An important feature that LUCI provides to its users is named caches. From a recipe perspective, a named cache is a symlink in [CACHE]/<path> to a named permanent location on the bot. A path is typically a directory name, such as "git", "goma", "builder". There are built-in caches (git), and user-defined ones.

Currently caches are available only in swarming task view. E.g. table in https://chromium-swarm-dev.appspot.com/task?id=34f66e733b6d8910&refresh=10

builder_7d53502990028433c920d01dffdd0c154e5847549971a954e8bc862fab65a940:cache/builder
git:cache/git
goma:cache/goma

Here LHS of : is name of the cache and RHS is path. Only caches with paths that start with "cache/" are exposed to a recipe, e.g. "cache/git" is accessible as "[CACHE]/git" in a recipe, and it is being used by bot_update. Caches with paths that don't start with "cache/" are not exposed (but we don't have any today, too).

A LUCI build should display a list of caches available to the recipe, at least paths. E.g. https://luci-milo-dev.appspot.com/swarming/task/34f66e733b6d8910?server=chromium-swarm-dev.appspot.com
should communicate to the user that there are cache dirs "git", "goma", etc and they are are mapped to caches named "git", "goma", respectively.