Docker engine and runtime - Step-by-Step Execution

Process Flow - Docker engine and runtime

User runs docker command

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Docker CLI sends request

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Docker Engine receives request

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Docker Engine communicates with Runtime

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Runtime creates/starts container

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Container runs isolated process

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Docker Engine monitors container

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User gets output or status

The Docker CLI sends commands to the Docker Engine, which uses the runtime to create and manage containers running isolated processes.

Execution Sample

Docker

docker run --name myapp alpine echo Hello

Runs a container named 'myapp' from the Alpine image that prints 'Hello' and then stops.

Process Table

Step	Action	Component	Result	Notes
1	User runs 'docker run --name myapp alpine echo Hello'	Docker CLI	Command sent to Docker Engine	CLI parses command and sends request
2	Docker Engine receives command	Docker Engine	Validates image 'alpine'	Checks local cache or pulls image
3	Docker Engine requests Runtime to create container	Docker Engine -> Runtime	Container 'myapp' created	Runtime sets up container environment
4	Runtime starts container process	Runtime	Runs 'echo Hello' inside container	Process runs isolated from host
5	Container outputs 'Hello'	Container process	Output sent to Docker Engine	Output captured by Docker Engine
6	Docker Engine sends output to CLI	Docker Engine -> CLI	User sees 'Hello'	Command completes
7	Container stops after command finishes	Runtime	Container 'myapp' exits	Container lifecycle ends
8	Docker Engine updates container status	Docker Engine	Container marked as stopped	Ready for inspection or removal

💡 Container process finishes, container stops, and Docker Engine updates status

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 7	Final
Container State	none	created	running	exited	exited
Process Output	none	none	Hello	Hello	Hello
Image Availability	may need pull	available	available	available	available

Key Moments - 3 Insights

Why does the container stop after printing 'Hello'?

Because the container runs the command 'echo Hello' which finishes immediately, so the runtime stops the container process as shown in execution_table step 7.

What role does the Docker Engine play between CLI and runtime?

Docker Engine acts as the middleman: it receives commands from CLI, manages images, and instructs the runtime to create and run containers, as seen in steps 2 and 3.

How does the output 'Hello' reach the user?

The container process outputs 'Hello', Docker Engine captures it, then sends it back to CLI to display to the user, shown in steps 5 and 6.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the container start running?

A Step 4

B Step 5

C Step 3

D Step 7

💡 Hint

Check the 'Action' and 'Result' columns for when the container process begins.

According to the variable tracker, what is the container state after step 7?

A created

B running

C exited

D none

💡 Hint

Look at the 'Container State' row under 'After Step 7' in variable_tracker.

If the image 'alpine' was not available locally, which step would change?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Refer to the 'Notes' in step 2 about image validation and pulling.

Concept Snapshot

Docker Engine receives commands from CLI
It manages images and container lifecycle
Runtime creates and runs containers
Containers run isolated processes
Output flows back through Engine to CLI
Container stops when process ends

Full Transcript

When you run a docker command like 'docker run', the Docker CLI sends this command to the Docker Engine. The Engine checks if the image is available locally or pulls it if needed. Then it asks the runtime to create a container and start the process inside it. The container runs the command isolated from the host. The output from the container process is captured by the Engine and sent back to the CLI so you can see it. When the command finishes, the container stops and the Engine updates its status. This flow ensures containers run quickly and isolated, managed by the Docker Engine and runtime.