Introduction

An event can be defined as a time-stamped message containing typed data that is delivered from a source to a set of subscribed listeners. It may contain additional information for message management, like a sequence number or a time-to-live value.

Event-based systems provide an elegant and robust mechanism to connect software components in distributed computing. They can help in debugging, monitoring, and dynamically negotiating communications protocols in heterogeneous environments. Events can be broadly classified to belong to one of the two categories: system events and application events. System events typically are used by the component framework to report occurrences such as component instantiation or connection. Application events are for application-specific information like file activity or application state such as ``iteration $k$ has completed''. An event system should be simple and leverage the benefits provided by existing standards. It should be extensible, language-independent, platform-independent, and provide ready integration with applications that use events internally. There are several scenarios where event services can be useful:

• A process is interested in when a remote process has finished writing a file.
• A drag-and-drop component framework is waiting for successful instantiation of a remote component so that it can be connected to other components.
• A Grid Monitoring System (GMA) [3] needs to collect data for fault detection and performance tuning from a computational Grid.
• Distributed objects need an alternative mechanism to their usual communications mechanisms, for debugging or for (re)negotiating transport protocols for large-scale or specialized data transfer.

This paper addresses the following questions:

• What are the requirements of events in Grid [8] based applications?
• How should event types be defined so that the event framework is extensible?
• What are the minimal requirements that publishers and subscribers should satisfy?
• What designs will allow events to work through firewalls?
• Is it possible to design an event system that has built-in recovery mechanisms?
• What is rate at which SOAP based events can be generated and consumed ?

Any new event framework should address five requirements: simplicity, extensibility, rapid deployment and integration, interoperability, and performance. For some time now, it has been clear that systems using XML and SOAP can meet the first three requirements; this paper shows that it can meet the last two as well. In order to specify a minimal standard that satisfies these requirements, we first need to summarize existing ones.