docker / getting-started

Image Layer Caching

Docker leverages image layer caching to optimize build times and reduce image size. When you build a Docker image, Docker creates layers for each instruction in your Dockerfile.

  • Layers are cached: When you rebuild an image, Docker checks for existing layers and only rebuilds the layers that have changed.

  • Caching is based on dependencies: The caching process works by looking at the input to each instruction. If the input is the same as a previous build, the layer is reused.

Key considerations for layer caching:

  • Order of instructions: The order of instructions in your Dockerfile can significantly impact layer caching.

  • Cache busting: Intentionally busting the cache can be helpful for specific situations.

  • Understanding layer dependencies: To optimize your build process, you need to understand the dependencies between Dockerfile instructions.

Example:

# This Dockerfile builds a Node.js app image
          FROM node:16-alpine
          
          # Install dependencies
          WORKDIR /app
          COPY package.json package-lock.json ./
          RUN npm install
          
          # Copy the source code 
          COPY . .
          
          # Start the app
          CMD ["npm", "start"]

  • Building for the first time: When you run docker build ., Docker will execute each instruction in the Dockerfile, creating layers for each one.

  • Rebuilding the image: If you change the package.json or package-lock.json file, the RUN npm install instruction will need to be rebuilt. However, the COPY . . instruction will use the cached layer from the previous build.

Caching Considerations:

  • Order matters: By placing the COPY instruction before RUN npm install, you increase the chance that the RUN npm install instruction will be able to use the cached layer.

  • Cache busting: If you want to force a rebuild of the entire image, you can use the --no-cache flag when you run docker build.

Example:

docker build --no-cache .
  • Understanding dependencies: It’s essential to understand which instructions depend on each other. In this example, the RUN npm install instruction depends on the COPY package.json package-lock.json ./ instruction. This dependency is due to the input for the RUN command.

Important Resources:

Code Files:

  • docker/getting-started/examples/nodejs/Dockerfile
  • docker/getting-started/examples/python/Dockerfile

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