[trishume]

I was talking purely about humans finding messages they don't have the schema for during the course of debugging. Computers do equally badly with JSON data and Cap'n Proto data they don't have the schema for.

A fake example scenario of what I'm talking about: Someone notices a server is producing way too many errors, they check the logs and see a bunch of "invalid request: ...." messages. The messages are JSON like {"api_version":2,"cpu_usage":0.57}. They figure out that the server monitoring system is forwarding stats to the wrong IP address. Even though they don't know the full schema of the messages, they get the gist and it helped debug. With Cap'n Proto that would have been a few nonsensical bytes.

I don't think this is a very big downside, so in general I like protocols like Cap'n Proto.

Perhaps a much bigger downside is that in most languages integrating code generation into the build pipeline is a pain in the ass, so it is way easier to use dynamic serialization protocols that don't require compiling a schema.