Docker - Data Storage

---

---

By design, data should not generally be persisted directly in a Docker container for a few reasons. First, containers were always intended to be transient. In other words, they can be stopped, started, or, in theory, destroyed at any time. Data that are stored inside a container is consequently lost each time a container stops existing. With that said, this makes data persistence and recovering your data hard.

Second, the writable layer of a container can be heavily coordinated with the host machine on which it is running, often making it hard to move it to another machine or to extract data. Furthermore, the writing in this layer is usually performed using a storage driver and a union file system, which may cause performance overhead compared to the writing of the host's file system.

Data can be stored within a container, too. This can lead to problems with scaling and sharing, as more than one container may wish to access the same data, making management and keeping the said data in synchronization complex. That is why it is much better to use the Docker volumes or bind mounts for storing data out of the container, which will provide persistence, portability, and easy access.

In this chapter, let’s discuss on how volumes and bind mounts can be used to persist data in Docker containers.

Different Ways to Persist Data in Docker Containers

Whether you use mount types volume, bind mount, or tmpfs, the data inside the container is presented as a directory or file within the container's filesystem. Here is the crucial difference: the location on the Docker host where the persistent data resides.

Volumes live in a Docker-managed part of the host filesystem, usually at /var/lib/docker/volumes/ on Linux. This area is not accessible to natively running Docker processes, so volumes are the only applicable mechanism for holding data persistently in Docker.

Bind mounts, on the other hand, can be located anywhere in a host system, even some crucial system files, and therefore, can be changed by a process not managed by Docker. This makes them more flexible but less isolated. Finally, tmpfs mounts exist only in the host system's memory and never touch the underlying filesystem - perfect for ephemeral, non-persistent data.

The -v or --volume flag allows specifying a mounting point for volumes or bind mounts. The syntax is slightly different: use the --tmpfs flag for tmpfs mounts. But for maximum readability and clarity, whenever possible, use --mount with all the options merged and nested inside.

Docker Volumes

Volumes are the preferred way for persisting data generated by and used in Docker containers. Docker manages them and is independent of whatever the host machine's filesystem is. There are also several benefits to using them over other storage strategies like bind mounts.

Key Features of Docker Volumes

• Persistence − Data stored in volumes will outlive the lifecycle of a stopped, removed, or replaced container.
• Portability − It's easy to backup, migrate, or share among multiple containers with volumes.
• Management − Control and manage Docker volumes with Docker CLI commands or via the Docker API.
• Cross-platform compatibility − Runs on Linux and Windows containers with remarkable consistency.
• Performance − Volumes have more optimal performance with Docker Desktop than bind mounts from Mac and Windows hosts.

Creating a Volume

This is the basic command to create a new volume with the name "my-vol."

$ docker volume create my-vol

Attach a Volume to a Container

The below command attaches the "my-vol" volume to the "/app/data" directory within the container. If any data is written to this directory, it will be stored in the volume persistently.

$ docker run -d --name my-container -v my-vol:/app/data my-image

Listing Volumes

This command lists all the volumes that are available in your Docker environment.

$ docker volume ls

Inspecting a Volume

This command gives detailed information about the volume, including the mount point, driver, and other details.

$ docker volume inspect my-vol

Removing a Volume

This command removes the "my-vol" volume. Warning: The data in the volume is destroyed irreversibly.

$ docker volume rm my-vol

Real-World Use Cases of Docker Volumes

• Databases − The database files of the data should be stored in a volume that will make it persistent across all container restarts.
• Web Server Content − Storing website files or user uploads within a volume, so even when the web server container is replaced, they remain accessible.
• Application Logs − Store logs in a volume for easy analysis and persistence.

Docker volumes bring strong and flexible management of persistent data inside contained applications. Data remains secured and accessible even with the leverage of volumes in dynamic container environments.

Bind Mounts

Bind mounting in Docker is a way to directly share files or directories from the host machine into a Docker application. Bind mounts directly associate a file or directory from the host machine to a path in the container; unlike volumes, they do not need to be managed since Docker manages them.

Key Features of Mount Bind

• Direct Access − Any changes made to the files on the host are immediately reflected within the container, and vice versa.
• Flexibility − You can mount any location on your host system, including system files, configuration files, or your project's source code.
• Development Workflow − In development, bind mounts prove to be a boon for you, as you can edit code on your host drive, and the changes taking place in the running container are seen close to immediately.

Mount Host Directory

The below command mounts the current directory on your machine to the container's '/app' directory. Any changes to the files inside the current directory will reflect inside the container and vice versa.

$ docker run -d --name my-container -v $(pwd):/app my-image

Mount a Single File

This would mount the host file "file.txt" to the path "/etc/config.txt" in the container.

$ docker run -d --name my-container -v /path/to/file.txt:/etc/config.txt my-image

Using the --mount Flag

The --mount flag allows for more verbose specification on a bind mount, stating its type, source, and target explicitly.

