Dockerfiles, the blueprints of containerized applications, offer a powerful set of instructions to craft consistent and reproducible environments. Among these instructions, VOLUME stands out for its crucial role in data persistence and management. Understanding how to effectively leverage VOLUME is essential for any Docker practitioner seeking to build robust and scalable applications. This article delves into the intricacies of the VOLUME instruction, exploring its syntax, use cases, and best practices. Mastering this instruction will empower you to control your application’s data lifecycle and ensure data integrity across different environments.

What is the VOLUME Instruction?

The VOLUME instruction in a Dockerfile creates a mount point in the container, designated for storing persistent data independent of the container’s lifecycle. This means that even if a container is stopped, removed, or replaced, the data within the volume remains untouched. This is critical for preserving application data, configurations, and databases, ensuring data continuity and avoiding data loss.

Unlike binding mounts, which link a directory on the host machine to a directory in the container, volumes are managed by Docker itself. This offers greater flexibility and portability, as volumes can be easily shared between containers and managed through Docker commands.

Docker expert, Bret Fisher, emphasizes the importance of volumes: “Volumes are the preferred mechanism for persisting data generated by and used by Docker containers.” This highlights the industry-wide recognition of VOLUME as the standard for data management in Dockerized environments.

Syntax and Usage

The VOLUME instruction has a straightforward syntax:

VOLUME ["/path/to/directory1", "/path/to/directory2", ...]

This command creates one or more directories within the container at the specified paths, designating them as volumes. These directories are then managed by Docker, ensuring their persistence outside the container’s filesystem.

• Use absolute paths within the container for specifying volume locations.
• Multiple directories can be specified within a single VOLUME instruction.

Example: Creating a Volume for Application Data

VOLUME /app/data

This instruction creates a volume at /app/data inside the container. Any data written to this directory will persist even after the container is removed. This is ideal for storing application logs, user-uploaded files, or database data.

Use Cases for VOLUME

The VOLUME instruction finds application across diverse scenarios:

Data Persistence: The primary use case is to preserve application data across container restarts and deployments. This ensures data integrity and prevents data loss.

Sharing Data Between Containers: Volumes can be shared between multiple containers, allowing for seamless data exchange and collaboration. This facilitates microservice architectures and simplifies data management.

Backup and Restore: Docker provides commands to manage volumes, including backing up and restoring data. This simplifies data management and disaster recovery procedures.

Best Practices for Using VOLUME

To maximize the effectiveness of the VOLUME instruction, consider these best practices:

1. Plan your volume strategy: Determine which directories require persistence early in the development process.
2. Use named volumes: Assign names to your volumes for easier management and identification using docker volume create.
3. Avoid hardcoding paths: Use environment variables to define volume paths for greater flexibility.

By adhering to these best practices, you can ensure a robust and efficient data management strategy for your containerized applications.

VOLUME vs. Bind Mounts

While both VOLUME and bind mounts facilitate data persistence, they differ in their implementation and use cases. VOLUME offers better portability and is managed by Docker. Bind mounts, on the other hand, directly link directories on the host to the container, providing tighter control over data location on the host but sacrificing some portability.

Choosing between the two depends on your specific needs and priorities. For improved portability and easier management, VOLUME is generally preferred. Bind mounts are useful when precise control over host directory mapping is required.

[Infographic Placeholder: Comparing VOLUME and Bind Mounts]

Frequently Asked Questions (FAQ)

Q: How can I list existing volumes?

A: Use the command docker volume ls.

Q: How can I remove a volume?

A: Use the command docker volume rm <volume_name>.

Understanding and effectively utilizing the VOLUME instruction is fundamental to building robust and scalable Dockerized applications. By leveraging its capabilities for data persistence, sharing, and management, you can ensure data integrity and streamline your development workflow. Explore further resources and documentation to deepen your understanding of Docker volumes and unlock their full potential. Dive deeper into Docker best practices with this helpful resource: Learn more about Docker. This article provides a comprehensive understanding of the VOLUME instruction, empowering you to effectively manage data persistence in your Docker containers. For more in-depth information on Docker, refer to the official Docker documentation and explore best practices on writing efficient Dockerfiles. Consider further research into container orchestration platforms like Kubernetes for managing containerized applications at scale. Explore the Kubernetes documentation on Volumes to expand your knowledge.

Question & Answer :
Below is the content of my “Dockerfile”

FROM node:boron # Create app directory RUN mkdir -p /usr/src/app # Change working dir to /usr/src/app WORKDIR /usr/src/app VOLUME . /usr/src/app RUN npm install EXPOSE 8080 CMD ["node" , "server" ] 

In this file I am expecting VOLUME . /usr/src/app instruction to mount contents of present working directory in host to be mounted on /usr/src/app folder of container.

Please let me know if this is the correct way?

In short: No, your VOLUME instruction is not correct.

