98 lines
3.3 KiB
Markdown
98 lines
3.3 KiB
Markdown
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---
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description: How to control service startup and shutdown order in Docker Compose
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keywords: documentation, docs, docker, compose, startup, shutdown, order
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title: Control startup and shutdown order in Compose
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notoc: true
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---
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You can control the order of service startup and shutdown with the
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[depends_on](compose-file/index.md#depends_on) option. Compose always starts and stops
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containers in dependency order, where dependencies are determined by
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`depends_on`, `links`, `volumes_from`, and `network_mode: "service:..."`.
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However, for startup Compose does not wait until a container is "ready" (whatever that means
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for your particular application) - only until it's running. There's a good
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reason for this.
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The problem of waiting for a database (for example) to be ready is really just
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a subset of a much larger problem of distributed systems. In production, your
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database could become unavailable or move hosts at any time. Your application
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needs to be resilient to these types of failures.
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To handle this, design your application to attempt to re-establish a connection to
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the database after a failure. If the application retries the connection,
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it can eventually connect to the database.
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The best solution is to perform this check in your application code, both at
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startup and whenever a connection is lost for any reason. However, if you don't
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need this level of resilience, you can work around the problem with a wrapper
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script:
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- Use a tool such as [wait-for-it](https://github.com/vishnubob/wait-for-it),
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[dockerize](https://github.com/jwilder/dockerize), sh-compatible
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[wait-for](https://github.com/Eficode/wait-for), or [RelayAndContainers](https://github.com/jasonsychau/RelayAndContainers) template. These are small
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wrapper scripts which you can include in your application's image to
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poll a given host and port until it's accepting TCP connections.
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For example, to use `wait-for-it.sh` or `wait-for` to wrap your service's command:
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```yaml
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version: "2"
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services:
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web:
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build: .
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ports:
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- "80:8000"
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depends_on:
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- "db"
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command: ["./wait-for-it.sh", "db:5432", "--", "python", "app.py"]
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db:
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image: postgres
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```
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> **Tip**
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>
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> There are limitations to this first solution. For example, it doesn't verify
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> when a specific service is really ready. If you add more arguments to the
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> command, use the `bash shift` command with a loop, as shown in the next
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> example.
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- Alternatively, write your own wrapper script to perform a more application-specific
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health check. For example, you might want to wait until Postgres is ready to
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accept commands:
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```bash
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#!/bin/sh
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# wait-for-postgres.sh
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set -e
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host="$1"
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shift
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cmd="$@"
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until PGPASSWORD=$POSTGRES_PASSWORD psql -h "$host" -U "postgres" -c '\q'; do
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>&2 echo "Postgres is unavailable - sleeping"
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sleep 1
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done
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>&2 echo "Postgres is up - executing command"
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exec $cmd
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```
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You can use this as a wrapper script as in the previous example, by setting:
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```yaml
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command: ["./wait-for-postgres.sh", "db", "python", "app.py"]
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```
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## Compose documentation
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- [User guide](index.md)
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- [Installing Compose](install.md)
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- [Getting Started](gettingstarted.md)
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- [Command line reference](reference/index.md)
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- [Compose file reference](compose-file/index.md)
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- [Sample apps with Compose](samples-for-compose.md)
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