Influencing the run-time behaviour of complex services using contexts

Abstract

Service oriented architecture (SOA) and web services make it possible to construct rich and complex distributed systems which operate at internet scales. However, the underlying design principles of SOA can lead to management problems for processes over web services.

This thesis identifies several potential problems with the management of processes over web services, and proposes the use of explicit context as a possible solution. The available options are explored, and the WS-Context specification is implemented and evaluated.

The SOA design principles of loose coupling, interaction at an interface, autonomy, and composablity can lead to management problems for processes over web services. Processes over web services where one composite service invokes other composite services which in turn invoke other composite services can lead to complex invocation trees. These invocation trees may be different at different times due to the shifting effect of loose coupling, as new services are swapped in to replace those in previous invocations. In such an environment how well can we define the interface of the top level service in a static document such as a WSDL? Because there is a separation between the ultimate service consumer, and the ultimate service provider how can the service consumer correctly assign fault when a service fails? When concurrency is used, and encouraged, how can we deal with the inevitable race conditions and deadlock? In distributed systems where portions of processes execute on systems beyond our organizational control, how can we pause, or kill these processes? Many of these systems model long-running business processes. How do we communicate changes in process requirements?

The use of an explicit context is a potential solution to these types of problems. The abstraction context provides an environment in which the process participants can describe their requirements, query those of other process participants, and react to changes in the environment.

A sample context server, based on the WS-Context specification, was implemented using the Erlang language. The sample context server provides the basic operations required to manage and store contextual information about a process.

The sample context server was evaluated to determine the cost of employing a context as part of a web service based software system. The performance of the sample server was also evaluated.

Test were conducted on the time costs of the basic operations of the context server, and they were found to have a constant time cost. The operations for getting and setting the contents of the context were found to have a time cost dependant on the size of the context. The cost of propagating the context along a chain of service invocations was tested and found to have an overhead which increased linearly with the length of the service invocation chain.

The context server was stress tested using a closed loop test which simulated the interaction of a number of concurrent clients, and an open loop test which simulated bursts of arriving requests. The open loop testing showed that the context server could handle 75 concurrent clients. Beyond 75 concurrent clients, the response times of the context server began to slowly increase. The closed loop testing showed that the context server had a maximum throughput of 190 requests per second for bursts of 200 requests with an interarrival time of 4 milliseconds.