[sitkack]

I have version N of a struct and then having version N+1 of a struct in-flight at the same time is almost impossible with current statically typed languages. In a dynamically typed language, as long as the contents of version N+k struct are additive and don't change the semantics, old code can read new data.

What needs to happen is both, immutable code, and versioned structs with pure functions that can upgrade and possibly downgrade structs as needed. The larger the distributed system, the versions of a struct (message) will be in-flight at a time. Services need to contain no state, so that they can be micro-rebooted and brought up with the new version.

Joe Armstrong had a comment on globally accessible but immutable code, which I think would go a long way towards the ability to statically type the inputs to a function in a distributed system. Interposition and routing would be the only way to upgrade or deprecate old code paths.