Why you must not modify existing fields in future Protobuf versions

Protobuf doesn't prevent you from changing data types on your fields. But not all data types are compatible with each other. If you change the data type of just one of your fields to a data type that isn't compatible with it, you will make your whole interface incompatible with the existing clients.

In the following list, each bullet contains the data types that can be interchanged with each other:

• int32, uint32, int64, uint64, and bool
• int32, uint32, int64, uint64, and enum values
• sint32 and sint64
• string and bytes, but only if the bytes value uses UTF-8 encoding
• fixed32 and sfixed32
• fixed64 and sfixed64

However, just because you can change the data type of a field, it doesn't mean that you should. For example, what would happen if you sent a negative value as int32, but consumed it as a positive-only uint32 data type on the other side? The original value cannot...