syntax = "proto3"; option java_multiple_files = true; option java_package = "kr.heek"; package kr.heek; message Book { string isbn = 1; string title = 2; optional string description = 3; optional string author = 4; optional string publisher = 5; optional PublishDate publish_date = 6; } message PublishDate { int32 year = 1; optional int32 month = 2; optional int32 day = 3; }