什么是Protocol Buffers

先看官网定义：

protocol buffers – a language-neutral, platform-neutral, extensible way of serializing structured data for use in communications protocols, data storage, and more.

Protocol Buffers 是一种结构化数据的存储格式，可以用于结构化数据的序列化和反序列化。它很适合做数据存储、 RPC 数据交换格式。

Protocol Buffers的作用可以类比JSON、XML。

对于传输双方，如果约定好使用Protocol Buffer为数据传输的格式，那么这将是一种比JSON和XML都高效轻便的途径。

Protocol Buffers支持多种编程语言，包括C++、Java、Python、Go、C#等。

安装Protocol Buffers

1、官网下载压缩包

2、解压

tar -zxvf protobuf-all-3.6.0.tar.gz

3、protobuf的包需要自己编译

cd protobuf-3.6.0
./configure
make
make install

4、在protobuf-3.6.0目录下有对应着各种语言的文件夹，每个文件夹下都有README.md，里面有相应的安装步骤。
对于python：

cd protobuf-3.6.0/python
python setup.py build
python setup.py test
python setup.py install

5、到此为止，protobuf的python版本已经安装完成，可在命令行键入protoc试试。

Demo

1、创建addressbook.proto文件。.proto文件定义了要序列化的数据结构。

syntax = "proto2";

package tutorial;

message Person {
  required string name = 1;
  required int32 id = 2;
  optional string email = 3;

  enum PhoneType {
    MOBILE = 0;
    HOME = 1;
    WORK = 2;
  }

  message PhoneNumber {
    required string number = 1;
    optional PhoneType type = 2 [default = HOME];
  }

  repeated PhoneNumber phones = 4;
}

message AddressBook {
  repeated Person people = 1;
}

2、编译addressbook.proto

protoc --python_out=/Users/ya/developer/protoc-test  addressbook.proto

这一条命令执行完后会在你指定的目录下生成addressbook_pb2.py文件。

addressbook_pb2.py会提供关于所定义的数据结构的操作方法。

3、编写writer.py，负责序列化数据并保存到文件

# See README.txt for information and build instructions.
import addressbook_pb2
import sys

try:
  raw_input          # Python 2
except NameError:
  raw_input = input  # Python 3


# This function fills in a Person message based on user input.
def PromptForAddress(person):
  person.id = int(raw_input("Enter person ID number: "))
  person.name = raw_input("Enter name: ")

  email = raw_input("Enter email address (blank for none): ")
  if email != "":
    person.email = email

  while True:
    number = raw_input("Enter a phone number (or leave blank to finish): ")
    if number == "":
      break

    phone_number = person.phones.add()
    phone_number.number = number

    type = raw_input("Is this a mobile, home, or work phone? ")
    if type == "mobile":
      phone_number.type = addressbook_pb2.Person.MOBILE
    elif type == "home":
      phone_number.type = addressbook_pb2.Person.HOME
    elif type == "work":
      phone_number.type = addressbook_pb2.Person.WORK
    else:
      print("Unknown phone type; leaving as default value.")


# Main procedure:  Reads the entire address book from a file,
#   adds one person based on user input, then writes it back out to the same
#   file.
if len(sys.argv) != 2:
  print("Usage:", sys.argv[0], "ADDRESS_BOOK_FILE")
  sys.exit(-1)

address_book = addressbook_pb2.AddressBook()

# Read the existing address book.
try:
  with open(sys.argv[1], "rb") as f:
    address_book.ParseFromString(f.read())
except IOError:
  print(sys.argv[1] + ": File not found.  Creating a new file.")

# Add an address.
PromptForAddress(address_book.people.add())

# Write the new address book back to disk.
with open(sys.argv[1], "wb") as f:
  f.write(address_book.SerializeToString())

5、执行writer.py

python writer.py addressbook.data 

执行完脚本后数据将会序列化并存储到addressbook.data中。

6、编写reader.py，负责读取文件并反序列化数据

import addressbook_pb2
import sys


# Iterates though all people in the AddressBook and prints info about them.
def ListPeople(address_book):
  for person in address_book.people:
    print "Person ID:", person.id
    print "Name:", person.name
    if person.email != "":
      print "E-mail address:", person.email

    for phone_number in person.phones:
      if phone_number.type == addressbook_pb2.Person.MOBILE:
        print "Mobile phone :",
      elif phone_number.type == addressbook_pb2.Person.HOME:
        print "Home phone :",
      elif phone_number.type == addressbook_pb2.Person.WORK:
        print "Work phone :",
      print(phone_number.number)


# Main procedure:  Reads the entire address book from a file and prints all
#   the information inside.
if len(sys.argv) != 2:
  print("Usage:", sys.argv[0], "ADDRESS_BOOK_FILE")
  sys.exit(-1)

address_book = addressbook_pb2.AddressBook()

# Read the existing address book.
with open(sys.argv[1], "rb") as f:
  address_book.ParseFromString(f.read())

ListPeople(address_book)

7、执行reader.py

python reader.py addressbook.data

输出结果示例：

Person ID: 1
Name: kanon
E-mail address: 540004716@qq.com
Mobile phone : 123456

两个很重要的方法

• SerializeToString( )：serializes the message and returns it as a string.
• ParseFromString( data )：parses a message from the given string.
  
  
  

Protocol Buffers 相比 XML

• are simpler
• are 3 to 10 times smaller
• are 20 to 100 times faster
• are less ambiguous
• generate data access classes that are easier to use programmatically
  
  
  

在ODPS（MaxCompute）中的应用

ODPS的Tunnel HTTP Server采用了Protobuf作为其序列化机制。客户端在上传数据时，需要先对结构化数据进行序列化（生成二进制流），Tunnel服务端接收到数据后，（对二进制流）执行反序列化，还原出结构化数据，写到ODPS表中。