Hybrid build

Hybrid builds were implemented in response to dealing with large module graphs in a project and not all of them would be migratable or requires much refactoring effort to make them build with both the build systems. Hybrid builds helps in performing incremental migrations on large projects where one shot migration might not possible.

It works in tandem with Migration Criteria and builds only part of the dependency graph with Bazel and ensures remaining graph nodes are built with Gradle while consuming artifacts built by Gradle.

How it works

During Gradle's configuration phase, Grazel does bazelisk build //... on the project directory to build all targets. This action would generate build artifacts like .jar or .aar in the configuration phase. Then Grazel sets up dependency substitution rules on the entire dependency graph such that during resolution if Grazel finds an build artifact for a project, it removes the project dependency and replaces it with the built artifact.

Doing this process on the entire graph lets the execution phase use prebuilt artifacts by Bazel in the part of the graph and rest with Gradle.

Run Hybrid builds

Hybrid builds can be triggered by settting bazelEnabled boolean property in either local.properties or gradle.properties. Then when building any gradle module like app:assembleDebug, Grazel would build and register substitution rules. Substitution rules occur lazily and only when configuration it is replacing dependency of is needed in current build.

Debugging

To perform dependency substitution, Grazel relies on strong naming conventions. If there is a module called :quiz, then the corresponding artifact that would be searched for would be quiz.aar and quiz.jar.

During build to debug whether substituions are happening correctly look for Dependency Substitution table in build logs as shown below.

┌───────────────────────────────────────────────────┐
│                       :app                        │
├──────────────┬─────────────┬──────────────────────┤
│Configuration │   Project   │Substitution Artifacts│
├──────────────┼─────────────┼──────────────────────┤
│implementation│:quiz        │[quiz.aar]            │
├──────────────┼─────────────┼──────────────────────┤
│implementation│:base        │[base.aar]            │
└──────────────┴─────────────┴──────────────────────┘

Error

Note that when a build artifact for a module can't be found, Grazel fails the build. This is to ensure every module that has BUILD.bazel is able to build successfully