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89 lines
3.0 KiB
Markdown
89 lines
3.0 KiB
Markdown
<!--[metadata]>
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+++
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title = "Controlling startup order"
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description = "How to control service startup order in Docker Compose"
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keywords = "documentation, docs, docker, compose, startup, order"
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[menu.main]
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parent="workw_compose"
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weight=90
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+++
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<![end-metadata]-->
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# Controlling startup order in Compose
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You can control the order of service startup with the
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[depends_on](compose-file.md#depends-on) option. Compose always starts
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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, Compose will 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, your application should 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 should eventually be able to 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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or [dockerize](https://github.com/jwilder/dockerize). These are small
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wrapper scripts which you can include in your application's image and will
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poll a given host and port until it's accepting TCP connections.
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Supposing your application's image has a `CMD` set in its Dockerfile, you
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can wrap it by setting the entrypoint in `docker-compose.yml`:
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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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entrypoint: ./wait-for-it.sh db:5432
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db:
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image: postgres
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- Write your own wrapper script to perform a more application-specific health
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check. For example, you might want to wait until Postgres is definitely
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ready to accept commands:
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#!/bin/bash
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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 psql -h "$host" -U "postgres" -c '\l'; 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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You can use this as a wrapper script as in the previous example, by setting
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`entrypoint: ./wait-for-postgres.sh db`.
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## Compose documentation
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- [Installing Compose](install.md)
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- [Get started with Django](django.md)
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- [Get started with Rails](rails.md)
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- [Get started with WordPress](wordpress.md)
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- [Command line reference](./reference/index.md)
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- [Compose file reference](compose-file.md)
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