Consolidated Audit Trail
(Also see the entry for Consolidated Audit Trail on MarketsReformWiki here: http://www.marketsreformwiki.com/mktreformwiki/index.php/SEC_Final_Rule:_Consolidated_Audit_Trail)
The Securities and Exchange Commission ordered the creation of the Consolidated Audit Trail (CAT) in 2012 after regulators found they didn’t have enough information to explain the “flash crash” that occurred in May 2010. It is designed to compile detailed information on orders and trades for the US equity and options markets. One SEC commissioner, Kara Stein, said the database could become the “Hubble telescope of securities markets.” 
SEC Rule 613 required FINRA and the U.S. securities exchanges to jointly submit a National Market System (NMS) plan detailing how they would develop, implement and maintain a consolidated audit trail that would track orders throughout their life cycle and identify the broker-dealers handling them, thus allowing regulators to more efficiently track activity in Eligible Securities throughout the U.S. markets.
The Consolidated Audit Trail (CAT) collects and accurately identifies every order, cancellation, modification and trade execution for all exchange-listed equities and options across all U.S. markets.
Implementation of the CAT was delayed for several years. It was finally launched on November 15, 2018, although some "bugs" still remained to be ironed out. The first phase began that day with US exchanges including Nasdaq, the New York Stock Exchange and Cboe Global Markets reporting market information. That came after a one-year delay on the original timetable of milestones for the scheme amid calls by exchanges for the postponement. FINRA had been working with the exchanges to develop an NMS plan that met the requirements of Rule 613.
Thesys Group is the company hired to build the database.
Phase II is scheduled to begin in November of 2019 with large brokers reporting. 
- Stock-Market Supercomputer to Launch With Glitches. The Wall Street Journal.
- Consolidated Audit Trail. FINRA.
- US stock market surveillance system goes live. The Financial Times.