It doesn’t try to tally the number by querying the individual accounts of all the people who have posted those “Likes.” Since these accounts sit in databases that run on hard disk, that would take too long. Instead, the company keeps a “Like” count for each photo in a single database cell, and it accesses this single cell as needed. “That’s just one disk access of about ten micro-seconds,” says Instagram engineer Lisa Guo.

Facebook a bien racheté des utilisateurs et non une technologie… !

Source : How Instagram Solved Its Justin Bieber Problem | WIRED