While looking for ways to improve our service-startup in docker-compose I stumbled upon a nice feature introduced with Docker 1.12, which lets images expose a command to check that the created container is still working properly.
HEALTHCHECK --interval=5s --timeout=5s --start-period=15s --retries=5 CMD curl http://localhost:8080 || exit 1
The above line configures a healthcheck for a HTTP service running on
localhost:8080 inside the container. This check does a HTTP GET request every
5 seconds (
--interval). If the check passes, the status of the container will
be set to
healthy. If it fails, it will try again 5 times (
then set the health-status to
Services that take longer to start up (like most Java based applications) are
given a grace-period (
--start-period) in which healthcheck failures don’t
count to the retry-limit until either the period is over or the healthcheck has
passed at least once.
Healthchecks with docker run
Let’s say, we have a simple Postgres container that we want to monitor. The
official image doesn’t support this feature by default, but luckily the
docker run command has all the options discussed above exposed as command-line flags:
--health-cmd string Command to run to check health --health-interval duration Time between running the check (ms|s|m|h) (default 0s) --health-retries int Consecutive failures needed to report unhealthy --health-start-period duration Start period for the container to initialize before starting health-retries countdown (ms|s|m|h) (default 0s) --health-timeout duration Maximum time to allow one check to run (ms|s|m|h) (default 0s)
Taking our Postgres container as an example, you can add healthchecks like this:
docker run --rm \ -e POSTGRES_DB=testdb \ -e PGDATA=/var/lib/postgresql/data/pgdata \ -v $PWD/postgres_data:/var/lib/postgresql/data/pgdata \ --health-cmd "pg_isready -U postgres" \ --health-interval 5s \ postgres:9.6
If you now inspect the global docker events you can see lots of
container exec_start and
container health_status entries:
... 2017-09-02T19:14:00.033560520+02:00 container exec_create: /bin/sh -c pg_isready -U postgres 015f289346c02e3ac131948f39e0ca044a41641b9b41c23951862806db888c29 (image=postgres:9.6, name=pedantic_heisenberg) 2017-09-02T19:14:00.033701347+02:00 container exec_start: /bin/sh -c pg_isready -U postgres 015f289346c02e3ac131948f39e0ca044a41641b9b41c23951862806db888c29 (image=postgres:9.6, name=pedantic_heisenberg) 2017-09-02T19:14:00.165324872+02:00 container health_status: healthy 015f289346c02e3ac131948f39e0ca044a41641b9b41c23951862806db888c29 (image=postgres:9.6, name=pedantic_heisenberg) 2017-09-02T19:14:05.170512928+02:00 container exec_create: /bin/sh -c pg_isready -U postgres 015f289346c02e3ac131948f39e0ca044a41641b9b41c23951862806db888c29 (image=postgres:9.6, name=pedantic_heisenberg) ...
You can also see the healthiness of each running container in the output of
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 015f289346c0 postgres:9.6 "docker-entrypoint..." 5 minutes ago Up 5 minutes (healthy) 5432/tcp pedantic_heisenberg
It is also available through
$ docker inspect 015f289346c0 | jq '..State.Health.Status' "healthy"
Doing healthchecks in docker-compose
Getting back to my original use-case, healthchecks are also supported within docker-compose (since version 2.1 of the configuration file format):
version: '2.1' services: db: image: postgres:9.6 healthcheck: test: ["CMD", "pg_isready", "-U", "postgres"] interval: 5s retries: 10 start_period: 10s timeout: 2s app: # ... depends_on: db: condition: service_healthy
In the example above, the service “app” will wait for the db-service to be healthy before starting up. Normally, it would only wait until the container has been launched.
condition option within
depends_on has been removed with version
3 (See docker-compose (version 3): depends_on contains an invalid type, it should be an array
this comment by shin- the
reason behind this is that Docker seems to be slowly deprecating docker-compose
in favour of
docker stack and for that a unified approach is preferred.
What about docker-compose and v3?
If you cannot easily migrate up to docker stacks but need a way to start a service only if another service has reported that it’s ready, there are currently only some workarounds available. As described in the official documentation (Controlling startup order in Compose) you should use some kind of wrapper script to wait for a resource to become available within your own container.
Personally, I will most likely take a look at
docker stack next as I want to
play around with the
new secrets manager in Docker
Swarm anyway 😊