A curious detail of Protocol Buffers is that it does not have a list or array datatype, but instead has a repeated marker for fields (which is a third option alongside required and optional). As you can see in Figure 4-4, the encoding of a repeated field is just what it says on the tin: the same field tag simply appears multiple times in the record. This has the nice effect that it’s okay to change an optional (single-valued) field into a repeated (multi-valued) field. New code reading old data sees a list with zero or one elements (depending on whether the field was present); old code reading
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