But this is not true for events. If you change the body such that you now need to maintain two versions of an event, then you have to publish both events simultaneously, which means double the server side effort, storage etc for each event version. It’s pretty inefficient, and painful. You can work out who subscribes to the old event but there is still a big efficiency hit.

You might be right about many reads per event in a simplistic way; if you have a lot of clients then it could be expensive if you don’t have a server side cache. But there would typically be a lot of temporality in such a system so it seems like an easy problem to solve for most use cases; you don’t have to cache for long, but caches are of course tricky if your use case is not very simple. That said, if there is already a HTTP connection open then the additional latency and bandwidth hit cause by this events are going to be minimal in most cases, and probably drowned out entirely if you need to push multiple versions.

As I said in another thread, I should have said that thin is my default. There are cases when fat makes more sense, but normally I’d start with thin and see if I need to flesh it out. Whenever I’ve started fat I’ve ended up reverting.