| Service Oriented Computing is a new paradigm for accessing, integrating and coordinating loosely coupled software systems in a standardized way. It aims to reduce the cost of building and maintaining complex software systems while increasing their re-usability. It is increasingly being employed by large scientific collaborations to "wrap" existing command-line scientific applications as web services i.e. to create an additional "web services layer" on top of existing command-line scientific applications. The "web services layer" is called an application service and is described using the Web Services Description Language (WSDL) [25]. It serves as an interface to the underlying application. Thus, when a user invokes an application service with some input parameters, the service runs its application with those input parameters, monitors the application and returns the output results to the user. Since web services can be easily described, published, discovered and consumed in a standard way, by wrapping command-line scientific applications as application services, scientists can describe, publish, discover and consume those applications also in a standard way. This enables scientists to easily compose complex scientific workflows from a distributed set of command-line scientific applications, run and monitor them on a distributed set of heterogeneous resources. However, during the execution of such complex scientific workflows, application services often become unavailable primarily due to the unreliable nature of the resources that host them. When an application service becomes unavailable, all workflows that are accessing it must be stopped and can be resumed only after it becomes available, and thus wastes a lot of time and resources. This is an important and widespread problem and no good solutions to this problem currently exist. This thesis offers a new solution to this problem, by providing a mechanism by which application services can be created on-demand from workflows in the event that they are unavailable. |
