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Docker layers and optimizing your Dockerfile

dockerdockerfile

One small problem i recently came across was a project that had a slow build time to run, ~7 mins approx. Config changes required a rebuild, so 7 mins per change. Something i noticed was npm packages being installed for config changes and even HTML changes - why do i need to download 1000+ npm packages if i'm only changing HTML? Madness!

Docker can cache commands but only if the Dockerfile is written in a certain way. Take the following scenario:

FROM node:9
COPY /src /src
RUN npm install
RUN npm build

Docker can cache each command and create a Layer. Layers are cached if Docker detects that no files have been changed for that command.

By splitting the COPY into 2 commands, we can utilize caching and speed up the build time for builds that haven't modified npm packages:

FROM node:9
COPY /src/package*.json /src
# Docker will cache these 2 layers if package.json and package.lock.json are unmodified
RUN npm install

# copy the rest of our code and run a fresh build
COPY /src /src
RUN npm build

Just by adding 1 line, we can speed up most of our builds - in my scenario it was local builds and CI server builds (npm packages rarely change in a mature project)

This 1 line Dockerfile change has saved our team a few minutes per build (we had some large / slow npm dependencies for things like integration testing such as cypress)

That's all folks!

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© Steve Baker 2021, Built using Gatsby. Last updated: 1/31/2021