Maps

Finally, we can talk about how maps are encoded in Protobuf. In Chapter 3 on Protobuf text format, I briefly mentioned that a map is a list of objects that contains the key and value fields. In this section, we are going to dive deeper into this and see how maps are encoded.

First, let’s not take for granted that a map is a list of objects. Let’s investigate that. We can define a message containing a map field (map/encoding.proto):

syntax = "proto3";
message Encoding {
  map<string, int32> m = 1;
}

Now, to see how this translates internally, we can turn that proto file into a descriptor file. Protoc has a flag called --descriptor_set_out for doing that. Let’s create a descriptor file called encoding.desc:

$ protoc --descriptor_set_out=encoding.desc encoding.proto

This file contains a binary of FileDescriptorSet, which is a message defined in the descriptor.proto file provided with protoc. Now, we can decode this descriptor...