Caching dependency compilation in Docker build

tao

I’m deploying a Phoenix app with Elixir releases and Docker. The Phoenix docs have an example dockerfile you can use – mine is based on that one.

I’ve set up the file so that dependency downloads and compilation should be cached by Docker, but this isn’t happening. This is slowing down deploys a bit. Can anyone spot why Docker isn’t caching dependencies? Even when mix.exs is unchanged, it redownloads and rebuilds them.

FROM elixir:1.9.0-alpine as build

# install build dependencies
RUN apk add --update git build-base

# prepare build dir
RUN mkdir /app
WORKDIR /app

# install hex + rebar
RUN mix local.hex --force && \
  mix local.rebar --force

# set build ENV
ENV MIX_ENV=prod

# install mix dependencies
COPY mix.exs mix.lock ./
RUN mix deps.get
RUN mix deps.compile
COPY config config

# build assets
# COPY assets assets
# RUN cd assets && npm install && npm run deploy
# RUN mix phx.digest

# build project
COPY priv priv
COPY lib lib
RUN mix compile

# build release
COPY rel rel
RUN mix release

20 comments

#docker #deployment

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2024-06-01 13:22:37 UTC

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NobbZ

Do you touch the mix.lock or mix.exs between builds?

From a theoretical point of view, your file should work.

Post #1

tao

Interesting. Nope, the dependencies get redownloaded and rebuilt even when I haven’t touched those files – that’s why I’m confused and why I opened this thread!

The Phoenix docs actually have a slightly different order:

COPY mix.exs mix.lock ./
COPY config config
RUN mix deps.get
RUN mix deps.compile

I moved this around since I didn’t want to redownload deps after changing configuration, but now I’m starting to wonder if there’s a reason to their ordering?

Post #2

NobbZ

Oh, yes. Config needs to be available, as it might affect how dependencies are built.

Post #3

cnck1387

Can you give an example where a config’s value would determine how dependencies are built?

I know you can choose to install packages based on the environment with mix but isn’t that controlled by the mix command itself with --only prod along with mix.exs?

Post #4

NobbZ

Compile time configuration is the magic word.

Post #5

tao

Okay, I’ll try going back to the original config from the docs!

I’m not sure I understand how this could make Docker not cache things, though. All that Docker knows is that nothing has changed in mix.exs and mix.lock, yet it still re-runs the dependency steps instead of using the cached versions.

Post #6

NobbZ

Yeah, I do not understand that as well… Weird.

Post #7

jola

As an example, Logger lets you remove log calls from modules at compile time, even in dependencies, through the config option :compile_time_purge_matching.

Post #8

cnck1387

Thanks, but would this change which dependencies get mix installed?

Post #9

jola

Oh, sorry, I was just respond to your comment “determine how dependencies are built” and giving an example of the compile time configuration @NobbZ mentioned. Not sure how config would affect which dependencies get installed though, since that’s the job of the mix.exs and mix.lock.

Post #10

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