What to use for a backend tech stack?

Yeah, these are very cool problems you don't really get to see building a common thing like an e-commerce site.

I have a single top-level document open at a time right now, but that has many documents referenced by it. Firestore's data model has concepts of documents and collections. The root of the db is your base collections. For me, that is /users and /spaces. A space is a users workspace that can have many pages, and each page can have many blocks (paragraphs). A firestore document under /spaces holds the top-level data, with a collection /pages that references all of the firestore documents representing pages in my app. Each firestore document holding a page holds a reference to another collection /elements. Firestore documents under elements hold the text and metadata for each paragraph. This data model creates a tree structure:

// Sorry for this ill-formatted code, I thought it would be monospaced
/spaces <= collection
    abdc123 (spaceId) <= document
        /pages <= collection
            fghj456 (pageId) <= document
                /elements <= collection
                     uiyt789  (elementId) <= document

The document ids can be generated by firebase, or you can set them yourself. The reason I had to make it so nested is that each firestore document has a size limit of 1Mb. A paragraph would likely never get there, but a single page could.

Right now, on save, I'm essentially just doing a full replace starting at the space id. A user is really only updating one element at a time, though. Unless they delete a lot at once. I just need to be smarter about managing the database references down to each element, identifying what actually changed, and saving only that. That will significantly cut down on writes to the DB.

I'm excited about websockets! I'd love to hear how that implementation goes for you. I think I'm going to stick with firebase for this (for a while at least). The js SDK provides an implementation where you can subscribe to real-time updates of any firestore document. It uses websockets under the hood, but you don't have to do any channel implementation. Just have to manage subscriptions which play nicely with a redux model.

As for the problem of having the same user logged in on multiple devices, you could maybe handle it how it seems google docs does. Generate some deviceId fingerprint and combine that with the user name. If you log into google docs on two devices, you'll see a collaborator and the name is yours. On only one device, there is no other collaborator. Each device always having the latest via websockets definitely helps this be less of a problem, too.

Sorry for the book! I can quickly get out of hand when talking about implementation details. Thanks for the back and forth