Concurrency

Introduction

Once upon a time there was a soda shop that had an original jukebox.   Regulars would come into the restaurant and hope that they would get to play a song on the jukebox.   In order to do this they would have to add their quarter to the line of quarters accumulating on top of the machine.   They would memorize where their quarter was in line, and no regular was ever rude enough to put a quarter that wasn't their own into the jukebox.   When one song ended, the owner of the quarter next in line would put their quarter in and choose their song.

Now you ask, wouldn't that get frustrating and tedious?   You may even think that the design for that jukebox wasn't very clever.   This jukebox was designed with only one thread of control.   This means that only one thing could be done at a time.   This jukebox could only take one quarter and then play one song.   It wasn't until the task of playing the song was finished that the task of handling another quarter and song selection could occur.

If that design seems silly to you, you are in favor of a multi-threaded design, or a design that allows the jukebox to perform more than one task at a time.   In the jukeboxes we are accustomed to, you can insert a quarter to reserve your song while another song is playing.   One thread is in charge of playing the current song while another thread is in charge of putting requested songs on queue.   This thread takes the place of lining quarters up on the outside of the machine.

Concepts of Concurrency