Docker

Run Context Flow of a Request When Executing Docker Commands

When you type a docker command, the request flows through multiple layers before the command is executed. Understanding this flow helps diagnose issues, optimize performance, and improve security.

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🔹 Context Flow of a Docker Command Execution

Let's break down what happens when you run a typical Docker command, such as:

shCopyEditdocker run -d -p 8080:80 --name mycontainer myimage

1️⃣ CLI (Command Line Interface) Processing

• The Docker CLI parses the command and validates syntax.

• If the command is incorrect, the CLI returns an error immediately.

• Otherwise, it sends the request to the Docker Daemon.

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2️⃣ Communication with Docker Daemon

• The CLI communicates with the Docker Daemon (dockerd) via:

  • UNIX socket (/var/run/docker.sock) on Linux/macOS.

  • Named pipe (npipe:////./pipe/docker_engine) on Windows.

  • TCP connection if the daemon is remote (DOCKER_HOST is set).

• The Daemon processes the request and checks:

  • Does the container name (mycontainer) already exist?

  • Is the requested image (myimage) available locally?

  • If not, does it need to pull the image from a registry?

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3️⃣ Image Resolution & Pulling

• If the image (myimage) is not found locally, Docker pulls it from:

  • The Docker Hub (default).

  • A private registry (if configured).

  • A local cache (if available).

• If the image is available locally, Docker skips the pull and proceeds.

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4️⃣ Container Creation & Configuration

Docker prepares the container environment by:

• Allocating a unique container ID.

• Assigning a namespace (to isolate processes, networking, and mounts).

• Setting up networking (bridge, host, or custom).

• Setting up volumes or bind mounts.

• Mapping ports (8080 → 80 in this case).

• Defining environment variables (if passed with -e).

• Assigning a cgroup for resource limits (CPU, memory).

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5️⃣ Container Start & Execution

• Docker initializes the container runtime (like runc).

• It creates the root filesystem based on the image layers.

• The entrypoint or command in the image is executed inside an isolated process.

Example of what happens internally:

shCopyEdit/usr/bin/docker-runc exec -d mycontainer /bin/sh -c "start-command"

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6️⃣ Container Lifecycle Management

• The container is now running.

• The process runs inside a sandboxed environment.

• Docker manages logs, health checks, and networking.

• If the container crashes or stops, Docker follows the defined restart policy (--restart flag).

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7️⃣ Output & Response to CLI

• If the command is successful:

  • The Docker daemon returns the container ID to the CLI.

  • If running in interactive mode (-it), the CLI attaches to the container’s STDOUT/STDERR.

• If there’s an error, Docker provides an error message (e.g., image not found, port conflict).

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🔹 Summary of the Request Flow

1. CLI Parsing → Validates command and sends request.

2. Daemon Communication → Sends request over UNIX socket/TCP.

3. Image Handling → Pulls image if not found locally.

4. Container Creation → Sets up filesystem, networking, volumes, cgroups.

5. Container Execution → Runs entrypoint and executes the process.

6. Lifecycle Management → Monitors logs, crashes, and health.

7. CLI Response → Returns container ID, logs, or errors.

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🔹 Example: Docker Run with Debug Mode

To see this context flow in action, you can use Docker’s debug mode:

shCopyEditDOCKER_HOST=unix:///var/run/docker.sock DOCKER_CLI_TRACE=1 DOCKER_CLI_DEBUG=1 docker run -d -p 8080:80 --name mycontainer myimage

• DOCKER_CLI_TRACE=1 → Traces CLI-to-daemon communication.

• DOCKER_CLI_DEBUG=1 → Shows detailed logs of the execution.

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💡 Key Takeaways

• Every docker command flows through the CLI → Daemon → Container Runtime.

• Docker manages networking, storage, and execution in an isolated environment.

• Debugging the execution flow can help resolve issues like:

  • Image not found

  • Port conflicts

  • Mounting issues

  • Network failures

Docker Bind Mounts vs. Volumes

Both bind mounts and volumes are used in Docker to persist data, but they have different use cases, benefits, and performance considerations.

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1. Bind Mounts

A bind mount is a direct mapping of a file or directory on the host machine to a container.

How It Works

• Uses an existing directory or file on the host.

• The container accesses the same data directly from the host’s filesystem.

• Changes in the container are reflected in the host and vice versa.

Syntax

shCopyEditdocker run -v /host/path:/container/path myimage

or (newer syntax):

shCopyEditdocker run --mount type=bind,source=/host/path,target=/container/path myimage

Pros

✅ Simple and fast since it directly uses the host’s filesystem.
✅ Useful for development when you want files to sync between host and container.

Cons

❌ Tightly coupled to the host machine’s directory structure.
❌ Less portable (paths may not exist on different hosts).
❌ Less secure (gives direct access to host files).

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2. Volumes

A volume is a Docker-managed storage system that persists data independently of the container.

How It Works

• Stored in /var/lib/docker/volumes/ on the host.

• Not directly tied to a specific host path (Docker manages it).

• Data persists even if the container is deleted.

Syntax

Creating a volume:

shCopyEditdocker volume create myvolume

Using a volume in a container:

shCopyEditdocker run -v myvolume:/container/path myimage

or (newer syntax):

shCopyEditdocker run --mount type=volume,source=myvolume,target=/container/path myimage

Pros

✅ More portable (works across different hosts).
✅ Better security (isolated from host filesystem).
✅ Docker manages volume lifecycle.

Cons

❌ Slightly more complex than bind mounts.
❌ Takes up disk space even after containers are removed (until manually deleted).

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Key Differences

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When to Use What?

• Use Bind Mounts when:

  • You need real-time sync with files on the host (e.g., development).

  • You want direct control over the files in your host.

• Use Volumes when:

  • You need persistent storage for a production app.

  • You want portability across different machines.

  • You need better security and performance.

🚨 Important: Avoid sending unnecessary files to the build context (use .dockerignore to exclude files).