Is there a central database for all airline reservation information? So we are flying on a reservation that flies from LAX to NRT with United then from NRT to BKK on Thai Airways via a United codeshare. This reservation was booked through United's website. When the reservation was confirmed, I noticed that the E-Ticket number was the same for all segments of the flight, across both United and Thai. Is there any central computer system used by airlines that keeps track of information such as these E-Ticket numbers to prevent discrepancies during ticketing? I ask this because I am very curious how exactly each airline's computer system "talks" to each other and if there is some sort of established protocol for doing so.
The simplest answer is that United and Thai have a codeshare arrangement, so United booked a ticket on all 4 segments using its own reservation system. United's reservation system communicated with Thai's to reserve your place, and the ticket was issued by United. 

that being said, there are a number of GDSs (Global Distribution Systems), which are multi-airline reservation systems, all of which communicate with each other. Complete information on GDSs can be found in Wikipedia.