I dont know full details of Cap'n Proto, so please correct me if I am wrong. My impression is that Cap'n Proto is a re-implementation of protocol buffers with additional functionality for RPC wrapped in a futures/promises abstraction. Would that be a reasonable summary of Cap'n Proto ?

Guessing from the name I expected FlatBuffers to be a unified file format for on-disk and on-RAM presence, so that you can just mmap the disk file and you have the data-structure in memory. No need to translate between on disk and on disk representations. I am not claiming that it is so, this is what I expected it to be, given the name. But after spending some time looking around I am still confused if that's what it is. Would appreciate confirmation either way. Have to look at the code when I get around to it.

I have an use case in mind for a tree of appendable/insertable matrices. HDf5 comes close, but isnt quite what I am looking for.

EDIT @vertex-four thanks for the clarification much appreciated.