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only aware of the interface of the programs and does not access the actual
binary i les. Jobs are submitted to the grid by the services. The WSDL is
used to describe the services. In the service-oriented approach, the work-
l ow description stipulates that a particular output of a program is linked
to a particular input of another. Hence, the iteration of such workl ows
on a number of input datasets is straightforward and does not require
and rewriting of the workl ow [51]. In the job-based approach, data
dependencies between programs are not explicitly described. Iterating a
single workl ow on many datasets requires writing specii c jobs. The
service-based approach is more independent from the infrastructure
than the job-based one. Services themselves are responsible for the sub-
mission of jobs but in the job-based approach it is the workl ow manager
that submits jobs to the grid.
11. 3.1. 2
Workfl ow Languages
The workl ow language is a language that specii es the rules for
connecting components to produce workl ows. It uses XML notation to
represent the intercomponent dependencies. In the workl ow, each
component has an input and outputs. The input receives tokens from a
predecessor component, and the outputs send tokens to successor com-
ponents. Receipt of a token from a predecessor triggers the component
to perform its task. When the tasks completed, that component gener-
ates a token at each output, along with a result from the task. The work-
l ow language can be used to coordinate the operation of the component
within the workl ow.
There are many existing standards for workl ow such as XLANG, WSFL,
BREL, ebXML, XPDL, and so on. Among these workl ow languages, busi-
ness process execution language (BPEL) is a leading candidate for the stan-
dard workl ow language for business-oriented Web services. BPEL replaced
IBM's WSFL and Microsoft's XLANG languages and is currently in the
process of standardization in OASIS [55]. It is a combination of graph-ori-
ented (WSFL) and procedural (XLANG) workl ow languages and provides
a convenient language to express the majority of workl ow processes.
BPEL has become a very attractive workl ow language to use in grid
environments due to its strong support for Web services [55]. A BPEL work-
l ow is designed to present as a Web service. It can easily interact with
existing Web services by combining them into a new service. The BPEL
workl ow engine can receive messages, determine their destination, and
send messages to other Web services depending on the workl ow's execu-
tion state. BPEL uses the WS-Addressing Endpoint References to simplify
passing addresses of Web services participating in a workl ow. One very
important advantage of using BPEL in grids is that it can easily and seam-
lessly interact with standard Web services (that may not be part of grids)
and grid services provided by OGSI, WSRF, and so on.
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