  In the massive star-forming region IRAS 21078+5211, a highly fragmented
cluster (0.1~pc in size) of molecular cores is observed, located at the density
peak of an elongated (1~pc in size) molecular cloud. A small (1~km/s per
0.1~pc) LSR velocity (Vlsr) gradient is detected across the axis of the
molecular cloud. Assuming we are observing a mass flow from the harboring cloud
to the cluster, we derive a mass infall rate of about 10^{-4}~M_{sun}~yr^{-1}.
The most massive cores (labeled 1, 2, and 3) are found at the center of the
cluster, and these are the only ones that present a signature of protostellar
activity in terms of emission from high-excitation molecular lines or a
molecular outflow. We reveal an extended (size about 0.1~pc), bipolar
collimated molecular outflow emerging from core 1. We believe this is powered
by a (previously discovered) compact (size <= 1000~au) radio jet, ejected by a
YSO embedded in core 1 (named YSO-1), since the molecular outflow and the radio
jet are almost parallel and have a comparable momentum rate. By means of
high-excitation lines, we find a large (14~km/s over 500~au) Vlsr gradient at
the position of YSO-1, oriented approximately perpendicular to the radio jet.
Assuming this is an edge-on, rotating disk and fitting a Keplerian rotation
pattern, we determine the YSO-1 mass to be 5.6+/-2.0~M_{sun}. The water masers
(previously observed with VLBI) emerge within 100-300~au from YSO-1 and are
unique tracers of the jet kinematics. Their three-dimensional (3D) velocity
pattern reveals that the gas flows along, and rotates about, the jet axis. We
show that the 3D maser velocities are fully consistent with the
magneto-centrifugal disk-wind models predicting a cylindrical rotating jet.
Under this hypothesis, we determine the jet radius to be about 16~au and the
corresponding launching radius and terminal velocity to be about 2.2~au and
200~km/s, respectively.
