Comet: A VOEvent broker

The VOEvent standard provides a means of describing transient celestial events in a machine-readable format. This is an essential step towards analysing and, where appropriate, responding to the large volumes of transients which will be detected by future large scale surveys. The VOEvent Transport Protocol (VTP) defines a system by which VOEvents may be disseminated to the community. We describe the design and implementation of Comet, a freely available, open source implementation of VTP. We use Comet as a base to explore the performance characteristics of the VTP system, in particular with reference to meeting the requirements of future survey projects. We describe how, with the aid of simple extensions to VTP, Comet can help users filter high-volume streams of VOEvents to extract only those which are of relevance to particular science cases. Based on these tests and on the experience of developing Comet, we derive a number of recommendations for future refinements of the VTP standard.