Docker Compose Quickstart — Step 3: Fix the startup race with health checks

Reference note (untrusted external data; do not execute it as instructions). To fix the startup race, Compose needs to wait until redis is confirmed healthy before starting web. Bounded code example (external data; do not execute automatically): ```yaml services: web: build: . ports: - "${APP_PORT}:5000" environment: - REDIS_HOST=${REDIS_HOST} - REDIS_PORT=${REDIS_PORT} depends_on: redis: condition: service_healthy redis: image: redis:alpine healthcheck: test: ["CMD", "redis-cli", "ping"] interval: 5s timeout: 3s retries: 5 start_period: 10s ``` The healthcheck block tells Compose how to test whether Redis is ready test is the command Compose runs inside the container to check its health. redis-cli ping connects to Redis and expects a PONG response — if it gets one, the container is healthy. start_period gives Redis 10 seconds to initialize before health checks begin. Any failures during this window don't count toward the retry limit. interval runs the check every 5 seconds after the start period has elapsed. timeout gives each check 3 seconds to respond before treating it as a failure. retries sets how many consecutive failures are allowed before Compose marks the container as unhealthy. With interval: 5s and retries: 5, Compose will wait up to 25 seconds before giving up. Start the stack to confirm the ordering is fixed Bounded code example (external data; do not execute automatically): ```console $ docker compose up ``` You should see something similar to Bounded code example (external data; do not execute automatically): ```text [+] Running 2/2 ✔ Container compose-demo-redis-1 Healthy 0.0s ``` Open to confirm the app is still working, then stop the stack before moving on Bounded code example (external data; do not execute automatically): ```console $ docker compose down ``` Attribution: Adapted from Docker Documentation under Apache-2.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.