  We describe the simulated sky survey underlying the second data challenge
(DC2) carried out in preparation for analysis of the Vera C. Rubin Observatory
Legacy Survey of Space and Time (LSST) by the LSST Dark Energy Science
Collaboration (LSST DESC). Significant connections across multiple science
domains will be a hallmark of LSST; the DC2 program represents a unique
modeling effort that stresses this interconnectivity in a way that has not been
attempted before. This effort encompasses a full end-to-end approach: starting
from a large N-body simulation, through setting up LSST-like observations
including realistic cadences, through image simulations, and finally processing
with Rubin's LSST Science Pipelines. This last step ensures that we generate
data products resembling those to be delivered by the Rubin Observatory as
closely as is currently possible. The simulated DC2 sky survey covers six
optical bands in a wide-fast-deep (WFD) area of approximately 300 deg^2 as well
as a deep drilling field (DDF) of approximately 1 deg^2. We simulate 5 years of
the planned 10-year survey. The DC2 sky survey has multiple purposes. First,
the LSST DESC working groups can use the dataset to develop a range of DESC
analysis pipelines to prepare for the advent of actual data. Second, it serves
as a realistic testbed for the image processing software under development for
LSST by the Rubin Observatory. In particular, simulated data provide a
controlled way to investigate certain image-level systematic effects. Finally,
the DC2 sky survey enables the exploration of new scientific ideas in both
static and time-domain cosmology.
