Docker containers are designed with immutability in mind, meaning that the code and data within them are not meant to change after the container is built. This immutability is beneficial in ensuring consistency between development, testing, and production environments. It guarantees that the code running in production is the same as what passed quality assurance. However, this concept becomes less practical when you need to persist data or allow for changes to data over the lifetime of the containerized application.
Typically, data persistence can be handled by relying on external databases. However, there are scenarios where an application within a container requires access to local file systems or needs to maintain data in a way that mimics a local file system. This is where Docker volumes come into play, providing a solution for persisting data in a more flexible manner within the container environment.
Docker volumes are designed to handle local storage in a way that allows containerized applications to write and retrieve data through a file system-like interface. A volume maps a directory within the container (referred to as the mount point) to an external file system path. By doing this, any data written to the volume is stored externally, ensuring persistence beyond the lifecycle of a single container. One of the key benefits of volumes is that multiple containers can access the same volume simultaneously, making it easier to share data between containers—though this access must be carefully managed to prevent conflicts.
Under the hood, Docker volumes are managed by a volume driver, which determines where and how the data is stored. For example, a volume driver like Blockbridge allows access to iSCSI targets, while a more robust solution like REX-Ray can integrate with a variety of storage vendors and standards. REX-Ray works with Docker’s volume plug-in system or adheres to the Container Storage Interface (CSI) specification, providing greater flexibility for containerized applications that require more advanced storage solutions. However, while volumes offer a great way to manage persistent data, they are not a catch-all solution, and should be used with care to avoid pitfalls related to data integrity and performance.