SOA Journal Module: Web Services Grade Mark: Distinction of 90% Date: Apr 2012 Sheffield Hallam University Assessment

1. Critical Context-based Analysis of Top-down and
Bottom-up Approaches in SOA Development
Traitet Thepbandansuk
Abstract
Service-Oriented Architecture (SOA) is widely recognised as a potential architecture for aligning business
and IT strategies by heterogeneous systemintegration. Development of SOA by using a top-down
approach focuses on business-driven strategiesto align IT with business, whereas that by using a bottom-
up approach focuses on technology-driven processes to align business with IT. To successfully develop
SOA, top-down and bottom-up approaches have been discussed in various contexts. This paper provides
critical context-based analysis of two commonly usedapproachesin SOA development.
Introduction
SOA is extensively accepted as a suitable architectural style for designing and developing software in
terms of interoperable services.In general, they are often implemented as Web servicesto support
interoperability between various application development platforms (Margolis 2008). There are two
oppositescommonly used as approachesto develop SOA,namely top-down and bottom-up. These days,
an appropriate approach that should be employed in developing SOA and Web services is a controversial
issue leading to much debate. There is a widespread perception that using a top-down approach can
enhance reusability and enable SOA projects to meet business requirements; however, others take a
different stance. This paper criticallyanalyses and evaluates both views in three distinctive contexts:
strategic level, service delivery and operational level, as illustrated in Figure 1 and the following sections.
Figure 1 Top-down and bottom-up approachesin SOA development
(Josuttis 2007; Erl 2008; Krafzig, Banke and Slama 2005)
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2. Context of the Strategic Level in Terms of Project Initiation
A top-down approach is a methodologyto align IT infrastructure with business strategies. As can be seen
in Figure 1, in terms of project creation, this approach is initiated by top management who would like to
build a flexible business model, which can rapidly adapt to changes in the market-place (Sharbanoo, Ali
and Mehran 2012). Then, execution teams implementSOA solutions following business strategies by
developing IT infrastructure in-line with thebusiness process modelrequriements.Conversely, a bottom-up
approach aligns business processes with IT infrastructure,especially business applications. This
methodstarts with a small group of SOA enthusiasts, such as a particular department or business unit.
After the first pilot project is successful, execution teams gradually implement other projects, and
ultimately end with an enterprise-wide rollout (Josuttis 2007).
However, Abdollah (2007) suggests that building SOA solutionsin practice by only working downward or
upward it is not easy to become successful, especially for medium to large businesses.Therefore,
combining both approaches is an effective way. The bottom-up method can reduce time, effort and cost
of developing SOA services, whereas the top-down initiated by top management can develop business
process-driven services, which enable business agility and flexibility to make sure that long-term business
goals, business strategies and requirements are addressed (Josuttis 2007; Walker 2007).
Context of Service Delivery
Atop-down approach in terms of service delivery starts with inventory analysis in order to designa service
inventory blueprint known as a service enterprise model. Then,the services required by a new business
model are developed and delivered.On the contrary, a bottom-up approach focuses on service-oriented
analysis to fulfill immediate business requirements as the first priority. Although using the top-down
approach needs much higher investment and time to analyse business processes to produce a blueprint
comprising of a collection of standardised services, the overall burden of subsequent service governance
is reduced because the SOA services had been modeled since the first implementation stage(Erl 2008).
However, Worker (2007) suggests that using the top-down approach for large businessesrequiresan
effective collaboration between multiple business units with various business strategies.
Furthermore, using a bottom-up approach starts with service-oriented analysis that does not need to
produce a blueprint. This method can save costs, effort and time to deliver initial services (Erl 2008).
Nevertheless, it requires more expenses and time for updating and reworking when developing new
services or changing business requirements. Moreover, it tends to result in redundant SOA services;also
the services developed may not support the business requirements(Pomares 2010; Delsing 2011).
Context of the Operational Level in Terms of Implementing Web Services
Web Service Definition Language (WSDL) is an XML-based language for explainingdetails of Web
services (Margolis 2007). WSDL is also used to define anSOA contract including interface information
between a Web serviceand its consumers (Tere 2012). Building Web services by using a top-down or
contract-first approach begins with defining a WSDL contract, and then coding Web services as shown in
Figure 1 (Suriarachchi 2010). In contrast, a bottom-up or code-first approach starts with coding Web
services. Next, a WSDL contract can be automatically generated when deploying the Web services to a
Web service engine, such as Apache Axis2 for java and .NET (Ganguly and Goswami 2011).
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3. Developing a Web service by using a code-first approach makes it easy to convert a legacy code into a
Web service, and it also takes much less time to create a WSDL contract (Suriarachchi 2010). However,
by using this approach, XML Schema Definitions (XSDs) will be contained in the WSDL contract.
Therefore, when a XML schema definition is changed according to a business objecte.g. changing a data
type,developers must not only regenerate WSDL code, but they are also needed to inform this change to
all consumers in order to modify their consuming code. On the other hand, by using a contract-first
approach, developers can include business objects into their Web service classes. As a result, they are
flexible to modify those business objects without republishing WSDL files. Furthermore, the first-code
approach enables a XSD file can be used by several services. It is an effective way of enhancing
reusability of services in developing SOA (Vivek 2009).
Conclusion
To sum up, this paper analysesusing top-down and bottom-up approaches to developing SOA in three
different contexts: strategic level, service delivery and operational level.Firstly, in the context of business
strategy, a top-down approach initiated by top management enables SOA solutions to align with overall
business strategies and requirements, but it takes more effort and cost. Secondly, in terms of service
delivery, a top-down approach needs a lot of time to establish a service inventory blueprint, whereas a
bottom-up approach starting with immediate business requirements can reduce time and the scope ofthe
SOA project.However, development of consequent SOA services will take more time, expenditure and
effortthan a bottom-up approach. Lastly, in terms operational level in developing Web services, a bottom-
up or code-first approach is easier to develop than a top-down or contract-first approach because WSDL
can be generated automatically, but using the top-down method can enhance reusability of services and
flexibility to change business objectsor XML schema definitionswithout republishing a WSDL contract.
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