  Fast radio bursts, or FRBs, are transient sources of unknown origin. Recent
radio and optical observations have provided strong evidence for an
extragalactic origin of the phenomenon and the precise localization of the
repeating FRB 121102. Observations using the Karl G. Jansky Very Large Array
(VLA) and very-long-baseline interferometry (VLBI) have revealed the existence
of a continuum non-thermal radio source consistent with the location of the
bursts in a dwarf galaxy. All these new data rule out several models that were
previously proposed, and impose stringent constraints to new models. We aim to
model FRB 121102 in light of the new observational results in the active
galactic nucleus (AGN) scenario. We propose a model for repeating FRBs in which
a non-steady relativistic $e^\pm$-beam, accelerated by an impulsive
magnetohydrodynamic (MHD)-driven mechanism, interacts with a cloud at the
centre of a star-forming dwarf galaxy. The interaction generates regions of
high electrostatic field called cavitons in the plasma cloud. Turbulence is
also produced in the beam. These processes, plus particle isotropization, the
interaction scale, and light retardation effects, provide the necessary
ingredients for short-lived, bright coherent radiation bursts. The mechanism
studied in this work explains the general properties of FRB 121102, and may
also be applied to other repetitive FRBs. Coherent emission from electrons and
positrons accelerated in cavitons provides a plausible explanation of FRBs.
