https://www.cockroachlabs.com/blog/living-without-atomic-clocks/
blog-post-living-without-atomic-clocks#linearizability-vs-serializability1 2While Spanner provides linearizability, CockroachDB's external consistency guarantee is by default only serializability, though with some features that can help bridge the gap in practice. blog-post-living-without-atomic-clocks#linearizability-vs-serializability1 2
blog-post-living-without-atomic-clocks#causality-token1 2 3For situations where reordering could be problematic, CockroachDB returns a causality token, which is just the maximum timestamp encountered during a transaction. If passed from one actor to the next in a causal chain, the token serves as a minimum timestamp for successive transactions and will guarantee that each has a properly ordered commit timestamp blog-post-living-without-atomic-clocks#causality-token1 2 3
In Spanner blog-post-living-without-atomic-clocks#spanner-node-must-wait-7ms1Before a node is allowed to report that a transaction has committed, it must wait 7ms. blog-post-living-without-atomic-clocks#spanner-node-must-wait-7ms1