Observed behavior

flagd supports selectors, so separate flags into different buckets, eg. each file source, http source, etc, is its own selector.
If flagd is started with multiple sources, the selector param can be used to evaluate/sync only the set of flags from the source in question:

{
    "flags": {
        "flagb": {
            "state": "ENABLED",
            "variants": {
                "foo": "foo",
                "bar": "bar"
            },
            "defaultVariant": "foo"
        }
    },
    "metadata": {
        "flagSetId": "b"
    }
}

{
    "flags": {
        "flaga": {
            "state": "ENABLED",
            "variants": {
                "foo": "foo",
                "bar": "bar"
            },
            "defaultVariant": "foo"
        }
    },
    "metadata": {
        "flagSetId": "a"
    }
}

docker run \
  --rm -it \
  --name flagd \
  -p 8015:8015 \
  -v $(pwd):/flags \
  ghcr.io/open-feature/flagd:latest start \
  --uri file:./flags/a.json \
  --uri file:./flags/b.json

 grpcurl -import-path  <path-to-sync-proto-dir> -proto sync.proto -plaintext  -d '{"selector":"./flags/b.json"}'  localhost:8015 flagd.sync.v1.FlagSyncService/FetchAllFlags

grpcurl -import-path <path-to-sync-proto-dir> -proto sync.proto -plaintext  -d '{"selector":"./flags/a.json"}'  localhost:8015 flagd.sync.v1.FlagSyncService/FetchAllFlags

grpcurl -import-path <path-to-sync-proto-dir> -proto sync.proto -plaintext  localhost:8015 flagd.sync.v1.FlagSyncService/FetchAllFlags

This is effective, however, it requires API users to have knowledge of the sources, and means that if a source location is changed, often, application code needs to be changed accordingly (brittle!).

Instead, the selector should work on a logical identifier for a flag set, the flagSetId, which can be maintained separately from the source.

Definition of Done:

• flagSetId is used to select flag sets, not the source
• updated documentation
• discuss whether or not this should be done in a non-breaking way