.tip

Jun 8, 2026 · one of the small things

Vary tells the cache which headers change the response

If a response differs by Accept-Language or by a custom header, any cache that doesn't know that will serve it to the wrong user.

Vary: Accept-Encoding, Accept-Language

Never put Vary: Cookie on a cached response, because it makes the cache useless. If a response depends on the cookie, it should be no-store or private.

httpcaching

~/tips/today — 2026-10-19

~cat tips/today

Never build an index on a live table without CONCURRENTLY

postgresqlmigrations

A plain CREATE INDEX takes a lock that blocks every write to the table for the whole build. On a big table that means minutes of outage that look like a hung app.

CREATE INDEX CONCURRENTLY orders_customer_idx ON orders (customer_id);

It's slower and can't run inside a transaction, so you'll have to tell your migration tool. If it fails halfway, it leaves an invalid index behind, so check pg_index.indisvalid and drop it before you retry.

↵ open y yesterday l ls /tips

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