DOCKER · MEDIUM CODEX

Docker Interview Questions — Medium

Q: Detail the architecture of the Docker Engine.

Client-Server Architecture: Composed of three primary parts: Docker Client (docker): The CLI tool that accepts user commands and communicates with the daemon over a REST API (via Unix sockets or TCP).

Q: How does BuildKit differ from the legacy Docker builder?

BuildKit: The modern, highly optimized build engine (enabled via DOCKERBUILDKIT=1). Advantages over legacy: Parallel Execution: Builds independent stages concurrently. Cache Mounts: Supports mounting

Q: Explain how Docker image layers work and how they share space.

Union File System (UnionFS): Docker images are stacked layers where each layer represents a instruction in the Dockerfile. Layer Immutability: Each layer is read-only. Storage Sharing: If multiple ima

Q: Explain the HEALTHCHECK directive and its parameters.

Purpose: Instructs Docker how to test a container to verify it is actively functioning (not just running as a zombie process). Key Parameters: --interval=30s: Time between checks. --timeout=5s: Maximu

Q: How do Docker networks handle service discovery?

Embedded DNS: Docker runs an internal DNS server at 127.0.0.11 inside each container. Service Name Resolution: Within a user-defined network, containers resolve neighboring container IPs by using thei

Real-world mechanisms, state management, edge cases, performance trade-offs, and practical coding.

50 Theory Questions5 Free Model Answers

THEORY QUESTIONS & SOLUTIONSShowing 5 of 50 questions

Q1Detail the architecture of the Docker Engine.

Client-Server Architecture: Composed of three primary parts:
- Docker Client (docker): The CLI tool that accepts user commands and communicates with the daemon over a REST API (via Unix sockets or TCP).
- Docker Daemon (dockerd): A background service that listens for API requests and manages Docker objects (images, containers, networks, volumes).
- Containerd & Runc: The daemon relies on containerd (container lifecycle management) which spawns runc (the lightweight OCI-compliant runtime) to create and run containers.

Q2How does BuildKit differ from the legacy Docker builder?

BuildKit: The modern, highly optimized build engine (enabled via DOCKER_BUILDKIT=1).
Advantages over legacy:
- Parallel Execution: Builds independent stages concurrently.
- Cache Mounts: Supports mounting compilation caches (RUN --mount=type=cache) that persist between builds.
- Secrets Mounts: Mounts build secrets (RUN --mount=type=secret) securely without baking them into final image layers.
- Garbage Collection: Intelligent, automatic cleanup of intermediate build states.

Q3Explain how Docker image layers work and how they share space.

Union File System (UnionFS): Docker images are stacked layers where each layer represents a instruction in the Dockerfile.
Layer Immutability: Each layer is read-only.
Storage Sharing: If multiple images share the same parent layer, that layer is stored only once on disk.
Writable Layer: At runtime, Docker adds a thin, writable container layer. Read operations fall back to the image layers, while modifications copy the file up to the writable layer (Copy-on-Write).

Q4Explain the HEALTHCHECK directive and its parameters.

Purpose: Instructs Docker how to test a container to verify it is actively functioning (not just running as a zombie process).
Key Parameters:
- --interval=30s: Time between checks.
- --timeout=5s: Maximum time allowed for a single check.
- --start-period=10s: Initial grace period for slow-starting apps.
- --retries=3: Number of consecutive failures before marking as unhealthy.
Exit Codes: The test script must exit with 0 (healthy) or 1 (unhealthy).

Q5How do Docker networks handle service discovery?

Embedded DNS: Docker runs an internal DNS server at 127.0.0.11 inside each container.
Service Name Resolution: Within a user-defined network, containers resolve neighboring container IPs by using their container names or network aliases.
Limitation: The default bridge network does not support embedded DNS and requires outdated --link flags.

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