Are docker containers stateless?

With the continuous development of cloud computing technology, containerization technology, as the basic technology of cloud native, has attracted more and more attention and use. In containerization technology, Docker containers are the basic running environment for applications and one of the most commonly used container technologies. However, there are different opinions and misunderstandings about whether Docker containers are "stateless". This article will discuss this issue.

1. The concept of Docker container

Docker container technology is a lightweight virtualization technology that relies on the environment, configuration files, and code of applications or services. And data, etc. are packaged to build a runnable image, and then the image is deployed to different host nodes, ultimately forming a containerized service. Docker containers are similar to virtual machines, but containerization technology is more lightweight and flexible than virtualization technology, has faster startup time, and is more portable.

2. Stateless and Stateful

In cloud computing, we often hear the concept of "stateless". The so-called "stateless" means that the state information of the application is not stored within the application itself, but is stored in an external storage device (such as a database). This means that there is a gap between multiple instances of an application. can be replaced with each other, thus achieving high availability and horizontal expansion. Relative to "stateless", "stateful" means that the state information of the application is stored in the internal or shared storage device of the application instance, so that multiple instances of an application cannot be replaced with each other, because Their status information is different.

3. The "statefulness" of Docker containers

Since Docker containers are built based on images, Docker containers are generally considered to be "stateless", that is, The container itself does not carry any state information. Such a container can be deleted and rebuilt at any time without affecting the application's state information. Although the Docker container itself is stateless, we cannot assume that the applications in the Docker container are also stateless.

In actual applications, an application in a Docker container may need to read and write some status information, such as database files, etc. These data cannot be moved with the container, which means that these data must Stored in some form on the host machine. This also creates a strong coupling relationship between the container and the host, making the Docker container actually "stateful".

4. How to solve the "stateful" problem of Docker containers

There are two solutions to the "stateful" problem of Docker containers:

(1) Data volume technology: Data volume refers to a specific directory or file, which can be mounted from the host machine to the Docker container, and can realize data sharing between the host machine and the Docker container. In this way, we can mount the state information that the application depends on to the data volume to solve the "stateful" problem of Docker containers.

(2) Service orchestration technology: Service orchestration technology refers to managing multiple instances of an application through some orchestration tool (such as Kubernetes), and implementing functions such as load balancing and failover between these instances. . Through service orchestration technology, we can automatically deploy multiple application instances to achieve high availability and horizontal scalability.

5. Summary

The Docker container itself is stateless, but in actual applications, we need to store the status information of the application on the host machine, which requires Use data volume technology or service orchestration technology to solve the "stateful" problem of containers. Therefore, we cannot simply classify Docker containers as "stateless" or "stateful", but need to make judgments based on specific application scenarios. At the same time, when using Docker containers, you also need to consider the coupling relationship between the container and the host to ensure the operating efficiency and reliability of the application.

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