This experiment tries to build a container using this Docker file:
FROM lambdalinux/baseimage-amzn:2016.09-000
COPY ./bundle /opt/bundle/
VOLUME /bundle
Then inside the container, create a /opt/bundle/file.txt and put some text in it. But that file did not show up in the bundle directory on the host as I expected after reading Should I include my code with COPY/ADD or a volume last paragraph:
There may be cases where you’ll want to use both. You can have the image include the code using a COPY, and use a volume in your Compose file to include the code from the host during development. The volume overrides the directory contents of the image.
Doesn't Dockerfile VOLUME do the same as docker-compose.yml VOLUME? If so, how can this be done so that changes in the host directory is reflected inside the container directory in this case?
I also created a file on the host bundle/play.txt but that did not show up inside the container /opt/bundle/...
A VOLUME instruction in the dockerfile creates a mount point but initially only maps it to Docker's internal data directory.
In order to map the volume to the host filesystem, you need to specify which path on the host should be mapped to the volume. You can do this in the docker-compose file using the volumes parameter. (Note: you can create volumes using docker-compose without declaring them in the Dockerfile.)
Note that when mapping a directory from the host to a container, the directory contents on the host will replace the contents in the container, not vice versa.
Related
Docker run command has option to mount host directory into container
-v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].
If "host-dir" is missing, then docker creates a new volume.
And Dockerfile has VOLUME instruction
VOLUME ["/data"] - The VOLUME instruction will add one or more new volumes
to any container created from the image.
From what I see, there is no way to specify host-dir or rw/ro status when using Dockerfile.
Is there any other use of VOLUME in docker file other than wanting to share it with some other container?
Dockerfiles are meant to be portable and shared. The host-dir volume is something 100% host dependent and will break on any other machine, which is a little bit off the Docker idea.
Because of this, it is only possible to use portable instructions within a Dockerfile. If you need a host-dir volume, you need to specify it at run-time.
A common usage of VOLUME from Dockerfile is to store configuration or website sources so that it can be updated later by another container.
I have a docker container image that requires me to mount a volume containing a specific configuration file, in order for that container to properly start (this image is not one that I have control over, and is vendor supplied). If that volume is not mounted, the container will exit because the file is not found. So I need to put a configuration file in /host/folder/, and then:
docker run --name my_app -v /host/folder:/container/folder image_id
The application will then look in /container/folder/ for the file it needs to start.
I want to create/commit a new image with that file inside /container/folder/, but when that folder is mounted as volume from the host, docker cp will not help me do this, as far as I have tried. I think, as far as docker is concerned, the file copied there is no different than the files in the mounted volume, and will disappear when the container is stopped.
Part of the reason I want to do this, is because the file will not be changed, and should be there by default. The other reason is that I want to run this container in Kubernetes, and avoid using persistent volumes there to mount these directories. I have looked into using configmaps, but I'm not seeing how I can use those for this purpose.
If you can store the file into the ConfigMap you can mount the file to volume and use it inside the Kubernetes.
I am not sure with the type of file you have to use.
ConfigMap will inject this file into the volume of a POD so the application could access and use it.
In this case there will be no PVC required.
You can also follow this nice example showing how to mount the file into a volume inside a pod.
OR
Also, I am not sure about the docker image but if you can use that docker image you can add the file into the path, something like:
FROM <docker image>
ADD file ./container/folder/
In this case, you might have to check you can use the vendor docker image as a base and add the file into it.
I have the following in my docker-compose.yml file:
volumes:
- .:/var/www/app
- my_modules:/var/www/app/node_modules
I do this because I don't have node_modules on my host and everything gets installed in the image and is located at /var/www/app/node_modules.
I have two questions regarding this:
An empty folder gets created in my host named node_modules. If I add another volume (named or anonymous) to the list in my .yml file, it'll show up in my host directory in the same folder that contains my .yml file. From this answer, it seems to have to do with the fact that there's these two mappings going on simultaneously. However, why is the folder empty on my host? Shouldn't it either a) contain the files from the named volume or b) not show up at all on the host?
