Using monorepo dependencies

In a trunk-based (or “live at HEAD”) development model, you want changes to libraries in a monorepo to be immediately live in the application that depends on them. There shouldn’t be a bumpversion, wheel publishing step, extra package registry, etc.

Scaffold a library

This exercise uses a code generator to scaffold a new library into our repository. Here’s an example repository that gives a minimal python library: https://github.com/alexeagle/aspect-template-python-lib As the README of that repo says, it’s a copier template. Thanks to direnv, we already have copier on the PATH, so you can stamp out a copy of the library into a new folder named mylib with this command:

copier copy gh:alexeagle/aspect-template-python-lib mylib

And the library is now in the source tree:

Now we need to deal with the transitive dependencies. First, it uses a new Bazel module (jq.bzl) so we can run a command to install it:

buildozer 'new bazel_dep jq.bzl' //MODULE.bazel:all
buildozer 'set version 0.4.0' //MODULE.bazel:jq.bzl

It also happens that the new mylib depends on a library (cowsay). This should be constraint-solved along with the rest of the monorepo, so that you’re certain there’s no “diamond dependency” version conflict when an application includes two different libraries. So you need to add that to the monorepo requirements. Add a line to the requirements/all.in file, with a -r line that points to the new requirements.txt file:

echo "-r ../mylib/requirements.txt" >> requirements/all.in

Now Bazel wants to know the dependency graph, and the new file is a dependency of the uv pip-compile step. So you must update the requirements/BUILD file. For this, rather than edit by hand, you can use the buildozer tool which is a machine-editor for BUILD files that can apply simple instructions. It’s already on the PATH thanks to direnv.

buildozer "add data //mylib:requirements" //requirements:requirements.all

Now repin the dependencies again with this change:

./tools/repin

You can see that it worked as this new dependency is now listed along with the others in the monorepo.

requirements/all.txt

This all.txt file is registered with the pip.parse extension in /MODULE.bazel which is how Bazel knows to install this library when needed.

Use the library from the app

Let’s go back to __main__.py and use this. Start typing “from mylib” at the top of the file - your editor should give you assistance in completing the import statement to get the say function. Then replace the print call with say.marvin:

import requests
from mylib import say

x = requests.get("https://httpbin.org/bytes/1")

say.marvin(x.text.encode().hex())

You have to update the dependency graph for Bazel, so run aspect gazelle again. Now run the application again and see the library is working:

Swap out for a locally-built service

We’ll run copier again to get a FastAPI service that serves our data.

copier copy gh:alexeagle/aspect-template-python-service myservice

This time the template provides no pinned requirements or BUILD files. That’s okay, we do the same thing to add its requirements and generate the BUILD file. This time you can do the exercise with less help!

​Scaffold a library

​Use the library from the app

​Swap out for a locally-built service

Scaffold a library

Use the library from the app

Swap out for a locally-built service