Control startup and shutdown order in Compose
You can control the order of service startup and shutdown with the depends_on attribute. Compose always starts and stops containers in dependency order, where dependencies are determined by depends_on, links, volumes_from, and network_mode: "service:...".
A good example of when you might use this is an application which needs to access a database. If both services are started with docker compose up, there is a chance this will fail since the application service might start before the database service and won't find a database able to handle its SQL statements.
Control startup
On startup, Compose does not wait until a container is "ready", only until it's running. This can cause issues if, for example, you have a relational database system that needs to start its own services before being able to handle incoming connections.
The solution for detecting the ready state of a service is to use the condition attribute with one of the following options:
service_startedservice_healthy. This specifies that a dependency is expected to be “healthy”, which is defined withhealthcheck, before starting a dependent service.service_completed_successfully. This specifies that a dependency is expected to run to successful completion before starting a dependent service.
Example
services:web:build:.depends_on:db:condition:service_healthyrestart:trueredis:condition:service_startedredis:image:redisdb:image:postgreshealthcheck:test:["CMD-SHELL","pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}"]interval:10sretries:5start_period:30stimeout:10sCompose creates services in dependency order. db and redis are created before web.
Compose waits for healthchecks to pass on dependencies marked with service_healthy. db is expected to be "healthy" (as indicated by healthcheck) before web is created.
restart: true ensures that if db is updated or restarted due to an explicit Compose operation, for example docker compose restart, the web service is also restarted automatically, ensuring it re-establishes connections or dependencies correctly.
The healthcheck for the db service uses the pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB} command to check if the PostgreSQL database is ready. The service is retried every 10 seconds, up to 5 times.
Compose also removes services in dependency order. web is removed before db and redis.
