DOCKER · MEDIUM CODEX
Docker Interview Questions — Medium
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.
- Docker Client (
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 asunhealthy.
- Exit Codes: The test script must exit with
0(healthy) or1(unhealthy).
Q5How do Docker networks handle service discovery?
- Embedded DNS: Docker runs an internal DNS server at
127.0.0.11inside 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
bridgenetwork does not support embedded DNS and requires outdated--linkflags.
Unlock the remaining 45 Docker (Medium) questions
You've completed the 5 free sample questions. Get unrestricted lifetime access to every question, model answer, implementation challenge, and all 27+ technologies for a single payment.
₹399 India / $9 International · One-time settlement · Zero subscription