The Protobuf binary format is compact and efficient, and it has clever features that allow for a wide variety of schema changes to be both backward- and forward-compatible.

However, it is not possible to make meaningful sense of the data without a schema. Not only is it not human-friendly, since all fields are identified by an integer instead of a semantic name, but it also uses a very simple wire format which re-uses various value encoding strategies for different value types. This means it is not even possible to usefully interpret encoded values without a schema — for example, one cannot know (with certainty) if a value is a text string, a binary blob, or a nested message structure.

But there exists a category of systems and use cases where it is necessary or useful to decode the data at runtime, by a process or user agent that does not have prior (compile-time) knowledge of the schemas:

1. RPC debugging. It is useful for a human to be able to meaningfully interpret/examine/modify RPC requests and responses (with tools like tcpdump, Wireshark, or Charles proxy). But without the schema, these payloads are inscrutable byte sequences.
2. Persistent store debugging (includes message queues): This is similar to the above use case, but the human is looking at data blobs in a database or durable queue. A key difference between this case and the one above is that it is likely to observe messages produced over a longer period of time, using many versions of the schema as it evolved over time.
3. Data pipeline schemas and transformations: This is less for human interaction and more for data validation and transformation. A producer may be pushing binary blobs of encoded Protobuf into a queue or publish/subscribe system. The system may want to verify that the blob is actually valid for the expected type of data, which requires a schema. The consumer may need the data in an alternate format; the only way to transform the binary data into an alternate format is to have the schema. Further, the only way to avoid dropping data is to have a version of the schema that is no older than the version used by the publisher. (Otherwise, newly added fields may not be recognized and then silently dropped during a format transformation.)

All of these cases call for a mechanism by which the schema for a particular message type can be downloaded on demand, for interpreting the binary data.

The Buf Reflection API provides exactly that mechanism. It provides a means of downloading the schema for any module in the BSR, and even any specific version of a module.

In addition to querying for the schema by module name and version, this API also allows the caller to signal what part of the schema in which they are interested, such as a specific message type or a specific service or method. This is used to filter the schema, allowing the client to ignore parts of a module that it does not need. In many cases, the client only needs a small subset of the module's schema (espcially for large modules), so this can greatly reduce the amount of data that a client needs to download.