Owl the Engineer

How it actually works

The engineering questions you get asked, and the bugs you actually hit. Drawn, one mechanism at a time, in the order the videos ship.

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Backend

Cache invalidation: you cannot reach into a cache

You changed the price an hour ago and somebody just paid the old one. HTTP gives a server exactly one way to remove something from a cache, and it is a side effect rather than a command.

5 Oct 2026 views0 Read the post →

Backend

How HTTP caching works, and the line you have to write yourself

The page said welcome back, with somebody else's name on it. Four questions decide whether something in the path may answer instead of your server — and the default, when you say nothing at all, is not 'don't cache'.

12 Oct 2026 views0 Read the post →

System design

How CDNs work: distance is latency you can't optimise away

A CDN is not a faster server. It's a copy of your response nearer the person asking — and everything that surprises you about it follows from what HTTP lets a shared cache keep.

8 Sept 2026 views0 Read the post →