Rolling Upgrade — Unknown Fields Can Be Lost Diagram

Rolling Upgrade

New code deploys to a few nodes at a time while old nodes keep serving
Old and new code, and old and new data formats, coexist for a while
The system needs both backward and forward compatibility

The Hazard

Old code decodes a record, updates a few fields, writes it back
If the decoder drops fields it does not understand, they are gone forever
The loss is silent: no error is raised at write time

The Fix

Schema-driven encodings identify fields by tag or name
Old code skips unknown fields but preserves their bytes on round-trip
Readers fill defaults for fields the writer never sent