Docker-compose: how to do version 2 "mem_limit" in version 3? - docker

Question

Recently, I tried upgrading a version 2 docker-compose yaml file to version 3. Specifically, I was going from 2.1 to 3.4. Using docker-compose version 1.18.0 and docker version 18.06.01.
The first attempt caused docker-compose to abort because of the presence of the Version 2 option: mem_limit. Reading these Version 3 docs, it clearly states mem_limit was removed and to see "upgrading" to guide usage away from this option. These instruction tell you to use the deploy section with resources. Making these changes to the docker-compose.yml file and the system started normally.
Unfortunately, I missed the disclaimer in there where it states that deploy is ignored by docker-compose! My question: is there a way to use Compose file reference 3 and docker-compose while still enforcing a container memory limit?

Answer 1

No, there is not.
Between versions 2.x and 3.x...several options have been removed
...mem_limit, memswap_limit: These have been replaced by the resources key under deploy. deploy configuration only takes effect when using docker stack deploy, and is ignored by docker-compose.
See Compose: Upgrading from 2 to 3
And also you don't have to upgrade, you don't even have any reason to upgrade if you don't use swarm.
Sadly in the official docker docs, there is stated
Version 3 (most current, and recommended)
which isn't actually really true, if you use docker-compose without swarm, there is hardly any reason to switch or to use on new project v3. In the official repository you can see comments like this [2][3].
Also in the compatibility-matrix you can see that v2 is still upgraded even when v3 is out for quite some time. And only v1 is marked as deprecated.

Answer 2

Here's an example Docker Compose file version 3.8 with memory limitations:
version: '3.8'
services:
web:
image: nginx:latest
deploy:
replicas: 3
update_config:
parallelism: 2
delay: 10s
restart_policy:
condition: on-failure
resources:
limits:
memory: 256M
ports:
- "80:80"
volumes:
- type: volume
source: myapp-data
target: /var/www/html
networks:
- myapp-net
environment:
- NGINX_HOST=example.com
- NGINX_PORT=80
db:
image: mysql:latest
deploy:
replicas: 1
restart_policy:
condition: on-failure
resources:
limits:
memory: 512M
volumes:
- type: volume
source: myapp-db
target: /var/lib/mysql
networks:
- myapp-net
environment:
- MYSQL_ROOT_PASSWORD=root
- MYSQL_DATABASE=myapp
volumes:
myapp-data:
myapp-db:
networks:
myapp-net: