When scaling, run some docker container startup commands only once - docker

Question

I have the following docker-compose.yml.
version: "3.1"
services:
db:
container_name: ${MYSQL_CONTAINER}
image: mysql:5.7.30
volumes:
- ${VOLUMES_DIR}/mysql_data:/var/lib/mysql
- ./slow_log.cnf:/etc/mysql/my.cnf
- ${VOLUMES_DIR}/mysql_logs:/var/log/mysql
environment:
- MYSQL_ROOT_PASSWORD=${MYSQL_PASSWORD}
- MYSQL_USER=${MYSQL_USER}
ports:
- ${MYSQL_PORT}:3306
entrypoint: ""
command: bash -c "chown -R mysql:mysql /var/log/mysql && exec /entrypoint.sh mysqld --default-authentication-plugin=mysql_native_password"
restart: on-failure
backend:
container_name: ${BACKEND_CONTAINER}
image: ${BACKEND_IMAGE}
depends_on:
- db
ports:
- ${BACKEND_PORT}
command: >
bash -c "command A
&& command B
&& ... "
restart: unless-stopped
I am scaling backend service so my startup command is sudo docker-compose -p ${COMPOSE_PROJECT_NAME} up -d --scale backend=10.
The problem I am facing is command A, command B in service backend was running for all 10 containers startup(means they were being run 10 times).
But I want command A to run only once for all the backend service-related containers but Command B should run for all containers.
Any suggestions in accomplishing this?

Answer 1

I'm not entirely sure that there would be an out-of-the-box solution for your requirement.
However, I can suggest you a workaround like this. You can duplicate your backend service in docker-compose and run one backend service with both Command A and Command B, while the other backend has only Command B.
Then when you want to scale, you scale the backend which has only Command B.
version: "3.1"
services:
db:
container_name: ${MYSQL_CONTAINER}
image: mysql:5.7.30
volumes:
- ${VOLUMES_DIR}/mysql_data:/var/lib/mysql
- ./slow_log.cnf:/etc/mysql/my.cnf
- ${VOLUMES_DIR}/mysql_logs:/var/log/mysql
environment:
- MYSQL_ROOT_PASSWORD=${MYSQL_PASSWORD}
- MYSQL_USER=${MYSQL_USER}
ports:
- ${MYSQL_PORT}:3306
entrypoint: ""
command: bash -c "chown -R mysql:mysql /var/log/mysql && exec /entrypoint.sh mysqld --default-authentication-plugin=mysql_native_password"
restart: on-failure
backend_default:
container_name: ${BACKEND_CONTAINER}
image: ${BACKEND_IMAGE}
depends_on:
- db
ports:
- ${BACKEND_PORT}
command: >
bash -c "command A
&& command B
&& ... "
restart: unless-stopped
backend:
container_name: ${BACKEND_CONTAINER}
image: ${BACKEND_IMAGE}
depends_on:
- db
ports:
- ${BACKEND_PORT}
command: >
bash -c "command B
&& ... "
restart: unless-stopped
Now you can use the scale option like below:
sudo docker-compose -p ${COMPOSE_PROJECT_NAME} up -d --scale backend=9
Now if there happens to be a scenario, where you need only 1 backend to be run, you can use profiles in docker-compose to only run backend when there is a specific profile is given with docker-compose command. That means only default_backend will run if that profile is not given and hence the scale is 1.
Hope this helps you. Cheers 🍻 !!!

Answer 2

If BACKEND_IMAGE is being built by you, you should do RUN command A in your Dockerfile. The RUN line will be executed only once during build time — so you will also need to make sure that this meshes with your needs — while the ENTRYPOINT and CMD lines will only be run upon execution of the container. The command in the docker-compose file overrides the CMD line.