spmallette commented on code in PR #3483:
URL: https://github.com/apache/tinkerpop/pull/3483#discussion_r3553190675
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docs/src/upgrade/release-4.x.x.asciidoc:
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@@ -800,6 +887,37 @@ Kryo's default `Map` serializer because it extended
`HashMap`, whereas in 4.x it
serializer. As a result, Gryo-serialized `Tree` data written by 3.x cannot be
read by 4.x and vice versa. The
Gryo type registration id (`61`) is unchanged and 4.x-to-4.x Gryo round-trips
correctly; this Gryo break is
expected for the 4.0.0 major release.
+===== Multi-Label Support
+
+Providers declare their supported label cardinality via
`Graph.Features.getLabelCardinality()`, which defaults to
+`LabelCardinality.ONE` (single label, immutable, the 3.x behavior). To support
multi-label, return `ONE_OR_MORE` or
+`ZERO_OR_MORE` from this method.
+
+The `LabelCardinality` enum exposes `min()`, `max()`, and `supportsMutation()`
for programmatic introspection of
+constraints. Constraint enforcement is handled by the
`LabelCardinalityValidator` utility class, which providers may
+use directly or replace with their own validation logic tailored to their
storage backend.
+
+Providers implementing multi-label must:
+
+- Store and return a `Set<String>` from `Element.labels()`
+- Implement `Element.addLabel(String, String...)` and
`Element.dropLabel(String, String...)`
+- Ensure `hasLabel("a", "b")` uses OR semantics (matches vertices with label
"a" or "b")
+- Serialize/deserialize the label set via the V4 GraphBinary format (which
sends labels as a list)
+
+The default `elementMap()` and `valueMap()` implementations determine label
output format solely from the
+`with("multilabel")` source option, not from the graph's `LabelCardinality`.
This means a multi-label graph still
+returns a single label string from these steps unless `with("multilabel")` is
explicitly configured. The intent is
+to eventually deprecate the single-string path entirely and always return
labels as a set, aligning `elementMap()`
+and `valueMap()` with `labels()`. Until then, `with("singlelabel")` allows
users to override a source-level
+`with("multilabel")` back to single-string output.
+
+Providers who want `elementMap()`/`valueMap()` to return the full label set by
default (without requiring users to
+set `with("multilabel")`) should override `PropertyMapStep` and
`ElementMapStep` to tie label output to their
+server-side label cardinality configuration. In that case,
`with("singlelabel")` should still be respected as an
+explicit user override back to single-string output.
+
+Older serialization formats (GraphSON V1/V2/V3, Gryo) only support a single
label and will silently use the value
Review Comment:
More references to old serialization formats and the single label thing....
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