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The special "all" schema

There is a special schema known as the all schema, defined in glean/schema/source/schema.angle.

The all schema is used to resolve names to particular predicates and types when the version in omitted. This matters in a few places:

  • When Thrift types are generated from the schema, the types and predicates from the all schema determine which names are unversioned in the generated Thrift. For example, if the all schema contains src.1, then the predicate src.File.1 will be simply called File in the generated Thrift; otherwise it would be called File_1.

  • When an Angle query mentions an unversioned predicate or type, the name is resolved to a particular version of that predicate or type using the current all schema.

  • When deriving a predicate with the CLI tool, like glean derive python.TargetUses.

The all schema is defined like this:

schema all.6 :
  src.1,
  python.4,
  code.18
note

Multiple versions of all

This is a rather specialised scenario, but it might be necessary to have multiple versions of the all schema during a schema migration. We can code-generate the Thrift types from each version of all separately, and clients can select at build time which version they want to use. This enables incremental migration of code from one schema to another schema.