Computer Laboratory

Technical reports

Monitoring the behaviour of distributed systems

Scarlet Schwiderski

July 1996, 161 pages

This technical report is based on a dissertation submitted April 1996 by the author for the degree of Doctor of Philosophy to the University of Cambridge, Selwyn College.


Monitoring the behaviour of computing systems is an important task. In active database systems, a detected system behaviour leads to the triggering of an ECA (event-condition-action) rule. ECA rules are employed for supporting database management system functions as well as external applications. Although distributed database systems are becoming more commonplace, active database research has to date focussed on centralised systems. In distributed debugging systems, a detected system behaviour is compared with the expected system behaviour. Differences illustrate erroneous behaviour. In both application areas, system behaviours are specified in terms of events: primitive events represent elementary occurrences and composite events represent complex occurrence patterns. At system runtime, specified primitive and composite events are monitored and event occurrences are detected. However, in active database systems events are monitored in terms of physical time and in distributed debugging systems events are monitored in terms of logical time. The notion of physical time is difficult in distributed systems because of their special characteristics: no global time, network delays, etc.

This dissertation is concerned with monitoring the behaviour of distributed systems in terms of physical time, i.e. the syntax, the semantics, the detection, and the implementation of events are considered.

The syntax of primitive and composite events is derived from the work of both active database systems and distributed debugging systems; differences and necessities are highlighted.

The semantics of primitive and composite events establishes when and where an event occurs; the semantics depends largely on the notion of physical time in distributed systems. Based on the model for an approximated global time base, the ordering of events in distributed systems is considered, and the structure and handling of timestamps are illustrated. In specific applications, a simplified version of the semantics can be applied which is easier and therefore more efficient to implement.

Algorithms for the detection of composite events at system runtime are developed; event detectors are distributed to arbitrary sites and composite events are evaluated concurrently. Two different evaluation policies are examined: asynchronous evaluation and synchronous evaluation. Asynchronous evaluation is characterised by the ad hoc consumption of signalled event occurrences. However, since the signalling of events involves variable delays, the events may not be evaluated in the system-wide order of their occurrence. On the other hand, synchronous evaluation enforces events to be evaluated in the system-wide order of their occurrence. But, due to site failures and network congestion, the evaluation may block on a fairly long-term basis.

The prototype implementation realises the algorithms for the detection of composite events with both asynchronous and synchronous evaluation. For the purpose of testing, primitive event occurrences are simulated by distributed event simulators. Several tests are performed illustrating the differences between asynchronous and synchronous evaluation: the first is ‘fast and unreliable’ whereas the latter is ‘slow and reliable’.

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BibTeX record

  author =	 {Schwiderski, Scarlet},
  title = 	 {{Monitoring the behaviour of distributed systems}},
  year = 	 1996,
  month = 	 jul,
  url = 	 {},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-400}