Dockerfile’s VOLUME specify one or more volumes given container-side paths. But it does not allow the image author to specify a host path. On the host-side, the volumes are created with a very long ID-like name inside the Docker root. On my machine this is /var/lib/docker/volumes.

Note: Because the autogenerated name is extremely long and makes no sense from a human’s perspective, these volumes are often referred to as “unnamed” or “anonymous”.

Your example that uses a ‘.’ character will not even run on my machine, no matter if I make the dot the first or second argument. I get this error message:

docker: Error response from daemon: oci runtime error: container_linux.go:265: starting container process caused “process_linux.go:368: container init caused “open /dev/ptmx: no such file or directory””.

I know that what has been said to this point is probably not very valuable to someone trying to understand VOLUME and -v and it certainly does not provide a solution for what you try to accomplish. So, hopefully, the following examples will shed some more light on these issues.

Minitutorial: Specifying volumes

Given this Dockerfile:

FROM openjdk:8u131-jdk-alpine VOLUME vol1 vol2 

(For the outcome of this minitutorial, it makes no difference if we specify vol1 vol2 or /vol1 /vol2 — this is because the default working directory within a Dockerfile is /)

Build it:

docker build -t my-openjdk 

Run:

docker run --rm -it my-openjdk 

Inside the container, run ls in the command line and you’ll notice two directories exist; /vol1 and /vol2.

Running the container also creates two directories, or “volumes”, on the host-side.

While having the container running, execute docker volume ls on the host machine and you’ll see something like this (I have replaced the middle part of the name with three dots for brevity):

DRIVER VOLUME NAME local c984...e4fc local f670...49f0 

Back in the container, execute touch /vol1/weird-ass-file (creates a blank file at said location).

This file is now available on the host machine, in one of the unnamed volumes lol. It took me two tries because I first tried the first listed volume, but eventually I did find my file in the second listed volume, using this command on the host machine:

sudo ls /var/lib/docker/volumes/f670...49f0/_data 

Similarly, you can try to delete this file on the host and it will be deleted in the container as well.

Note: The _data folder is also referred to as a “mount point”.

Exit out from the container and list the volumes on the host. They are gone. We used the --rm flag when running the container and this option effectively wipes out not just the container on exit, but also the volumes.

Run a new container, but specify a volume using -v:

docker run --rm -it -v /vol3 my-openjdk 

This adds a third volume and the whole system ends up having three unnamed volumes. The command would have crashed had we specified only -v vol3. The argument must be an absolute path inside the container. On the host-side, the new third volume is anonymous and resides together with the other two volumes in /var/lib/docker/volumes/.

It was stated earlier that the Dockerfile can not map to a host path which sort of pose a problem for us when trying to bring files in from the host to the container during runtime. A different -v syntax solves this problem.

Imagine I have a subfolder in my project directory ./src that I wish to sync to /src inside the container. This command does the trick:

docker run -it -v $(pwd)/src:/src my-openjdk 

Both sides of the : character expects an absolute path. Left side being an absolute path on the host machine, right side being an absolute path inside the container. pwd is a command that “print current/working directory”. Putting the command in $() takes the command within parenthesis, runs it in a subshell and yields back the absolute path to our project directory.

Putting it all together, assume we have ./src/Hello.java in our project folder on the host machine with the following contents:

public class Hello { public static void main(String... ignored) { System.out.println("Hello, World!"); } } 

We build this Dockerfile:

FROM openjdk:8u131-jdk-alpine WORKDIR /src ENTRYPOINT javac Hello.java && java Hello 

We run this command:

docker run -v $(pwd)/src:/src my-openjdk 

This prints “Hello, World!”.

The best part is that we’re completely free to modify the .java file with a new message for another output on a second run - without having to rebuild the image =)

Final remarks

I am quite new to Docker, and the aforementioned “tutorial” reflects information I gathered from a 3-day command line hackathon. I am almost ashamed I haven’t been able to provide links to clear English-like documentation backing up my statements, but I honestly think this is due to a lack of documentation and not personal effort. I do know the examples work as advertised using my current setup which is “Windows 10 -> Vagrant 2.0.0 -> Docker 17.09.0-ce”.

The tutorial does not solve the problem “how do we specify the container’s path in the Dockerfile and let the run command only specify the host path”. There might be a way, I just haven’t found it.

Finally, I have a gut feeling that specifying VOLUME in the Dockerfile is not just uncommon, but it’s probably a best practice to never use VOLUME. For two reasons. The first reason we have already identified: We can not specify the host path - which is a good thing because Dockerfiles should be very agnostic to the specifics of a host machine. But the second reason is people might forget to use the --rm option when running the container. One might remember to remove the container but forget to remove the volume. Plus, even with the best of human memory, it might be a daunting task to figure out which of all anonymous volumes are safe to remove.