In fact, the cron job will : -> select about 10 000 lines from a big table (>100 Gb of data). 1 user has about 10 lines. -> each line will be examinate by an algorithm -> at the end of each line, the cron job updates a few parameters for the user (add some points for example) -> Then, it inserts a line in another table to indicate to the user each transaction.
All updates and inserts can be inserted ONLY by the cron job ... Therefore ... the merge can be done easily : no one can be update these new datas.
But ... how big company like Facebook or Youtube can calculate on (a) dedicated server(s) without impacting users ?