I'm having a hard time understanding the shape of the state that's derived applying that entity's events vs a projection of that entity's data.
Is an Aggregate's state ONLY used for determining whether or not a command can successfully be applied? Or should that state be usable in other ways?
An example - I have a Post entity for a standard blog post. I might have events like postCreated, postPublished, postUnpublished, etc. For my projections that I'll be persisting in my read tables, I need a projection for the base posts (which will include all posts, regardless of status, with lots of detail) as well as published_posts projection (which will only represent posts that are currently published with only the information necessary for rendering.
In the situation above, is my aggregate state ONLY supposed to be used to determine, for example, if a post can be published or unpublished, etc? If this is the case, is the shape of my state within the aggregate purely defined by what's required for these validations? For example, in my base post projection, I want to have a list of all users that have made a change to the post. In terms of validation for the aggregate/commands, I couldn't care less about the list of users that have made changes. Does that mean that this list should not be a part of my state within my aggregate?
TL;DR: yes - limit the "state" in the aggregate to that data that you choose to cache in support of data change.
In my aggregates, I distinguish two different ideas:
There's not a lot of value in caching results that we are never going to use.
One of the underlying lessons of CQRS is that we don't need aggregates everywhere
An AGGREGATE is a cluster of associated objects that we treat as a unit for the purpose of data changes. -- Evans, 2003
If we aren't changing the data, then we can safely work directly with immutable copies of the data.