deploy.replicas doesn't work with environment variables

[kinghuang]

Description

services.service.deploy.replicas in Compose file version 3 with Docker Compose 1.10-rc2 doesn't seem to work with environment variables. If a variable instead of a literal integer is used, then Docker Compose says it contains an invalid type

Steps to reproduce the issue:

1. Create a compose file where the value for replicas is a variable instead of a literal integer, like the following.

version: '3'
services:
  node-chrome:
    image: selenium/node-chrome:3.0.1
    deploy:
      replicas: $NUM_NODES_CHROME

2. Define a value for NUM_NODES_CHROME. In my case, I declared NUM_NODES_CHROME=2 in a .env file.

Describe the results you received:

Docker Compose says that replicas contains an invalid type.

ERROR: The Compose file './selenium.yml' is invalid because:
services.node-chrome.deploy.replicas contains an invalid type, it should be an integer

Describe the results you expected:

Docker Compose should substitute $NUM_NODES_CHROME with 2, treating the value as an integer.

Output of docker-compose --version:

docker-compose version 1.10.0-rc2, build fb241d0

[shin-]

This is complex, as environment variables are always expanded to string values. Even if unintuitive, Compose is behaving according to the rules here.

cc @dnephin : You might have more thoughts on the issue. We could allow string values and parse them for integers potentially.

[dnephin]

This has been an issue for a while, see #2730.

There's an old proposal PR to address this and cast to the type in the schema. It is indeed a difficult problem, but I think using the schema type and casting is a reasonable approach.

If we had a different variable syntax we might be able to include type hints with the variable, but it's too late for that I think.

[GabKlein]

Do you guys have a workaround for this issue?

[hutchic]

Yeah it's not pretty though. Make a docker-compose.yml template using language / library of your choosing (Python / Jinja). Have it substitute the values then run docker-compose

[rca]

This just bit me, I'm sad. Solved the issue in a similar manner to hutchic above. I felt like throwing out this suggestion just in case it makes sense: why not perform a pre-pass, replacing variables with the environment values, then letting whatever YAML parser do what it does. This, more or less, sums up what I ended up doing to resolve my problem.

[bradmccormack]

This just bit me too. Hopefully, this gets implemented.

[ryanmasse]

+1

[magrossi]

+1

[shin-]

#5291

[bitsofinfo]

+1

[timansky]

+1