Understanding volumn in docker compose

Question

The following is an example given in https://docker-curriculum.com/

version: "3"
services:
  es:
    image: docker.elastic.co/elasticsearch/elasticsearch:6.3.2
    container_name: es
    environment:
      - discovery.type=single-node
    ports:
      - 9200:9200
    volumes:
      - esdata1:/usr/share/elasticsearch/data
  web:
    image: prakhar1989/foodtrucks-web
    command: python app.py
    depends_on:
      - es
    ports:
      - 5000:5000
    volumes:
      - ./flask-app:/opt/flask-app
volumes:
    esdata1:
      driver: local

and it says The volumes parameter specifies a mount point in our web container where the code will reside about the /opt/flask-app

I think it means, /opt/flask-app is a mount point and it points to the host machines ./flask-app

However it doesn't say anything about esdata1 and I can't apply the same explanation as given to /opt/flask-app since there's no esdata1 directory/file in the host machine.

What is happening for the esdata1 ? My guess is that it means creating a volume (The closest thing I can think of is a disk partition) and name it esdata1 and mount it on /usr/share/elasticsearch/data, am I correct on this guess?

score 0 · Accepted Answer · answered Feb 04 '19 at 15:09

These are a bit different things: volumes and bind mounts. Bind mounts let you specify folder on host machine, which would serve as a storage. Volumes (at lease for local driver) also have folders on host machines, but their location is managed by Docker and is sometimes a bit more difficult to find.

When you specify volume in docker-compose.yml, if your path starts with / or . it becomes a bind mount, like in web service. Otherwise, if it is a single verb, it is a volume, like for es service.

You can inspect all volumes on your host machine by running docker volume ls.

What is happening for the esdata1 ? My guess is that it means creating a volume (The closest thing I can think of is a disk partition) and name it esdata1 and mount it on /usr/share/elasticsearch/data, am I correct on this guess?

That's all correct.

I do not pretend on setting up the rules, but in general, volumes are more suitable for sharing common data between several containers, like in docker-compose, while bind mounts suite better for sharing data from host to container, like some initial configs for services.

Thanks for quick answer, what about persistant data such as DB or elasticsearch data? which of `volumes` or `bind mounts` are preferred for those? — eugene, Feb 04 '19 at 15:25
It depends whether you need this data outside or not. If you use it internally in `docker-compose`, using volume is a better solution. — grapes, Feb 04 '19 at 15:33
Just written an answer few minutes ago on persisting the data: https://stackoverflow.com/a/54519405/10524386 The last part of the answer relates to the topic — grapes, Feb 04 '19 at 15:35
heh, that's me again actually, I'll be reading the answers there .. Thanks! — eugene, Feb 04 '19 at 15:44

Understanding volumn in docker compose

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