Used them before and they're ok. They were missing some important features like sum types. The code output was a pain, but targeted a few languages. My suspicion is that Captain Proto would be technically superior but less documented than flatbuffers.

However, my preference is to use something like MsgPack or CBOR with compile time reflection to do serde directly into types. You can design the types to require minimal allocations and to parse messages within a few dozen nanoseconds. That means doing things like using static char arrays for strings. It wastes a bit of space but it can be very fast. Also skipping out on spaced used by 64bit pointers can replace a lot of shorter text fields.

That said, I should wrap or port this UPB to Nim. It'd be a nice alternative if it's really as fast as claimed. Though how it handles allocating would be the clincher.

> They were missing some important features like sum types.

They support unions now. I haven't had any trouble representing anything including recursive structures.

> The code output was a pain, but targeted a few languages.

You do need a day or so to get used to the code. Its a pointer based system with a 'flat memory'. Order of operations matters. If you have a parent and a child you need to write the child first, obtain a pointer to it and only then create/write the parent containing the pointer to the child. Once you get used to this it goes quickly. The advantage is that you don't have to create an intermediate copy in memory when reading (like protobuf) and you can read any particular part by traversing pointers without having to load the rest of the data into memory.