ERROR: In file './docker-compose.yml', volume must be a mapping not a string

Question

Question: Why do I get this error?

ERROR: In file \'./docker-compose.yml\', volume \'mariavolume\' must be a mapping not a string.

Answer 1

Unfortunately, there is no such a feature.

You can’t map a top-level volume in docker-compose.

Here are the options:

• Adding volume per container and map it there. (like what Daniel did here)

• Create a volume outside of the compose (with mapping) and use it in your compose.

  volumes:
     maria_volume: 
         external:
             name: volume-name
  

Answer 2

correct syntax for volume is
volumes: first:
where first is just a placeholder name to be used for volume

Answer 3

I've just tackled this issue myself. If you just want a volume to store data, then do as below. This will create/reuse a volume that is persisted to disk as part of the Docker graph driver.

The next question is where is this?.

You can find it inside the docker image - Default Location -

C:\Users\Public\Documents\Hyper-V\Virtual hard disks

db:
  image: mariadb
  restart: always
  environment:
    MYSQL_ROOT_PASSWORD: example
  volumes:
    - maria_volume: /var/lib/mysql

volumes:
  maria_volume:

Of course, if you are after mapping a host directory into docker rather than having it inside the Docker graph driver. Then you can do it as follows.

db:
  image: mariadb
  restart: always
  environment:
    MYSQL_ROOT_PASSWORD: example
  volumes:
    - maria_volume: /var/lib/mysql

volumes:
  maria_volume:
    driver: local
    driver_opts:
      o: bind
      type: none
      device: /C/mariadb

Please note, when mapping over host directories as volume (at least on windows) you can have issues with read/write permissions, something I have yet to resolve myself.

Answer 4

try this:

    volumes:
        - maria_volume: /var/lib/mysql
volumes:
    maria_volume: 
        external:
            name: ~/mariadb

Answer 5

I was running into the same issue as yourself and as a last act of despair I tried putting

volumes:
  - maria_volume: /var/lib/mysql

before

environment:
  MYSQL_ROOT_PASSWORD: example

I'm not sure what kind of magic applied here but in my case, it worked

Let me know!

Answer 6

In my case this was happening because I missed adding a : after the volume name.

Instead of:

volumes:
    - mysqldata:

I had typed:

volumes:
    - mysqldata

docker-compose up gave me the same error as above.