  The discovery of variable and transient sources is an essential product of
synoptic surveys. The alert stream will require filtering for personalized
criteria -- a process managed by a functionality commonly described as a
Broker. In order to understand quantitatively the magnitude of the alert
generation and Broker tasks, we have undertaken an analysis of the most
numerous types of variable targets in the sky -- Galactic stars, QSOs, AGNs and
asteroids. It is found that LSST will be capable of discovering ~10^5 high
latitude |b| > 20 deg) variable stars per night at the beginning of the survey.
(The corresponding number for |b| < 20 deg is orders of magnitude larger, but
subject to caveats concerning extinction and crowding.) However, the number of
new discoveries may well drop below 100/night within 2 years. The same analysis
applied to GAIA clarifies the complementarity of the GAIA and LSST surveys.
Discovery of variable galactic nuclei (AGNs) and Quasi Stellar Objects (QSOs)
are each predicted to begin at ~3000 per night, and decrease by 50X over 4
years. SNe are expected at ~1100/night, and after several survey years will
dominate the new variable discovery rate. LSST asteroid discoveries will start
at > 10^5 per night, and if orbital determination has a 50% success rate per
epoch, will drop below 1000/night within 2 years.
