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Use Docker's bind mounts to build a persistent log system

FireFrozen - - 4 mins read

Co-Authored with ctmbl

Before reading this article, it’s recommended to have a minimum knowledge of docker.
See, for example, the previous blog on docker: Get Started with Docker by ctmbl.

Also I strongly recommend you to read the Disclaimer.

A brief disclaimer

TLDR:
Volumes and bind mounts aren’t the same thing.
We can confuse --volume and --mount BUT we can’t confuse the concepts of volumes and bind mounts.
Volumes aren’t only managed by --volume, bind mounts aren’t only managed by --mount.

This article addresses the concept of bind mounts.

In Docker, two ways exist to persist/share data: volumes (named and anonymous) and bind mounts.

What we’re talking about in this blog aren’t volumes at all, they are bind mounts.
We’ll name them bind mounts but be aware that often on Internet you’ll encounter the word volume while the author is actually speaking of a bind mount.
This is certainly due to the fact that options --volume and --mount can be used almost interchangeably.

Even in docker’s doc the naming is confusing

Key differences exist between bind mounts and volumes, for more information see Manage data in Docker.

What is a Docker’s bind mount?

A docker’s bind mount is the action of mounting an existing file or directory of the host (the computer running the container) into a container. To rephrase, it enables a container to share a memory space with the host. So both the container and the user can access and modify it in real time and because it’s not “contained” in the container but only mounted, it’ll persist after the container’s death.

Why do we need bind mounts?

Docker containerization is very useful on lots of points for development but it’s very annoying if your application needs to produce persistent data, like logs. Indeed if you kill your container (to update it for example) or if it crashes, you lose everything that was inside and understanding why your application crashed without its logs can be very complicated or even impossible.

So we need to find a way to keep certain files after the container’s death and that can be achieved with bind mounts.

It’s also interesting to notice another really good usecase for bind mounts: during development you can share source code to execute it within the container while editing it in your IDE in the host

How to create a bind mount?

To create a bind mount, you need to add a specific argument to the docker run command (similar syntax exists for docker-compose.yml).

There is two different arguments which are really similar in their use. Those are --mount and --volume, both can be used to create bind mounts (but volumes too, see Disclaimer above).
However they slightly differ: if you enter a non existing file to mount, --volume will create it when --mount will return an error, also --volume needs fewer arguments.
For these reason and because we focus on simplicity in this post we’ll describe the --volume argument.

The exact syntax is:

docker run <other args> --volume path/in/host:path/in/container:<optional args>

Note that you can use -v instead of --volume

Note that you can mount a directory instead of a file with the same method

A similar syntax exists for docker-compose.yml

Optional args must be separated by comas, they are not used very often but lets detail the most useful one quickly :

  • ro : if used, the mounted file/folder will be in read-only mode inside the container

A concrete example with the code of our RootMe discord bot:

source ./.env.prod

docker run --rm --interactive --tty \
  --detach \
  --volume $(realpath -P ${LOG_FOLDER}):/opt/${NAME}/logs \
  --env-file .env.prod \
  --name ${NAME} \
  ${NAME}:latest

In this example LOG_FOLDER is an environment variable defined in the .env.prod file.
The code will write transparently to /opt/${NAME}/logs/<log file> but because this folder is shared with the host, logs will be available in the host and survive potential application crashes or updates.

WARNING : Be careful that the bot/application running in the container must have the right permission to modify the file/directory

Resources to go further

About volumes VS bind mounts:

About bind mounts specifically:

Other sources: