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

Protocol buffers的一个重要应用就是RPC服务，下面简单介绍一下相关的使用方法

定义服务

如果要使用RPC（远程过程调用）系统的消息类型，可以在.proto文件中定义一个RPC服务接口，protocol buffer编译器将根据您选择的语言生成服务接口代码和存根（记录）。例如，如果要使用一个使用SearchRequest并返回SearchResponse的方法定义RPC服务，可以在.proto文件中定义它，如下所示：

service SearchService {
  rpc Search (SearchRequest) returns (SearchResponse);
}

目前比较典型的使用protocol buffer的RPC是google的gRPC

JSON 映射

Proto3 支持JSON的规范编码，这使得不同系统之间共享数据更容易

如果在JSON编码中数据缺少值或者值为空，protocol buffer解析时会解析成字段的默认值

proto3	JSON	JSON 例子	说明
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