> OK, well, I've worked on lots of systems -- across three different companies -- where this feature is essential. Here's an actual real life example: Chrome u

> OK, well, I've worked on lots of systems -- across three different companies -- where this feature is essential. Here's an actual real life example: Chrome uses this in Chrome Sync feature that allows you to sync your browser configuration and state across various devices. The feature is implemented basically like this: Chrome sends its version of state to server in a proto, and the Sync server reconciles it with the one it has saved, updating both according to which one is more recent. The feature fundamentally depends on the fact that the client won't drop the fields it doesn't know, because it would then be data loss for some other more recent client who knows these fields and synced them to server: if the older client dropped these unknown fields, it would be an equivalent to syncing in an empty value of this field. Original designers of proto3 (the most recent protocol buffers definition language and semantics) actually decided to drop the unknown field preservation, for simplicity reasons. This made Googlers so unhappy that an internal doc was created listing many internal use cases for this feature, and after discussion, this was added back to proto3.