Introduction

Docker has become a pivotal tool in contemporary software development, offering a standardized platform for deploying applications. It achieves this by encapsulating an application, along with its dependencies, within a container. However, when working with Docker, it’s essential to understand the distinction between running a Docker container and starting one. This subtle difference has a significant impact on how applications are deployed and maintained. Let's delve deeper into these concepts.

Running a Docker Container

When we talk about "running" a Docker container, it generally refers to the command that both creates and starts a container in one go. Here’s the typical command used:

docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

Key Aspects of docker run:

• Creation and Execution: docker run does both—it initializes a fresh container from an image and then starts it. This is ideal for scenarios where you want to quickly deploy an application environment.
• Options: This command includes several options, such as:
  • -d: Run the container in detached mode.
  • -p: Publish a container's port(s) to the host.
  • -e: Set environment variables.
  • --name: Assign a name to the container.
• Execution of Default Command: By default, docker run executes the command specified in the Dockerfile's CMD or ENTRYPOINT. However, you can override this behavior by providing a different command at the end of the docker run command.
• Example:

  docker run -d --name mywebserver -p 8080:80 nginx

In this example, Docker creates a new Nginx container named mywebserver running in detached mode.

Starting a Docker Container

Once a container is created, you can simply start it again without re-creating it. The docker start command is used primarily for this.

docker start [OPTIONS] CONTAINER [CONTAINER...]

Key Aspects of docker start:

• Reusability: Unlike docker run, docker start doesn't create a new container; it uses an already existing one. This is beneficial for maintaining state between restarts.
• No New Options: The configuration from the container’s creation (e.g., port mappings, environment variables) persists, and cannot be altered using docker start.
• Faster Initialization: Since it skips the creation phase, starting a container is typically faster than running a new one.
• Example:

  docker start mywebserver

This command will start the existing mywebserver container with its initial configuration.

Technical Implications

• Stateless vs. Stateful: docker run is typically used when creating stateless applications where the state doesn't need to persist. On the other hand, docker start is more applicable for stateful applications that require persistent state throughout container lifecycles.
• Data Management: When running a new container, any data written to the container's filesystem that isn’t stored in volumes or bind mounts will be lost upon container termination. Starting an existing container retains prior data unless explicitly deleted or modified.

Summary Table

Additional Insights

• Integration Use: For seamless Continuous Integration/Continuous Deployment (CI/CD) systems, understanding when to use docker run versus docker start is critical. Utilizing docker start as part of a hot-reload development cycle can save significant time and compute resources.
• Resource Management: Efficiently using system resources is essential for infrastructure cost management. Running many containers simultaneously can lead to resource contention, which can be mitigated by effectively starting and stopping containers instead of creating new ones each time.

Conclusion

While both docker run and docker start are vital commands in the Docker ecosystem, they serve different purposes and are suited to various scenarios. A solid grasp of these commands, alongside knowledge of Docker's foundational principles, will enhance your ability to deploy and manage containerized applications efficiently.