Checkout is slow

Where checkout time goes

A checkout request spends time in four places: web's own handling (milliseconds), the database (two short queries, usually under 25 ms each), waiting for a database connection when the pool is busy, and the payments call. The payments call usually dominates.

Triage

1. Compare web's /checkout p95 with checkout's payment_call_duration_seconds p95 and payments' payment_provider_request_duration_seconds p95. If provider latency explains the checkout latency, the problem is the card provider (rb-008).
2. Check db_pool_waiting. Requests waiting for a connection add up to one second each before they fail.
3. If only some web requests are slow and product pages are slow too, the problem is in web or the host, not in checkout.

Do not

• Do not raise the pool size to fix latency caused by a slow provider: more connections held while waiting does not make the provider faster.