Protobuf Field Tags

Fields tags are numbers that serve as unique IDs for each fields in a binary format. They are important because Protocol Buffer do not store names to save space and instead, it uses those IDs. Essentially it:

• Encodes only field numbers and values
• Allows efficient binary serialisation (don’t have to convert the field name to binary)

Rules

• Numbers must be unique per message. You cannot use the same number for two different fields.
• Use small numbers that only take 1 byte (1-15)
• 16-2047 Take 2 bytes
• 19000-19999 - are reserved for internal use, so you cannot use it
• Do not change field numbers, because it will break compatibility. If you change age = 3 to age = 5, old clients will read the wrong data.

Example

syntax = "proto3"  
  
message User {  
	string id = 1;  
	string name = 2;  
	int32 age = 3;  
}  

When the User message is serialised the resulting binary data would look as follows

08 96 01 12 05 4a 6f 68 6e  

Field	ProtoBuf Binary Representation	Explanation
id = 150	08 96 01	08 = Field 1 + Varint, 96 01 = 150
name = "Joe"	12 03 4A 6F 65	12 = Field 2 + String, 03 = length, "Joe" = 4A 6F 65
age = 30	18 1E	18 = Field 3 + Varint, 1E = 30