  The Cherenkov Telescope Array (CTA) Observatory, with dozens of telescopes
located in both the Northern and Southern Hemispheres, will be the largest
ground-based gamma-ray observatory and will provide broad energy coverage from
20 GeV to 300 TeV. The large effective area and field-of-view, coupled with the
fast slewing capability and unprecedented sensitivity, make CTA a crucial
instrument for the future of ground-based gamma-ray astronomy. To maximise the
scientific return, the array will send alerts on transients and variable
phenomena (e.g. gamma-ray burst, active galactic nuclei, gamma-ray binaries,
serendipitous sources). Rapid and effective communication to the community
requires a reliable and automated system to detect and issue candidate science
alerts. This automation will be accomplished by the Science Alert Generation
(SAG) pipeline, a key system of the CTA Observatory. SAG is part of the Array
Control and Data Acquisition (ACADA) working group. The SAG working group
develops the pipelines to perform data reconstruction, data quality monitoring,
science monitoring and real-time alert issuing during observations to the
Transients Handler functionality of ACADA. SAG is the system that performs the
first real-time scientific analysis after the data acquisition. The system
performs analysis on multiple time scales (from seconds to hours). \abrb{SAG
must issue candidate science alerts within} 20 seconds from the data taking and
with sensitivity at least half of the CTA nominal sensitivity. These
challenging requirements must be fulfilled by managing trigger rates of tens of
kHz from the arrays. Dedicated and highly optimised software and hardware
architecture must thus be designed and tested. In this work, we present the
general architecture of the ACADA-SAG system.
