ARROW-9308: [Format] Add Feature enum for forward compatibility.

emkornfield · emkornfield · commit b990efe80605 · 2020-07-02T20:33:45.000-07:00
Will start a discussion on the mailing list shortly. Closes apache#7502 from emkornfield/feature_flag Authored-by: Micah Kornfield <emkornfield@gmail.com> Signed-off-by: Micah Kornfield <emkornfield@gmail.com>
diff --git a/format/Schema.fbs b/format/Schema.fbs
@@ -40,6 +40,35 @@ enum MetadataVersion:short {
   V5,
 }
 
+/// Represents Arrow Features that might not have full support
+/// within implementations. This is intended to be used in
+/// two scenarios:
+///  1.  A mechanism for readers of Arrow Streams
+///      and files to understand that the stream or file makes
+///      use of a feature that isn't supported or unknown to
+///      the implementation (and therefore can meet the Arrow
+///      forward compatibility guarantees).
+///  2.  A means of negotiating between a client and server
+///      what features a stream is allowed to use. The enums
+///      values here are intented to represent higher level
+///      features, additional details maybe negotiated
+///      with key-value pairs specific to the protocol.
+///
+/// Enums added to this list should be assigned power-of-two values
+/// to facilitate exchanging and comparing bitmaps for supported
+/// features.
+enum Feature : long {
+  /// Needed to make flatbuffers happy.
+  UNUSED = 0,
+  /// The stream makes use of multiple full dictionaries with the
+  /// same ID and assumes clients implement dictionary replacement
+  /// correctly.
+  DICTIONARY_REPLACEMENT = 1,
+  /// The stream makes use of compressed bodies as described
+  /// in Message.fbs.
+  COMPRESSED_BODY = 2
+}
+
 /// These are stored in the flatbuffer in the Type union below
 
 table Null {
@@ -369,6 +398,9 @@ table Schema {
   fields: [Field];
   // User-defined metadata
   custom_metadata: [ KeyValue ];
+
+  /// Features used in the stream/file.
+  features : [ Feature ];
 }
 
 root_type Schema;
