What is a Docker Container's Lifecycle?

A Docker container’s lifecycle is about the different stages a container goes through. This starts from when we create it until we delete it. Knowing the lifecycle of a Docker container helps us manage containers better. It also helps developers and system admins improve their work and fix problems faster. Each stage in a container’s lifecycle involves certain actions and events. These can really affect how well an application works and how we use resources.

In this article, we will look closely at the lifecycle of a Docker container. We will talk about the main stages in a Docker container’s lifecycle. We will explain how to create and start a container. Then, we will discuss what happens when a container is running. Finally, we will cover how to stop and remove a container. We will also show how to check a Docker container’s lifecycle events. This will give us a good understanding of how to manage Docker containers well. Here are the topics we will discuss:

Understanding the Lifecycle of a Docker Container
What are the Key States in a Docker Container’s Lifecycle?
How to Create a Docker Container and Start it?
What Happens When a Docker Container is Running?
How to Stop and Remove a Docker Container?
How to Inspect a Docker Container’s Lifecycle Events?
Frequently Asked Questions

If we want to learn more about Docker and how it works, we can read articles like What is Docker and Why Should You Use It? and What is Containerization and How Does it Relate to Docker?. These articles will give us useful information.

What are the Key States in a Docker Container’s Lifecycle?

A Docker container goes through some key states in its lifecycle. These states are:

Created: The container is made but not running yet. It is in the “created” state after we make it from a Docker image.
```
docker create <image_name>
```
Running: The container is running its tasks. It moves to this state when we start it with the docker start command or when we create it using the docker run command.
```
docker run <image_name>
```
Paused: The container’s tasks are frozen for a while. This state is good when we want to save system resources without stopping the container.
```
docker pause <container_id>
```
Stopped: The container has stopped working. This happens when the main task inside the container ends or if we stop the container ourselves.
```
docker stop <container_id>
```
Exited: The container has finished its tasks and is not running anymore. We see this state after the main task in the container is done.
```
docker ps -a  # To see exited containers
```
Dead: The container is in a bad state and cannot recover. This usually happens because of a failure. It is not common and means the container stopped unexpectedly.

Knowing these states helps us manage containers better. We can move between these states using commands like docker start, docker stop, docker pause, and docker rm for cleanup. If we need more details on making containers, we can check How to Create a Docker Container from an Image.

How to Create a Docker Container and Start it?

We can create and start a Docker container by using the docker run command. This command makes a container from an image and starts it right away. Here are the basic steps and examples for this.

Basic Command Structure

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

Example: Creating and Starting a Container

If we want to create and start a simple Docker container that runs an Nginx web server, we can use this command:

docker run --name my-nginx -d -p 80:80 nginx

Explanation of Options

--name my-nginx: This gives a name to the container. It is optional.
-d: This runs the container in the background.
-p 80:80: This links port 80 of the host to port 80 of the container.

Running an Interactive Container

If we need to create and start a container in interactive mode, for example, for debugging, we can use:

docker run -it ubuntu /bin/bash

Additional Options

--rm: This will remove the container automatically when it stops.
-v /host/path:/container/path: This mounts a volume from the host to the container.

Example with Volume Mounting

docker run --name my-app -v /my/host/data:/data -d my-image

This command creates and starts a container called my-app using my-image. It also mounts a volume from the host.

Environment Variables

We can pass environment variables to the container too:

docker run -e MY_ENV_VAR=value -d my-image

Starting an Existing Container

If we have a container that was created but is not running, we can start it with:

docker start my-nginx

This command will start the container called my-nginx.

For more information about creating containers, we can check How to create a Docker container from an image.

What Happens When a Docker Container is Running?

When we run a Docker container, it works as a separate environment. This environment has everything we need to run an application. The container uses the host operating system’s kernel but has its own filesystem, processes, and network. Here is what happens when it is running:

Process Execution: The container runs a command in its own space. This command can be anything that we define in the Docker image.
File System: The container uses a layered filesystem from its base image. This includes any extra files, libraries, or binaries that we add when we build the container.
Networking: Each container has its own network namespace. This means it can have its own IP address and port numbers. Containers can talk to each other through defined networks or open ports.
Resource Management: Docker sets limits on resources like CPU, memory, and I/O. This helps to prevent the container from using too many resources from the host.
Logs: We can access the standard output and error logs of a running container using Docker logs. This is important for checking errors and monitoring the application.
State Persistence: Changes made while the container is running do not get saved to the original image unless we commit them. We can stop and start containers while keeping their state. We can use volumes or bind mounts for storage that stays even after the container stops.

