Super interesting how many companies are building authorization systems based on Zanzibar suddenly! This is a bit of a shameless plug, but I just wrote a blog post earlier today talking through how to build Zanzibar from scratch in ~150 lines of code: https://news.ycombinator.com/item?id=28076549
Not many people talk about how Zanzibar requires you re-architect your application around authorization when you really don't need to do that at all. Any sufficiently powerful authorization framework can handle the same flexibility. If not more, since most Zanzibar implementations can't handle simple attribute-based controls (e.g. anyone can read a document if its public). Which means you'll end up implementing a bunch of authorization logic in your app anyway.
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Edit to add: I realise in hindsight I got a bit too absorbed in thinking about the Zanzibar part to say congrats on the launch! It's awesome to see the space heating up, and love to see more focus on the developer experience :)