02. Docker Layer

Docker Layer

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Prerequisites

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1. What is Docker Layer

A Docker layer is a read-only filesystem unit that represents a single step in building a Docker image.

A Docker image is essentially a stack of layers.

2. Docker Layer

• A Docker image = stack of layers
• Each Dockerfile instruction → creates a layer
• Layers are immutable and cacheable
• Containers add a writable layer on top
• Proper layer ordering improves build speed and efficiency

Docker images are read-only.

When a container runs, Docker adds a writable layer on top:

[ Writable Layer (Container) ]
[ Image Layer 4 ]
[ Image Layer 3 ]
[ Image Layer 2 ]
[ Image Layer 1 ]

All changes happen in the top writable layer

2-1. How Layers Are Created

Each instruction in a Dockerfile creates a new layer.

FROM ubuntu:22.04
RUN apt-get update
RUN apt-get install -y python3
COPY . /app

This results in:

Layer 1: Base image (ubuntu)
Layer 2: apt-get update
Layer 3: install python3
Layer 4: copy application files

Every step = one layer

2-2. Why Layers Matter

✔️ 1. Build Cache (Performance)

Docker caches layers.

If nothing changes:

• It reuses previous layers
• Avoids rebuilding everything

✔️ 2. Reusability

Multiple images can share layers:

Image A → ubuntu + python + app A
Image B → ubuntu + python + app B

ubuntu + python layers are shared

✔️ 3. Smaller Size

• Layers are reused instead of duplicated
• Saves disk space and network bandwidth

✔️ 4. Immutable

• Layers cannot be modified
• Any change creates a new layer

✔️ 5. Copy-on-Write

• When a file is modified:

  • It is copied to the writable layer
  • Original layer remains unchanged

3. Layer Optimization Tips

❌ Inefficient

RUN apt-get update
RUN apt-get install -y python3

👉 Creates multiple layers

✅ Optimized

RUN apt-get update && apt-get install -y python3

👉 Single layer

✅ Optimized

Docker rebuilds layers from the point of change.

COPY . /app

If this changes → all layers after it rebuild

✅ Best Practice

# Stable layers first
RUN apt-get install -y python3

# Frequently changing files last
COPY . /app