  This memo describes the system used to conduct commensal correlator and
beamformer observations at the Allen Telescope Array (ATA). This system was
deployed for ~2 years until the ATA hibernation in 2011 and was responsible for
collecting >5 TB of data during thousands of hours of observations. The general
system design is presented and the implementation is discussed in detail. I
emphasize the rationale for various design decisions and attempt to document a
few aspects of ATA operations that might not be obvious to non-insiders. I
close with some recommendations from my experience developing the software
infrastructure and managing the correlator observations. These include: reuse
existing systems; solve, don't avoid, tensions between projects, and share
infrastructure; plan to make standalone observations to complement the
commensal ones; and be considerate of observatory staff when deploying new and
unusual observing modes. The structure of the software codebase is documented.
