Docker: Defer startup actions relying on another container? - mysql

Question

I have two types of Docker containers: One with a web application (nginx/php-fpm) and one with a MySQL database. Both are connected through a network. The app container is aware of the DB container, however, the DB doesn’t know if zero, one or more app containers are available. Both types of containers use Supervisord.
The database container has to startup mysqld, which can take a few seconds.
The other container has to perform some startup actions, part of which require database access. As these actions depend on the DB container, I have put a loop at the top of the script, waiting for the DB server to become available:
try=0
ok=0
until mysql -h$dbhost -u$dbuser -p$dbpass -e "USE $dbname" && ok=1; do
[ $((++try)) -gt 30 ] && break
sleep 1
done
if [ $ok -gt 0 ]; then
# DO STUFF
else
exit 1
fi
While this does work, I see two downsides: First, the script will fail if a DB container is down or takes longer than a certain timeout to start when the app container comes up. Second, the app container won’t know if there are changes on the DB server (e.g. migrations).
While I’m aware of Supervisord events, I wonder: How can I notify an arbitrary amount of other containers in the same network of such events?
(NOTE: I’m not restricted to using Supervisord for this, I just feel that this is the most promising approach.)

Answer 1

You might want to use Compose.
Also you can add an healthcheck to your database container and add a condition for the web server container. Something like this.
healthcheck:
test: ["CMD-SHELL", "mysql_check.sh"]
interval: 30s
timeout: 30s
retries: 3
and
depends_on:
mysql-database:
condition: service_healthy
Compose will wait for the database to be ready before starting the webserver container.