Mods

Bazel rules to define modules in a monorepo not managed by Bazel.

Background

One of the problem when using monorepo approach is about running build of the modules in a Continuous Integration system. We often do not want to run build pipelines of all modules but only of those that are affected by changes made in commit.

There are build tools that support monorepo development workflow, for example Bazel. However, often we just want to use existing build tools instead of migrating to Bazel because it's more familar, or s more supported by IDE, or other reasons.

This Bazel rule, allows developers to (ab)use Bazel by using its excellent dependency tracking and query to support monorepo development workflow while still using more mainstream build tools. Here, Bazel is not used to build the modules but only to track changes of the modules.

Usage

Add the following into the WORKSPACE file

load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")

git_repository(
    name = "mods",
    remote = "https://github.com/fajran/mods_bazel.git",
    commit = "1048864b7dfe3d3d7f02d7c64103843ac4e38cbc",
)

To declare a module, add the following to the BUILD file of that module.

load("@mods//mods:def.bzl", "module")

module(
    name = "library",
    srcs = glob(["**"]),
)

To declare another module and set dependency, add the following.

load("@mods//mods:def.bzl", "module")

module(
    name = "service",
    srcs = glob(["**"]),
    deps = ["//common:library"],  # pointing to the previous 'library' module
)

Use Cases

Find all modules
```
bazel query "kind(module, //...)"
```
Find all buildable modules

For this we are going to use a custom rule called build_trigger available from //mods/extras:build.bzl.

Why new rule? The idea is of all modules that we define using module rule, some may want to be built in a CI system and some other don't. To be able to query these buildable modules, we can create a new rule with different name so they don't get mixed up.
```
bazel query "kind(build_trigger, //...)"
```
Find affected buildable modules from a commit

Run this from the root git project directory. Add --keep_going to recover from failures and get as much as possible.
```
bazel query --keep_going "kind(build_trigger, rdeps(//..., set($(git diff --name-only HEAD^))))"
```