Bazel on Android: The Good, The Bad and The Ugly
summary
Slack migrates Android builds to Bazel, builds the full app target and runs 45,000+ unit tests in CI, and measures ~70% faster test result times in CI with a p50 unit-test config under three minutes versus Gradle’s ~11–12 minutes. Migration uses rules_java/rules_kotlin/rules_android, rules_jvm_external, custom SQLDelight codegen, emulatorWTF for instrumentation, and a sync pipeline (Aaron plugin then a Bulldozer-backed Python script that syncs in ~3 seconds). It works for hermetic CI caching but fails to deliver acceptable local IDE experience and incremental compilation is slower, so rollout stalls.
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chapters (30)
- 0:00Introduction
- 1:08Considering Bazel for Android
- 3:34Gradle configuration pain
- 5:18Modularization and Bazel appeal
- 6:42Slack’s build goals
- 7:45Bazel without disrupting Gradle
- 8:46Gradle plugins and Bazel rules
- 11:58Replacing Gradle plugins
- 13:04External APIs and tool invocations
- 14:47Community rules challenges
- 15:29Migration progress
- 16:51Developer testing handoff
- 17:57Bazel to main branch
- 19:19CI builds and sync tools
- 20:20Gradle to Bazel sync
- 21:45Developer feedback and rollback
- 22:49PR-based Bazel workflow
- 24:11Bazel testing quirks
- 24:53Instrumentation tests issues
- 26:21Test runner limitations
- 27:45Hermetic builds benefits
- 28:48Bazel vs Gradle performance
- 31:14IDE integration challenges
- 33:41JetBrains H Garden support
- 34:22Developer preview feedback
- 36:07Pausing Bazel project
- 36:5015-month journey reflections
- 37:11Good, bad, and ugly of Bazel
- 39:39Incremental compilation limits
- 40:22Would we do it again
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