Protocol Buffers 學習（5）：在 rpc 中使用 protobuf

Protocol buffers的一個重要應用就是RPC服務，下面簡單介紹一下相關的使用方法

定義服務

如果要使用RPC（遠端過程呼叫）系統的訊息型別，可以在.proto檔案中定義一個RPC服務介面，protocol buffer編譯器將根據您選擇的語言生成服務介面程式碼和存根（記錄）。 例如，如果要使用一個使用SearchRequest並返回SearchResponse的方法定義RPC服務，可以在.proto檔案中定義它，如下所示：

service SearchService {
  rpc Search (SearchRequest) returns (SearchResponse);
}複製程式碼

目前比較典型的使用protocol buffer的RPC是google的gRPC

JSON 對映

Proto3 支援JSON的規範編碼，這使得不同系統之間共享資料更容易

如果在JSON編碼中資料缺少值或者值為空，protocol buffer解析時會解析成欄位的預設值

proto3	JSON	JSON 例子	說明
message	object	{"fBar": v, "g": null, …}	Generates JSON objects. Message field names are mapped to lowerCamelCase and become JSON object keys. null is accepted and treated as the default value of the corresponding field type.
enum	string	"FOO_BAR"	The name of the enum value as specified in proto is used.
map	object	{"k": v, …}	All keys are converted to strings.
repeated V	array	[v, …]	null is accepted as the empty list [].
bool	true, false	true, false
string	string	"Hello World!"
bytes	base64 string	"YWJjMTIzIT8kKiYoKSctPUB+"
int32, fixed32, uint32	number	1, -10, 0	JSON value will be a decimal number. Either numbers or strings are accepted.
int64, fixed64, uint64	string	"1", "-10"	JSON value will be a decimal string. Either numbers or strings are accepted.
float, double	number	1.1, -10.0, 0, "NaN", "Infinity"	JSON value will be a number or one of the special string values "NaN", "Infinity", and "-Infinity". Either numbers or strings are accepted. Exponent notation is also accepted.
Any	object	{"@type": "url", "f": v, … }	If the Any contains a value that has a special JSON mapping, it will be converted as follows: {"@type": xxx, "value": yyy}. Otherwise, the value will be converted into a JSON object, and the "@type" field will be inserted to indicate the actual data type.
Timestamp	string	"1972-01-01T10:00:20.021Z"	Uses RFC 3339, where generated output will always be Z-normalized and uses 0, 3, 6 or 9 fractional digits.
Duration	string	"1.000340012s", "1s"	Generated output always contains 0, 3, 6, or 9 fractional digits, depending on required precision. Accepted are any fractional digits (also none) as long as they fit into nano-seconds precision.
Struct	object	{ … }	Any JSON object. See struct.proto.
Wrapper types	various types	2, "2", "foo", true, "true", null, 0, …	Wrappers use the same representation in JSON as the wrapped primitive type, except that null is allowed and preserved during data conversion and transfer.
FieldMask	string	"f.fooBar,h"	See fieldmask.proto.
ListValue	array	[foo, bar, …]
Value	value		Any JSON value
NullValue	null		JSON null