Scaling Slack’s Mobile Codebases: Modularization

Slack's mobile codebase was slowing teams down. Their solution? Break the monolith into hundreds of independent modules.

Slack's engineering team documented their multi-year Duplo project, specifically the modularization phase that transformed how their iOS and Android teams build and ship features. This is a deep dive into the architectural decisions, tooling choices, and organizational strategies that made it work.

Key Takeaways

• Adopted Bazel for iOS, making module creation simple after XCode's painful process
• Android switched from nested Dagger graphs to Square's Anvil library, cutting boilerplate significantly
• Configuration caching alone saved 20-45 seconds on successive Android builds
• Three module types (Feature, Service, Library) with Interface/Implementation splits prevent tight coupling
• Involved pillar leads from each product team instead of isolating work

The article reveals why Slack's initial dependency injection strategy failed and what they learned from starting over.