$ docker run -d --name my-container --mount 
   type=bind,source="$(pwd)",target=/app my-image

Real-Life Applications of Bind Mounts in Docker

• Dev Environments − Make directories containing source codes mountable so that changes in the source can be live updated.
• Configuration Files − Mount your host's configuration files into the container to customize its behavior.
• Share Host Resources − mount files or directories that need access from the container - e.g., log files and data files.

Named Pipes and TMPFS

In Docker, you can store data in hosts’ system memory with the help of tmpfs mounts and named pipes, though they are implemented differently in different operating systems.

tmpfs Mounts (Linux)

When using Docker on Linux, a tmpfs mount is used to create a temporary filesystem held in memory. This implies that files written in a tmpfs mount are not persisted to disk and are hence ideal for the storage of sensitive information or temporary data, not needing to outlive the Container.

tmpfs operates from memory; therefore, it makes it a lot faster in reading and writing than the old, disk-based storage method. However, the data in tmpfs is volatile and will be lost in case the host system reboots or the container is stopped.

Named Pipes (Windows)

In Windows, named pipes work pretty similarly to tmpfs mounts to store data in memory. They enable processes to communicate with each other, which can store their data in the container's temporary memory.

Like tmpfs, the contents of named pipes are not written to a disk, and they are lost once the container stops. Named pipes are one of the basic mechanisms of inter-process communication in Windows, and Docker utilizes their functionalities to provide in-memory storage capabilities on Windows hosts.

Both tmpfs mounts and named pipes are designed to be supportive in use cases where performance, and not the persistence of data, is vital. They serve well to store temp files, cache, or sensitive information that should not be written on the disk.

When to Use Docker Volumes and Bind Mounts?

Volumes are the best way to handle persistent storage within Docker. It is perfect for sharing data between containers, where you cannot guarantee the host's file structure, storing data remotely, situations in which you have to back up, restore, or migrate data, and much more. In addition, volumes are more performant and natively provide the file system behavior for I/O-intensive applications on Docker Desktop.

Bind mounts, in contrast, link files/directories from the host directly to the container's path. Often, they are helpful by allowing a user to share a configuration file or source code between the host and the containers, especially in the development environment. But be cautious while using bind mounts with sensitive data because changes in the container directly impact the host.

Tmpfs mounts, being completely memory-based and temporary, fit exceptionally well with non-persistent data, like caches or sensitive information. They focus on speed and security, hence data persistence is not their concern.

Conclusion

So there you have it. You have mastered Docker storage options: volumes, bind mounts, and tmpfs mounts to optimize data management in containerized applications. Knowing their differences will let you make an educated choice of where and how to store your data.

Volumes give you persistence, portability, and isolation, making them suitable to hold precious data that needs to live longer than individual containers. Bind mounts are much more flexible and can offer real-time access to host files; they are helpful for the development and sharing of particular resources. Tmpfs can be mounted, prioritizing speed and security; it gives temporary storage in memory for sensitive or transient data.

A lot shall depend on the specific requirements and use case for which you'll need the proper storage mechanism. By considering factors like data persistence, access patterns, and performance needs, Docker's storage options enable one to build an efficient reliable, and secure containerized application.

FAQ About Docker Data Storage

Q 1. What happens to my data when a Docker container stops or is removed?

The data created directly in the container writable layer is lost when a container is stopped or removed. This is because the containers are supposed to be ephemerally created.

To ascertain the persistence of data, data storage mechanisms such as Docker volumes and bind mounts are put in place, whereby the data is stored externally on the host filesystem or using Docker-managed storage and linked to the container.

Q 2. What is the difference between Volumes and Bind Mounts in Docker?

Volumes are the Docker-preferred way to deal with persistent storage. Managed by Docker itself, they reside independently of any single container but in a way that some platforms provide excellent lifecycle features for them, like portability, easy backup, and better performance.

Bind mounts are direct links into a directory or file of the host from the container. This way, they serve to share files directly with the container and are, in fact, superior to volumes for this purpose: in most ways, more secure and certainly more portable.

Q 3. What is a tmpfs mount in Docker, and when should they be used?

Tmpfs mounts are temporary filesystems that solely reside in the host system's memory. As such, they are not persistent and are thus ideal for ensuring that sensitive data or temporary files do not outlive the container's life.

Though tmpfs mounts are suitable with their read and write operations, they are volatile: the data gets lost when the host reboots or the container is stopped.

Q 4. Can I use AWS S3 or Azure Blob storage solutions with Docker?

With working through Docker, you can still utilize any of the cloud storage solutions. Still, it's more about interfacing with the SDK or API for the cloud provider rather than mounting their cloud storage directly into a volume. This allows the user to store and retrieve data from the cloud; the solution is almost infinitely scaled and is durable.

Q 5. How can I secure the data stored in Docker volumes?

Potential measures of securing data in Docker volumes include isolating access to the volumes using proper permissions and avoiding shared volumes without discretion, having proper backup mechanisms in place for your volumes by regularly backing them up to external storage for disaster recovery, encrypting the contents of the volume in case you are storing sensitive data, or encrypt the whole file system of the host, and finally, keeping yourself constantly up to date with a new security update and best practice from Docker to secure and protect your environment from vulnerabilities.