  Lasair is the UK Community Broker for transient alerts from the Legacy Survey
of Space and Time (LSST) from the Vera C. Rubin Observatory. We explain the
system's capabilities, how users can achieve their scientific goals, and how
Lasair is implemented. Lasair offers users a kit of parts that they can use to
build filters to concentrate their desired alerts. The kit has novel lightcurve
features, sky context, watchlists of special sky objects and regions of the
sky, dynamic crossmatching with catalogues of known astronomical sources, and
classifications and annotations from other users and partner projects. These
resources can be shared with other users, copied, and modified. Lasair offers
real-time machine-to-machine notifications of filtered transient alerts. Even
though the Rubin Observatory is not yet complete, Lasair is a mature system: it
has been processing and serving data from the similarly formatted stream of the
Zwicky Transient Facility (ZTF) alerts.
