How to determine the number of replicas of scaled service - docker

Question

I have a docker-compose file that exposes 2 services, a master service and a slave service. I want to be able to scale the slave service to some number of instances using
docker-compose up --scale slave=N
However, one of the options I must specify on command run in the master service is the number of slave instances to expect. E.g. If I scale slave=10, I need to set --num-slaves=10 in the command on the master service.
Is there a way to determine the number of instances of a given service either from the docker-compose file itself, or from a customized entrypoint shellscript?
The problem I'm facing is that since there is no way I've yet found to specify the number of scaled instances from within the docker-compose file format itself, I'm relying on the person running the command to enter the scale factor consistently and to have that value align with the value I need to tell the master node to expect. And trusting users to do the right thing is a recipe for disaster. If I could continue to let the user specify the scale value on the command line, I need a way to determine what that value is at runtime.

Answer 1

scale is not added to up from compose version 3 but you may use replicas:
version: "3.7"
services:
redis:
image: redis:latest
deploy:
replicas: 1
and run it using:
docker-compose --compatibility up -d
docker-compose 1.20.0 introduces a new --compatibility flag designed
to help developers transition to version 3 more easily. When enabled,
docker-compose reads the deploy section of each service’s definition
and attempts to translate it into the equivalent version 2 parameter.
Currently, the following deploy keys are translated:
resources limits and memory reservations
replicas
restart_policy condition and max_attempts
but:
Do not use this in production!
We recommend against using --compatibility mode in production. Because
the resulting configuration is only an approximate using non-Swarm
mode properties, it may produce unexpected results.
see this
PS:
Docker container names must be unique you cannot scale a service beyond 1 container if you have specified a
custom name. Attempting to do so results in an error.

Answer 2

Unfortunately there is no way to define replicas for docker compose. IT ONLY WORKS FOR DOCKER SWARM The documentation specifies it link
Tip: Alternatively, in Compose file version 3.x, you can specify replicas under the deploy key as part of a service configuration for Swarm mode. The deploy key and its sub-options (including replicas) only works with the docker stack deploy command, not docker-compose up or docker-compose run.
So if you have the deploy section in the yaml, but run it with docker-compose, then it will not take any effect.
version: "3.3"
services:
alpine1:
image: alpine
container_name: alpine1
command: ["/bin/sleep", "10000"]
deploy:
replicas: 4
alpine2:
image: alpine
container_name: alpine2
command: ["/bin/sleep", "10000"]
deploy:
replicas: 2
So the only way to scale up in docker compose is by running the scale command manually.
docker-compose scale alpine1=3
Note I had a job in which they loved docker-compose so we had bash scripts to perform operations such as the ones you describe. So for example we would have something like ./controller-app.sh scale test_service=10 and it would run docker-compose scale test_service=10
UPDATE
To check the number of replicas you can mount the docker socket into your container. Then run docker ps --format {{ .Names }} | grep $YOUR_CONTAINER_NAME.
Here is how you would mount the socket.
docker run -v /var/run/docker.sock:/var/run/docker.sock -it alpine sh
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