docker-compose share named volume between services, with volume reading/writing from a specific directory

Question

This might be a duplicate, as i assume it's a common use case for docker-compose. However, i can't seem to find a similar question this specific scenario.

I have a docker-compose file with the following two services:

  processService:
    container_name: processservice
    image: mcr.microsoft.com/dotnet/core/sdk:3.1
    ports:
      - 5001:80
    volumes:
      - ./backend/packages/dotnet:/backend/packages/dotnet
      - ./backend/services/processService/src:/backend/services/processService/src
    working_dir: /backend/services/processService/src/ProcessService.Application
    command: dotnet watch run

  organizationService:
    container_name: organizationservice
    image: mcr.microsoft.com/dotnet/core/sdk:3.1
    ports:
      - 5002:80
    volumes:
      - ./backend/packages/dotnet:/backend/packages/dotnet
      - ./backend/services/organizationService/src:/backend/services/organizationService/src
    working_dir: /backend/services/organizationService/src/OrganizationService.Application
    command: dotnet watch run

As can be seen in the compose file, i use the same volume mount for both services, namely ./backend/packages/dotnet:/backend/packages/dotnet .

I want to know whether or not it is possible to declare this volume once in the volumes section, and then reuse it across services.

I have tried searching for an answer to this. The answers i come across suggest to create a volume manually with docker volume create foo and then using it as such:

...
volumes:
- foo:/backend/packages/dotnet
...

However, this doesn't make sense to me as I don't see how foo should be aware of the directory that I want it to read/write from when i havn't specified this at any point.

I have tried reading through the docker documentation, but I can't find anything related to creating a named volume mounting a specific directory.

Does anyone have any suggestions as to how to accomplish this?

Answer 1

These are two different volumes section in a docker-compose file:

1. local (service)

   Located into a service part (as you did in your file), local volumes sections map data volume(s) (or host path(s)) to a container location.

2. global

   Located at the same level as services: section, the global volumes section names and creates data volume(s).

Then, this is how to use data volumes within a docker-compose file:

service1:
  volumes:
    - vol_1:location_1

service2:
  volumes:
    - vol_1:location_2

volumes:
  vol_1:

Thus, in your case, the docker-compose part should be:

  processService:
    container_name: processservice
    image: mcr.microsoft.com/dotnet/core/sdk:3.1
    ports:
      - 5001:80
    volumes:
      - dotnet:/backend/packages/dotnet
      - processService:/backend/services/processService/src
    working_dir: /backend/services/processService/src/ProcessService.Application
    command: dotnet watch run

  organizationService:
    container_name: organizationservice
    image: mcr.microsoft.com/dotnet/core/sdk:3.1
    ports:
      - 5002:80
    volumes:
      - dotnet:/backend/packages/dotnet
      - organizationService:/backend/services/organizationService/src
    working_dir: /backend/services/organizationService/src/OrganizationService.Application
    command: dotnet watch run


volumes:
  dotnet:
  processService:
  organizationService: