step1：google 'protobuf required',嘗試了解required屬性跨版本兼容性的問題

http://stackoverflow.com/search?q=Protocol+Buffers+repeated

step2：繼續(xù)了解

http://stackoverflow.com/questions/31176621/how-to-define-repeated-field-as-required-in-google-protocol-buffer

step3：

https://capnproto.org/faq.html#how-do-i-make-a-field-required-like-in-protocol-buffers

How do I make a field “required”, like in Protocol Buffers?

You don’t. You may find this surprising, but the “required” keyword in Protocol Buffers turned out to be a horrible mistake.

step4：baidu 'protobuffer3.0 required'

https://www.oschina.net/news/75687/protocol-buffers-3-0

step5:

https://github.com/google/protobuf/releases/tag/v3.0.0

Explicit "optional" keyword are disallowed in proto3 syntax, as fields are

optional by default; required fields are no longer supported.

Protocol Buffers v3.0.0

released this on Jul 28 2016