• BPEL execution systems support multiple concurrent conversations

• Necessary to match messages to correct process instance (conversation)

• E.g. invoice correlated with correct instance of buyer process via apurchase order number received in earlier purchase order message.

Process

Partner Links

Variables

Correlation Sets

Fault Handlers


Event Handlers

Activities

Receive Reply

Invoke

Example: 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

BPEL design tools

• EclipseBPEL Designer• JBossIDE• NetBeansIDE• IBM WebSphere Integration Developer• Microsoft BizTalk Orchestration Designer • Oracle BPEL Designer (for JDeveloper and Eclipse)• Active Endpoints Active BPEL Designer

BPEL execution engines

• BPEL-Engine: Runtime environment for the interpretation of the BPEL documents und execution of the Processes

• Apache ODE

• IBM WebSphere Process Choreographer

• Microsoft BizTalk Server

• Oracle BPEL Process Manager (Intalio n3 Server

• ActiveBPEL Engine

• JBoss Application Server with jBPM

Advantages of WS-BPEL

• Portable, interoperable process model for long running business processes

• Flexible integration of Web services– WSDL abstract interfaces alone used to define

composition• Enables two levels of adaptive behavior

– Abstract partners can be bound to actual services at runtime– The process can choose a protocol for communicating with the

service at runtime

– Services whose data definitions do not match can be composed• Data transformations can be inlined in process definition

Disadvantages of WS-BPEL

• Static process composition.

• Process participants (partner‘s web services) must be defined and bound to the process flow at design time.

• BPEL standard is not about Semantic Web services:– Partner discovery and bounding at run time not possible.

– Message mediation not possible.




• Software development with services– Simple case: a client consumes a service

• WS APIs

– Common case: several services are composed with support of the service platform• Workflows

• BPEL

– Service Oriented Software Engineering particularitiesService Oriented Software Engineering particularities

Construction by composition

Formulateoutline

workflow

Discoverservices

Workflowdesign

Service list Servicespecifications

Workflowdesign

Selectservices

Refineworkflow

Createworkflowprogram

Executableworkflow

Testservice

Deployableservice

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

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.

outline concepts: services, soa, webservices services as reusable components service engineering...

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