Message queues & async processing

A user signs up, and your code sends them a welcome email before returning a response. The email service is having a slow day, so the signup request hangs for five seconds while it waits. The user just wanted an account; instead they are staring at a spinner because of a task that did not need to block them at all.

A message queue fixes this by letting you hand work off to be done later. Instead of doing the slow task while the user waits, you drop a message ("send a welcome email to this user") into a queue and return a response immediately. A separate worker picks up the message and does the job in the background.

That one move, do it later, off the request path, unlocks three big benefits: fast responses, the ability to absorb spikes, and decoupling services from each other. This unit covers all three, plus the one gotcha you must design around.