What are custom options?

Protobuf custom options are a way to annotate part of the schema with contextual information. Think of them as normal Protobuf options but with custom names. It is as simple as that. The reason they exist is also simple – it is extensibility. If you do not find the right option for your use case, just create one by yourself.

Now, let us face it, the first time we hear about Protobuf custom options, it is hard to understand why, except for the pretty abstract extensibility concept, we would need them. As such, I want to show you some examples of custom options and how they are used.

The first project that is very popular and uses Protobuf custom options is protovalidate (https://github.com/bufbuild/protovalidate/). If you have never heard of it, it is a “series of libraries designed to validate Protobuf messages at runtime based on user-defined validation rules”.

Let us see an example. Let us say that we want the name of the user...