[morelisp]

This lets you identify the version but it doesn't let old clients read the new messages. (Well, for avro and others they still can if the new fields aren't important or the old fields aren't required - but if you can do that you also don't really have a new incompatible version and you don't need the schema hash to begin with.)

The point is that with a pull-based API, I have a fixed number of requests. As clients migrate from /v1 to /v2, load on /v1 goes down and /v2 goes up, and I can adjust resource allocations accordingly to keep the total requirements relatively constant. I can even reimplement /v1 in terms of /v2 internally in many cases and have ~0 operational overhead.

But for an evented system, as soon as just a single client wants v2 I need to publish that, and as long as any client wants v1 I need to publish that. So my outbound "work" (at the very least i/o but probably also DTO conversions and god help you if it's any kind of storage or business logic) is doubled immediately and remains doubled until everything is migrated.