You've highlighted a very important part of the paper. A lot of the external consistency guarantees provided by Zanzibar are facilitated by Spanner and its TrueTime mechanism. Warrant doesn't currently support/use Spanner. However, for the databases we do support (MySQL and Postgres - which are both ACID compliant), we've implemented the zookie protocol using the incrementing transaction ids they provide. This approach works for single writer deployments of these databases, so know that write throughput and overall availability will be lower. We started with this approach because most teams still use MySQL/Postgres. Warrant is built to support running on different types of databases, so we will be working on support for Spanner and other multi-writer distributed databases like Cockroach and Yugabyte in the future. I hope that helps.