Vercel monorepo: properly cache Yarn installs
May 4, 2024
May 4, 2024
So you have a Vercel app that’s part of a monorepo. You may have noticed that by default it installs the whole monorepo dependencies, and you may have already addressed that!
But either way, you have another problem: Yarn downloads all your dependencies on every single build. That’s pretty time consuming. It doesn’t seem that dependencies are getting cached at all.
Vercel recommends
setting a ENABLE_ROOT_PATH_BUILD_CACHE=1 environment variable to
make build times faster in monorepos.
It sounds great, but in my experience it didn’t do anything, and I’m not the only one.
It seems
that regardless of ENABLE_ROOT_PATH_BUILD_CACHE, Vercel doesn’t
cache Yarn’s cache folder .yarn/cache, and Yarn 3 and greater will
download everything again if this directory is not present, regardless
of the state of node_modules.
So the key is to force Yarn’s cache folder to be inside a directory that Vercel actually caches.
I tried
setting it inside the root node_modules by doing
YARN_CACHE_FOLDER=../../node_modules/.yarn-cache yarn workspaces focus,
which worked at first, but quickly encountered some
issues
when the cache was reused across different build states.
Luckily, thanks to this comment
on a totally unrelated issue, I discovered I could put the cache in
.next/cache instead, and I never had issues since then!
{
"installCommand": "YARN_CACHE_FOLDER=.next/cache/yarn-cache yarn workspaces focus"
}