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Introduction to the

In the last article we had a basic understanding of Google’s Protobuf and were able to generate the appropriate code using the appropriate tools. However, the format of.proto files and the specific types supported are not clear. Today, this article will take you to find out.

Note that the protocol described in this article is proto3.

Define a message

A protobuf body is called a message and can be thought of as a class that we define in our program. We can define this message object in a.proto file and add attributes to it as follows:

syntax = "proto3";

message SearchRequest {
  string query = 1;
  int32 page_number = 2;
  int32 result_per_page = 3;
}
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The first line of the example above specifies the protocol type of the.proto file. Proto3 is used here, which is the latest version of the protocol. If not specified, proto2 is used by default.

The type definition

Here we define three properties of type String and int32 for the SearchRequest object.

String and int32 are both simple types. Protobuf supports the following simple types:

Protobuf type	instructions	The corresponding Java type
double	A double-precision floating point type	double
float	Floating point types	float
int32	Integer numbers, preferably not negative numbers	int
int64	Integer numbers, preferably not negative numbers	long
uint32	Unsigned integer	int
uint64	Unsigned integer	long
sint32	Signed integer	int
sint64	Signed integer	long
fixed32	A four-byte integer	int
fixed64	An 8-byte integer	long
sfixed32	A signed integer of 4 bytes	int
sfixed64	An 8-byte signed integer	long
bool	Boolean type	boolean
string	string	String
bytes	byte	ByteString

Protobuf also supports complex composite and enumerated types.

Enum types are represented as enum types in protobuf. Let’s look at the definition of an enum type:

message SearchRequest {
  string query = 1;
  int32 page_number = 2;
  int32 result_per_page = 3;
  enum Corpus {
    UNIVERSAL = 0;
    WEB = 1;
    IMAGES = 2;
    LOCAL = 3;
    NEWS = 4;
    PRODUCTS = 5;
    VIDEO = 6;
  }
  Corpus corpus = 4;
}
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We define an enumeration type Corpus above. Enumeration values defined in enumeration types start from 0, which is also the default value of enumeration types.

In enumerations, you can also define enumerations with the same value, but this requires the allow_alias=true option, as shown below:

message MyMessage1 {
  enum EnumAllowingAlias {
    option allow_alias = true;
    UNKNOWN = 0;
    STARTED = 1;
    RUNNING = 1;
  }
}
message MyMessage2 {
  enum EnumNotAllowingAlias {
    UNKNOWN = 0;
    STARTED = 1;
    // RUNNING = 1;  // Uncommenting this line will cause a compile error inside Google and a warning message outside.
  }
}
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In an enumerated type, if we follow-up to delete certain enumerated types, then deleted value may be used by subsequent users, this will cause potential code hidden trouble, in order to solve this problem, the enumeration provides a reserved keyword, enumerated types by the keyword statement, will not be used, as shown below:

enum Foo {
  reserved 2, 15, 9 to 11, 40 to max;
  reserved "FOO", "BAR";
}
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The reserved keyword can also be used in the field of message to indicate that these fields should not be used in the future, as follows:

message Foo {
  reserved 2, 15, 9 to 11;
  reserved "foo", "bar";
}
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The value of the field

As you can see, each field of message is assigned a value that is unique within message and is used to locate the field within the binary message format. So once you’ve defined it, don’t change it.

Note that values 1-15 are represented by 1 byte in binary and values 16-2047 by 2 bytes, so 1-15 is usually used for the most common fields and for fields that may be duplicated to save space after encoding.

The lowest value is 1, the highest value is 2 to the 29th power -1, or 536,870,911. This middle number from 19000 to 19999 is reserved and cannot be used.

When the message is compiled, each field will be converted to the corresponding type, and each field type will be given a different initial value.

The default value for strings is empty string, bytes is empty bytes, bools is false, numeric is 0, and enumeration is the first element of the enumeration.

Field descriptor

Each field of a message can have two descriptors, the first is called singular, which means that there can be zero or one of these fields in a message, which is the default definition in Proto3.

The second, called repeated, means that the field can be repeated in a message, meaning that it represents a collection.

Add comments

Comments in proto are similar in style to C++, and can be used: // or /*… */ style comments, as shown below:

/* This is a comment. */ message SearchRequest {string query = 1; int32 page_number = 2; // page number int32 result_per_page = 3; // Results per page}Copy the code

Nested types

You can also embed a message within a message, as follows:

message SearchResponse {
  message Result {
    string url = 1;
    string title = 2;
    repeated string snippets = 3;
  }
  repeated Result results = 1;
}
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In the above example, we define a Result type in SearchResponse, which in Java can actually be thought of as a nested class.

If you want to use this internal message outside of the message definition class, you can define it with _Parent_._Type_ :

message SomeOtherMessage {
  SearchResponse.Result result = 1;
}
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Nested types can be nested arbitrarily, as follows:

message Outer { // Level 0 message MiddleAA { // Level 1 message Inner { // Level 2 int64 ival = 1; bool booly = 2; } } message MiddleBB { // Level 1 message Inner { // Level 2 int32 ival = 1; bool booly = 2; }}}Copy the code

Map

If you want to define a map in proto, you can write:

map<key_type, value_type> map_field = N;
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Here value_type can be of any type except map. Note that maps cannot be repeated.

The order of data in a map is variable, so we cannot rely on the order in which the map is stored to determine the order in which the map is retrieved.

conclusion

These are the things you should pay attention to when using protobuf.

This article is available at www.flydean.com/02-protocol…

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