  The High Altitude Water Cherenkov observatory (HAWC) is an air shower array
devised for TeV gamma-ray astronomy. HAWC is located at an altitude of 4100 m
a.s.l. in Sierra Negra, Mexico. HAWC consists of 300 Water Cherenkov Detectors,
each instrumented with 4 photomultiplier tubes (PMTs). HAWC re-uses the
Front-End Boards from the Milagro experiment to receive the PMT signals. These
boards are used in combination with Time to Digital Converters (TDCs) to record
the time and the amount of light in each PMT hit (light flash). A set of VME
TDC modules (128 channels each) is operated in a continuous (dead time free)
mode. The TDCs are read out via the VME bus by Single-Board Computers (SBCs),
which in turn are connected to a gigabit Ethernet network. The complete system
produces ~ 500 MB/s of raw data. A high-throughput data processing system has
been designed and built to enable real-time data analysis. The system relies on
off-the-shelf hardware components, an open-source software technology for data
transfers (ZeroMQ) and a custom software framework for data analysis (AERIE).
Multiple trigger and reconstruction algorithms can be combined and run on
blocks of data in a parallel fashion, producing a set of output data streams
which can be analyzed in real time with minimal latency (< 5 s). This paper
provides an overview of the hardware set-up and an in-depth description of the
software design, covering both the TDC data acquisition system and the
real-time data processing system. The performance of these systems is also
discussed.
