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Efficient Layer Caching
When building container images, Docker caches the newly created layers. These layers can then be used later on when building other images, reducing the build time and minimizing bandwidth usage. However, to make the most of this caching mechanism, you should be aware of how to efficiently use layer caching.
How Docker Layer Caching Works
Docker creates a new layer for each instruction (e.g., RUN, COPY, ADD, etc.) in the Dockerfile. If the instruction hasn’t changed since the last build, Docker will reuse the existing layer.
For example, consider the following Dockerfile:
FROM node:14
WORKDIR /app
COPY package.json /app/
RUN npm install
COPY . /app/
CMD ["npm", "start"]
When you build the image for the first time, Docker will execute each instruction and create a new layer for each of them. If you make some changes to the application and build the image again, Docker will check if the changed instructions affect any of the layers. If none of the layers is affected by the changes, Docker will reuse the cached layers.
Tips for Efficient Layer Caching
- Minimize changes in the Dockerfile: Try to minimize the frequency of changes in your Dockerfile, and structure your instructions in a way that most frequently changed lines appear at the bottom.
-
Build context optimization: Use
.dockerignorefile to exclude unnecessary files from the build context that might cause cache invalidation. - Use smaller base images: Smaller base images reduce the time taken to pull the base image as well as the number of layers that need to be cached.
-
Leverage the Docker’s
--cache-fromflag: If you’re using a CI/CD pipeline, you can specify which image to use as a cache source. -
Combine multiple instructions: In some cases, combining instructions (e.g.,
RUN) can help minimize the number of layers, making caching more efficient.
By following these best practices, you can optimize the layer caching process and reduce the build time for your Docker images, making your development and deployment processes more efficient.
Visit the following resources to learn more: