Readers–writers problem

The readers–writers problem is a way of thinking about making sure that multiple people can access the same shared resource at the same time but in a way that prevents people from accidentally changing the same thing twice. It's like a group of kids all trying to write their names at the same time on a chalkboard. In the readers–writers problem, the chalkboard is the shared resource and "writing your name" is the action being protected.

To solve the problem, we need to make sure that each person can access the chalkboard one at a time. One way to do this is to give each kid two cards - one that says "reader" and one that says "writer". Everyone who wants to read from the chalkboard has to hold up the reader card, and whoever wants to write (or change) something on it has to hold up the writer card. That way, if multiple people all have their reader cards up at the same time, it's okay because they're not changing anything. But if someone has their writer card up, everyone else has to wait until they're finished before they can access the chalkboard.