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Module 11 · System Design · Five more whole designs
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Run this at nine (a distributed job scheduler)
Before you start. You need the six steps from Design the Whole Thing (the interview, assembled) : clarify, estimate, API, data, design, scale, with no box on the board until a number pays for it. You also need the idea from Who holds the pen (consensus & Raft) that a group of machines can agree on exactly one of them to be in charge, and that the one in charge is only trusted for a limited time. And you need a queue from The order slip rail (queues & the utilization cliff) : a waiting line of work that helpers take from.
The idea in 30 seconds
Cron woke once a minute on one machine and read one file. It never had to survive its own machine dying.
A cron expression is five fields: minute, hour, day of month, month, day of week.
The load is a wall at round times. Ten million jobs is 116 a second on average and three million in a single second at 09:00.
A leader runs the clock, or the jobs are split across pollers by a hash of the job id. A lease, an owner plus an expiry, lets a dead poller’s work be taken over without any vote.
Delivery is at-least-once, so every job needs a run id and must be safe to run twice.
Retries back off with jitter, and after N tries a run goes to the dead-letter queue.
Read the story
Lesson 54 of 73 in System Design
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In this lesson 1979: one machine, one minute, one list Counting the rush at 09:00:00 Who is the porter tonight? Leader election Hand the key card over: the lease Maybe twice: at-least-once Three strikes: retries and the dead-letter queue Terms it explains: cron expression, job store, poller, leader election, lease, at-least-once, idempotent, dead-letter queue, job definition store, run log, catch up, skip.
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