Protocol Buffers

Protocol Buffers are a serialization format with an interface description language developed by Google. The original Google implementation for C++, Java and Python is available under a free software, open source license. Various other language implementations are either available or in development.^[1]

The design goals for Protocol Buffers emphasized simplicity and performance. In particular, it was designed to be faster than XML.^[2]

Protocol Buffers are widely used at Google for storing and interchanging all kinds of structured information. Protocol Buffers serve as a basis for a custom remote procedure call (RPC) system that is used for practically all inter-machine communication at Google.^[3]

Protocol Buffers are very similar to Facebook’s Thrift protocol, except it does not include a concrete RPC stack to use for defined services.

Data structures (called "messages") and services are defined in the Proto Definition file (.proto) which is then compiled with protoc. This compilation generates code that matches the message definitions. For example, example.proto will produce example.pb.cc and example.pb.h which will define C++ classes for each Message and Service example.proto defines.

Canonically, Protocol Buffers are serialized into a binary wire format which is compact, forwards-compatible, backwards-compatible, but not self-describing. The officially supported implementation includes an ASCII serialization format, but this format -- although self-describing -- loses the forwards-and-backwards-compatibility behavior, and is thus not a good choice for applications other than debugging.

Though the primary purpose of Protocol Buffers is to facilitate network communication, its simplicity and speed make Protocol Buffers an alternative to data-centric C++ classes and structs, especially where interoperability with other languages or systems might be needed in the future.

Example

message Point {
  required int32 x = 1;
  required int32 y = 2;
  optional string label = 3;
}
 
message Line {
  required Point start = 1;
  required Point end = 2;
  optional string label = 3;
}
 
message Polyline {
  repeated Point point = 1;
  optional string label = 2;
}

The "Point" message defines two mandatory data items, x and y. The data item label is optional. Each data item has a tag. The tag is defined after the equal sign, e.g. x has the tag 1.

The "Line" and "Polyline" messages demonstrate how composition works in Protocol Buffers (they both use Point). Polyline has a repeated field, which behaves like a vector.

This is subsequently compiled with protoc. A C++ program can then use it like so:

#include "polyline.pb.h"  // generated by calling protoc polyline.proto (defined above)
 
Line* createNewLine(const std::string& name) {
  Line* line = new Line;
  line->mutable_start()->set_x(10);
  line->mutable_start()->set_y(20);
  line->mutable_end()->set_x(30);
  line->mutable_end()->set_y(40);
  line->set_label(name);
  return line;
}
 
Polyline* createNewPolyline() {
  Polyline* polyline = new Polyline;
  Point* point1 = polyline->add_point();
  point1->set_x(10);
  point1->set_y(10);
  Point* point2 = polyline->add_point();
  point2->set_x(10);
  point2->set_y(10);
  return polyline;
}

See also

• Comparison of data serialization formats

Notes and references

External links

• Official project page at code.google.com
• Another official home page at Google Open Source project hosting