Schedulers

An analogy

Picture some young teenagers. After school, they’d go to a courtyard and play soccer. That was an exciting sight. A random number of kids running around the yard without any orchestration. There was no offense and no defense. They’d just run after a ball.

Everyone moved forward towards the ball, someone kicked it to the left, and kids moved in that direction, only to start running back because someone kicked the ball again. The strategy was simple. Run towards the ball, kick it if you can, wherever you can, repeat. It’s hard to understand how anyone managed to score. It was complete randomness applied to a bunch of kids. There was no strategy, plan, and no understanding that winning required coordination.

If that was a “real” team, they’d need a coach. They’d need someone to tell them what the strategy is, who should do what, and when to go on the offense or fall back to defend the goal. They’d need someone to orchestrate them.

The field (a cluster) had a random number of people (services) with the common goal (to win). Since anyone could join the game at any time, the number of people (services) was continually changing. Someone would be injured and would have to be replaced, or when there was no replacement, the rest of us would have to take over his tasks (self-healing).

The following illustrations will give you a basic idea of a node and a cluster.