How does Docker know to check the underlying /var/www/app/node_modules from the image when initializing the volume rather than just saying "Oh, node_modules doesn't exist" (since I'm assuming the host bind mount happens before the named volume gets initialized, hence /var/www/app should no longer have a folder named node_modules. It seems like it even works when I create a sample node_modules folder on my host and a new volume while keeping my_modules:/var/www/app/node_modules—it seems to still use the node_modules from the image rather than from the host (which is not what I expected, although not unwanted).
As an implementation detail, Docker actually uses the Linux kernel filesystem mount facility whenever it mounts a volume. To mount a volume it has to be mounted on to a directory, so if the mount target doesn't already exist, it creates a new empty directory to be the mount point. If the mount point is itself inside a mounted volume, you'll see the empty directory get created, but the mount won't get echoed out.
(If you're on a Linux host, try running mount in a shell while the container is running.)
That is:
/container_root/app is a bind mount to /host_path/app; they are they same underlying files.
mkdir /container_root/app/node_modules creates /host_path/app/node_modules too.
Mounting something else on /container_root/app/node_modules doesn't cause anything to be mounted on /host_path/app/node_modules.
...which leaves an empty /host_path/app/node_modules directory.
The first time you start a container, and only then, if you mount an empty volume into a container, the contents from the image get copied into the volume. You're telling Docker this directory contains critical data that needs to be persisted for longer than the lifespan of the container. It is not a magic "don't use the host directory volume" knob, and if you do things like change your package.json file, Docker will not update the contents of this volume.
I've searched the docs but nothing came up so time to test it. But for a quick future reference...
Is the host folder populated with the container folder contents?
Is it the opposite?
Are both folder contents merged? (In that case: What happens when a file with the same name is in both folders?)
Or does it produce an error? Is the error thrown on launch or is it thrown when you try to build an image with a VOLUME pointing to an existing populated folder on the container?
Also, another thing that isn't in the docs: Do I have to define the container path as a VOLUME in the Dockerfile in order to use -v against it when launching the container or can I create volumes on the fly?
When you run a container and mount a volume from the host, all you see in the container is what is on the host - the volume mount points at the host directory, so if there was anything in the directory in the image it gets bypassed.
With an image from this Dockerfile:
FROM ubuntu
WORKDIR /vol
RUN touch /vol/from-container
VOLUME /vol
When you run it without a host mount, the image contents get copied into the volume:
> docker run vol-test ls /vol
from-container
But mount the volume from the host and you only see the host's content:
> ls $(pwd)/host
from-host
> docker run -v $(pwd)/host:/vol vol-test ls /vol
from-host
And no, you don't need the VOLUME instruction. The behaviour is the same without it.
Whenever a Docker container is created with a volume mounted on the host, e.g.:
docker run -v /path/on/host:/data container-image
Any contents that are already in /data due to the image build process are always completely discarded, and whatever is currently at /path/on/host is used in its place. (If /path/on/host does not exist, it is created as an empty directory, though I think some aspect of that behavior may currently be deprecated.)
Pre-defining a volume in the Dockerfile with VOLUME is not necessary; all VOLUME does is cause any containers run from the image to have an implicit -v /volume/path (Note lack of host mount path) argument added to their docker run command which is ignored if an explicit -v /host/path:/volume/path is used.
Docker run command has option to mount host directory into container
-v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro].
If "host-dir" is missing, then docker creates a new volume.
And Dockerfile has VOLUME instruction
VOLUME ["/data"] - The VOLUME instruction will add one or more new volumes
to any container created from the image.
From what I see, there is no way to specify host-dir or rw/ro status when using Dockerfile.
Is there any other use of VOLUME in docker file other than wanting to share it with some other container?
Dockerfiles are meant to be portable and shared. The host-dir volume is something 100% host dependent and will break on any other machine, which is a little bit off the Docker idea.
Because of this, it is only possible to use portable instructions within a Dockerfile. If you need a host-dir volume, you need to specify it at run-time.
A common usage of VOLUME from Dockerfile is to store configuration or website sources so that it can be updated later by another container.