Tech & Computing riddle: We come as a pair. One of us you hand out to strangers; the other you would burn a datacentre down to protect. Anything locked by the first can only be opened by the second.

I hold nothing of my own, yet almost everything passes through me. When I run out of room I forget the oldest thing first. You only notice I exist on the day I am wrong.

I make finding fast and writing slow. I am useless when you do not know what you are looking for, and I take my fee on every single insert.

Add more of me and the work finishes sooner — until it does not. Past a certain number I spend more time coordinating than working.