[GitHub] [incubator-pinot] mcvsubbu commented on a change in pull request #6710: DataTable V3 implementation and measure data table serialization cost on server

[GitBox <gi...@apache.org>]

mcvsubbu commented on a change in pull request #6710:
URL: https://github.com/apache/incubator-pinot/pull/6710#discussion_r605812512



##########
File path: pinot-common/src/main/java/org/apache/pinot/common/utils/DataTable.java
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@@ -80,4 +85,85 @@
   double[] getDoubleArray(int rowId, int colId);
 
   String[] getStringArray(int rowId, int colId);
+
+  /* The MetadataKey is used in V3, where we present metadata as Map<MetadataKey, String>
+   * ATTENTION:
+   *  - Don't change existing keys.
+   *  - Don't remove existing keys.
+   *  - Always add new keys to the end.
+   *  Otherwise, backward compatibility will be broken.
+   */
+  enum MetadataKey {
+    UNKNOWN("unknown"),

Review comment:
       I have found it useful to have one enum reserved that is never used. It is never sent by the sender, but the receiver, if needed, can set it to this value if it encounters a value that it does not know about. In that case, the special case handling is restricted to the layer that first scans the enums, and the other layers above don't need to worry about default cases.




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