FAQ.md

Frequently asked questions

Questions about Protocol Buffers

Protocol Buffers is designed and maintained by Google.

Google has very thorough documentation including FAQ.

If your question is not rust-protobuf-specific, it's likely you can find your answer there.

How to serialize a message to bytes?

The easiest way to do it is to invoke write_to_bytes function:

message.write_to_bytes()

How to deserialize a message from a byte array?

let my_message: MyMessage = protobuf::parse_from_bytes(bytes).unwrap()

What is cached_size field?

Before serializing protobuf need to know sizes of all nested structures.

So serialization of protobuf message is done in two steps:

• first, compute sizes of all nested messages
• second, write the message tree with each message prepended by serialized size

So before serialization protobuf implementation stores size of a message into cached_size field.

(The same strategy used at least in Google's C++ and Java implementation of Protocol Buffers)

Sometimes it's annoying. Previously rust-protobuf stored message sizes in external buffer, but it was slightly worse performance-wise.

rust-protobuf could have an option to skip generation of cached_size and use external buffer when there's no such field. If you need that could be useful to you, please open an issue.

What is unknown_fields field?

When unknown field is encountered during parsing (e. g. field added in newer version of .proto file) this field data is stored in unknown_fields. The purpose of this field is to be able to perform loseless read-modify-write operation even with older version of proto file which does not know about new fields.

You answer is not there?

Feel free to open an issue with a question.