Light in fibre is fast, but not fast enough to cross the world many times per page load. How a content delivery network caches your files at edges near users, what it can and can't cache, and how to control it with headers and purges.

Three numbers that describe how a page feels to a real user: how fast the main content appears, how quickly it responds, and how much it jumps around. What each one measures, the thresholds that count as good, and the fixes that move them.

No comparison sort can beat O(n log n) — but if you never compare, the rule doesn't apply. How counting sort sorts small-range integers in O(n + k), why stability matters, and how radix sort extends the trick to large numbers digit by digit.

ACID promises a lot, but the I — isolation — comes in levels, and the default one still lets concurrent transactions overwrite each other. What dirty reads, non-repeatable reads, phantoms and lost updates are, and the fixes that actually work.