Here is an example of how we can check the status of running containers:

docker ps

To see more details about a specific running container, we can use:

docker inspect <container_id>

Knowing what happens when a Docker container is running is very important for managing and fixing issues with containers. For more details on how to create and manage Docker containers, check out How to Create a Docker Container from an Image.

How to Stop and Remove a Docker Container?

We can stop and remove a Docker container easily with some Docker commands. Here are the steps to stop a running container and then take it away from our system.

Stopping a Docker Container

To stop a running Docker container, we use the docker stop command. We need to add the container ID or name after it. For example:

docker stop <container_id_or_name>

To find the container ID or name, we can list our running containers:

docker ps

Removing a Docker Container

After we stop the container, we can remove it with the docker rm command:

docker rm <container_id_or_name>

If we want to stop and remove a container using one command, we can do this:

docker rm -f <container_id_or_name>

This command stops the container forcefully if it is running. Then it removes it.

Removing All Stopped Containers

If we want to remove all stopped containers at once, we can use:

docker container prune

We will see a prompt to confirm. This command takes away all stopped containers. It helps free up space on our system.

Additional Considerations

Make sure we do not need the container data before we remove it. This action cannot be undone.
We can check for all containers, both running and stopped, with:

docker ps -a

For more details about managing Docker containers, we can read the article on how to stop and start Docker containers.

How to Inspect a Docker Container’s Lifecycle Events?

To inspect a Docker container’s lifecycle events, we can use the Docker events command or check the container’s logs. Docker keeps track of events that show us the changes and actions on containers.

Using Docker Events Command

The docker events command gives us real-time events from the Docker daemon. We can filter these events by container ID or name to see specific lifecycle events.

docker events --filter 'container=<container_name_or_id>'

Viewing Container Logs

We can also look at the logs of a container. This shows us the output and error messages. It helps us understand its lifecycle events.

docker logs <container_name_or_id>

Inspecting Container Details

To get more information about a container, like its status and settings, we can use the docker inspect command. This command gives us a JSON object with many details.

docker inspect <container_name_or_id>

Monitoring Container Status

We can check the current state of a container with this command:

docker ps -a

This command lists all containers and their status like running or exited. It helps us understand the lifecycle of a Docker container.

Example Workflow

Start a Docker container:
```
docker run -d --name my_container nginx
```

Inspect the container’s lifecycle events:

docker events --filter 'container=my_container'

View logs:
```
docker logs my_container
```
Inspect container details:
```
docker inspect my_container
```
Check the status:
```
docker ps -a
```

For more reading on Docker containers and how they work, we can look at this article on Docker containers.

Frequently Asked Questions

What is the lifecycle of a Docker container?

The lifecycle of a Docker container has different stages. It starts from creation to termination. First, we create a container from a Docker image. Then, it can be in states like running, paused, or stopped. Knowing this Docker container lifecycle is important for good deployment and management in containerized environments. For more details on how Docker works, you can check What is a Docker Container and How Does it Differ from a Virtual Machine.

How do you create and start a Docker container?

To create and start a Docker container, we use the Docker CLI. We can run the command docker run and add the image name to create and start a container at the same time. For example, docker run -d nginx creates and runs an Nginx container in detached mode. For more instructions, look at How to Create a Docker Container from an Image.

What are the primary states of a Docker container during its lifecycle?

A Docker container goes through several main states in its lifecycle. These states are created, running, paused, stopped, and deleted. Each state shows a specific phase in the container’s lifecycle. This affects how we manage and allocate resources. Knowing these states is important for good container orchestration. We can learn more in What is Containerization and How Does it Relate to Docker.

How can you stop and remove a Docker container?

To stop and remove a Docker container, we first use the docker stop command and add the container ID or name. Then we use the docker rm command to delete it. For example, we can write docker stop my_container and then docker rm my_container. This will stop and remove the container we choose. For more details, see How to Stop and Start Docker Containers.

How can you inspect a Docker container’s lifecycle events?

We can inspect a Docker container’s lifecycle events by using the docker events command. This command gives us a live stream of events about containers. Also, we can check a container’s logs with docker logs [container_id] to see what it outputs and its status. For deeper knowledge, visit How to Manage Docker Container Logs.