  Gravitational-wave observations became commonplace in Advanced LIGO-Virgo's
recently concluded third observing run. 56 non-retracted candidates were
identified and publicly announced in near real time. Gravitational waves from
binary neutron star mergers, however, remain of special interest since they can
be precursors to high-energy astrophysical phenomena like $\gamma$-ray bursts
and kilonovae. While late-time electromagnetic emissions provide important
information about the astrophysical processes within, the prompt emission along
with gravitational waves uniquely reveals the extreme matter and gravity during
- and in the seconds following - merger. Rapid communication of source location
and properties from the gravitational-wave data is crucial to facilitate
multi-messenger follow-up of such sources. This is especially enabled if the
partner facilities are forewarned via an early-warning (pre-merger) alert. Here
we describe the commissioning and performance of such a low-latency
infrastructure within LIGO-Virgo. We present results from an end-to-end mock
data challenge that detects binary neutron star mergers and alerts partner
facilities before merger. We set expectations for these alerts in future
observing runs.
