  We present a unified picture of open quantum systems, the theory of a system
probing a noisy thermal environment, distilling lessons learnt from previous
holographic analyses. Our treatment is applicable both when the system is
coupled to short-lived (Markovian), and long-lived (non-Markovian)
environmental degrees of freedom. The thermal environment is modeled using an
asymptotically AdS black hole, and the systems of interest are simple probe
field theories. The effective stochastic dynamics of the system is governed by
real-time thermal correlators, which we compute using the gravitational
Schwinger-Keldysh (grSK) geometry. We describe the structure of arbitrary
tree-level contact and exchange Witten diagrams in the grSK geometry. In
particular, we argue, that all such diagrams reduce to integrals supported on a
single copy of the exterior of the black hole. The integrand is obtained as a
multiple discontinuity of a function comprising ingoing boundary-bulk
propagators, monodromy functions which appear as radial Boltzmann weights, and
vertex factors. These results allow us to deduce the analytic structure of
real-time thermal n-point functions in holographic CFTs. We illustrate the
general statements by a two-dimensional toy model, dual to fields in the BTZ
background, which we argue captures many of the essential features of generic
open holographic QFTs.
