[GitHub] [beam] TheNeuralBit commented on a change in pull request #12426: [BEAM-7996] Add support for MapType and Nulls in container types for Python RowCoder

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TheNeuralBit commented on a change in pull request #12426:
URL: https://github.com/apache/beam/pull/12426#discussion_r464732307



##########
File path: model/pipeline/src/main/proto/beam_runner_api.proto
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@@ -855,10 +855,21 @@ message StandardCoders {
     //     BOOLEAN:   beam:coder:bool:v1
     //     BYTES:     beam:coder:bytes:v1
     //   ArrayType:   beam:coder:iterable:v1 (always has a known length)
-    //   MapType:     not yet a standard coder (BEAM-7996)
+    //   MapType:     not a standard coder, specification defined below.
     //   RowType:     beam:coder:row:v1
     //   LogicalType: Uses the coder for its representation.
     //
+    // The MapType is encoded by:
+    //   - An INT32 representing the size of the map (N)
+    //   - Followed by N interleaved keys and values, encoded with their
+    //     corresponding coder.
+    //
+    // Nullable types in container types (ArrayType, MapType) are encoded by:
+    //   - A one byte null indicator, 0x00 for null values, or 0x01 for present
+    //     values.
+    //   - For present values the null indicator is followed by the value
+    //     encoded with it's corresponding coder.
+    //

Review comment:
       That's right. Nullability is represented on the FieldType: https://github.com/apache/beam/blob/d5a6c2f0ceb7b3e609ba8d97e523b3bded7d0459/model/pipeline/src/main/proto/schema.proto#L47
   
   We _could_ document that schemas should not accept null keys/values in `MapType` and reject them in the SDKs. But there would still be an unused nullable field on the key and value FieldType:
   https://github.com/apache/beam/blob/d5a6c2f0ceb7b3e609ba8d97e523b3bded7d0459/model/pipeline/src/main/proto/schema.proto#L79-L82




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