Chapter 30: platforms, where build actions run, and a cache

Last chapter, and the most backstage. Every compile, link and test that buck2 runs is an action, and an execution platform says where actions may run: here, on this machine, or on a remote build farm, and whether a shared cache of results may be consulted first. .buckconfig names this folder's one platform, root//platforms:execution.

It is the prelude's own default (run everything locally on the host's CPU and OS) with one switch exposed: an opt-in remote action cache. The prelude's default hard-codes local-only execution, so no amount of configuration elsewhere can make it use a cache; this rule is the same platform with that one door unlocked.

flowchart LR
  action["a build action"] --> q{"[buildbuddy] cache = true?"}
  q -- no --> local["run it locally (the prelude default)"]
  q -- yes --> hit{"already in the remote cache?"}
  hit -- yes --> done["use the cached result"]
  hit -- no --> run["run it locally, upload the result"]

The switch is read_config("buildbuddy", "cache", "false"). Only a user-level buckconfig on the owner's development machine sets it, together with the cache's endpoint and key. Anywhere that file is absent (a fresh clone, CI, another machine) the flag is false and this is exactly the prelude default, so the repository never needs the cache, or its credentials, to build.

Aside: cache-only, never remote execution. Actions always run locally; the cache is only checked first, and local results uploaded. Measured 2026-09-23: a clean rebuild, and a second checkout at another path, both hit 100%.

Try it. grep -n -A1 '^\[build\]' .buckconfig shows .buckconfig selecting this platform. Without the user-level config it builds exactly as the prelude's default would.

That is the whole guide. Back to the prologue for the map.

For the people who maintain it

In this folder

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