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Manuals / The Buf Schema Registry (BSR) / Building with the BSR / Reflection


API Usage

The Buf Reflection API can be found in the public BSR: (sources are in GitHub). You can see the available remote packages for it here.

It contains a single RPC service: buf.reflect.v1beta1.FileDescriptorSetService. This service contains a single endpoint named GetFileDescriptorSet, which is for downloading the schema for a particular module (optionally, at a specific version). The response is in the form of a FileDescriptorSet. You can find reference documentation for all the request and response fields in the BSR.

For the general mechanics of how to use APIs exposed by the BSR, see API Access.

The endpoint accepts a module name, in <bsr-domain>/<owner>/<repo> format. For example, is the module name for the Eliza service (a demo service for Connect). The domain of the BSR is "" (the public BSR); the owner is the "bufbuild" organization; and the repo name is "eliza".

Here's an example API request for downloading the module:

> POST /buf.reflect.v1beta1.FileDescriptorSetService/GetFileDescriptorSet HTTP/1.1
> Host:
> Authorization: Bearer <insert-buf-token-here>
> Content-Type: application/json
> Connect-Protocol-Version: 1
> {"module": ""}

Assuming a valid BSR token is used in the Authorization header, this will return a FileDescriptorSet that describes the files in the requested module, which describe the Eliza RPC service and all related message types.

The above request does not contain a version field in the request, which means it will return the latest version. This is the same as asking for "version": "main", which also returns the latest version. The version can also refer to a commit, either via the commit name or an associated tag. Or the version can refer to the name of a draft.

These are the same ways one can pin a particular version in the deps section of a buf.yaml file.

See the Overview section on dependencies for more.

Filtering the Schema

The request may also include a field named symbols that is an array of fully-qualified names. If present and non-empty, the returned schema will be pruned to only include the data required to describe the requested symbols. All other content in the module will be omitted. This is particularly useful with large modules, to reduce the amount of schema data that a client needs to download. For example, let's say a client needs the schema for a single service, but it's defined in a large module that defines many services. The request can indicate the name of the service of interest in the symbols field and will get back only what they need and nothing else. Here's an example that returns only the google.longrunning.Operations service from the module:

> POST /buf.reflect.v1beta1.FileDescriptorSetService/GetFileDescriptorSet HTTP/1.1
> Host:
> Authorization: Bearer <insert-buf-token-here>
> Content-Type: application/json
> Connect-Protocol-Version: 1
> {
>    "module": "",
>    "version": "75b4300737fb4efca0831636be94e517",
>    "symbols": ["google.longrunning.Operations"]
> }

This currently returns a response that is about 11k. If we leave out the symbols field from the request, the response would be about 10x that size.

Dynamic Messages

Once you have downloaded a set of descriptors, the next step is what to do with it. Having the whole schema allows for building dynamic messages -- which are backed by a descriptor at runtime instead of by generated code.

The general shape of this solution is two-fold:

  1. Convert FileDescriptorProto instances to "rich" data structures that are cross-linked and indexed. This makes it easy to traverse type references in the schema. This process also validates the schema, to make sure it is not missing any necessary elements and is valid per the rules of the Protobuf language.
  2. Use a "rich" descriptor that describes a message to construct a dynamic message. This message acts on most ways like a regular generated message. You can unmarshal message data from an array of bytes or vice versa, marshal the message's data to bytes. You can examine the field values of the message, too. Since it is not a generated type, however, you can't access fields in the normal way since your code doesn't even know what fields the message has at compile-time.

The power of a dynamic message is that it enables an "appliance" that can process message data of arbitrary types in cross-cutting ways. A particularly powerful and common use case is to examine fields and field options to redact sensitive data/PII, convert to JSON, and then store in a data warehouse for use with business intelligence tools. Without a dynamic message, you have to write a bespoke message processor that must be recompiled and re-deployed whenever any of the message definitions are changed. With a dynamic message, you can compile and deploy the service once, but then must provide the service with updated message definitions as they change; that's where the BSR and the Buf Reflect API come in!

To get a sense of how the API can be used to perform functionality described in the above paragraph, take a look at our example client library.

Unfortunately, not all languages/runtimes have support for descriptors and dynamic messages. Here are ones that do, with links to relevant API documentation.

There are other languages (C#, PHP) that include some support for descriptors, but only for runtime reflection; they do not provide dynamic message support. There also may be third-party language runtimes that offer this support.