Challenge 2: Fix a depends_on Race Condition — Possible Solution
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DIAGNOSIS: this is the classic depends_on race condition this chapter
described. Plain depends_on: [db] only waits for the db CONTAINER to
start — it says nothing about whether the database software running
inside it has finished its own startup process and is actually
accepting connections yet. Because database startup (initializing
data files, running startup checks) genuinely takes some variable
amount of time, api sometimes wins the race and tries to connect
before db is truly ready — producing the intermittent (not constant)
"connection refused" behavior described, since the exact timing varies
build to build.

THE FIX:

services:
  db:
    image: mysql:8
    healthcheck:
      test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]
      interval: 5s
      retries: 10
      start_period: 30s

  api:
    depends_on:
      db:
        condition: service_healthy

Adding a healthcheck to db gives Compose a real way to know when MySQL
is actually ready (not just running), and changing api's depends_on to
condition: service_healthy means api won't even start until that
healthcheck passes — closing the race condition entirely, rather than
just making it statistically less likely with an arbitrary delay.