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Configuration transitions

Configuration transition is a mechanism for changing the configuration when depending on a target.

Currently, Buck2 has incoming and outgoing transitions:

  • Incoming transitions are specified per-target and take effect when depending on that target.
  • Outgoing transitions are specified on an attribute and take effect on dependencies that appear in that attribute.

Using incoming transitions​

Assuming that a transition rule called :my_transition exists (see the page on configuration transitions for rule authors for how to define one), it can be used in the following way:

cxx_library(
    name = "lib1",
    exported_headers = ["lib1.h"],
    exported_deps = [":lib2"],
    incoming_transition = [":my_transition"],
)

cxx_library(
    name = "lib2",
    exported_headers = ["lib2.h"],
)

When building lib1, whatever the current configuration is will be passed to my_transition, which will return a new configuration. This new configuration will be used to build both lib1 and lib2.

The transition is given the current configuration and returns a new one. For example, it could:

  • Add a new constraint to the existing configuration (e.g. add a release constraint so that it and all its dependencies are built in an optimized mode).
  • Return a blank configuration with no constraints at all (useful to deduplicate work when actions produce identical outputs, such as when downloading artifacts).
  • Make up an arbitrary configuration based on the constraints currently set.

Using outgoing transitions​

Outgoing transitions are used by rule authors, not end users.

See the page on configuration transitions for rule authors for more information.