Case Study: An In Memory Pub Sub Broker

A company mail room delivers each notice to every employee who asked to receive it. Each employee has a mailbox with room for a fixed number of letters. When one employee is away and that mailbox is full, the mail room does not stop delivering to everyone else. It sets the extra letters for that one mailbox aside and records how many there were.

A program often needs the same arrangement inside one process. An order service publishes "order placed" events, and an email sender, a stock counter, and an audit log each want a copy