I enjoyed reading this SpiceDB introduction blog post from 2021: https://authzed.com/blog/spicedb-is-open-source-zanzibar by @User I found the anecdote about database CPU and Quay to be an interesting way to frame the motivation: "While building the Quay container registry, which was eventually acquired by CoreOS, we took the standard path and delegated permissions to our application framework. This model brought us quickly to production, but would later constrain us in terms of flexibility and performance. There are some product features still in the icebox because the permissions system couldn’t model recursive relationships (think groups that belong to groups) out of the box. When our service became popular, computing and re-computing permissions became the single largest consumer of database CPU in our production Quay.io service." I also really liked the section that differentiates SpiceDB from Zanzibar as well as other implementations, including (1) support for multiple data stores, (2) treating users as just another type of object, (3) allowing computation of reverse permissions, (4) defining the SpiceDB Schema Language