You should think of the sequencer layer as a shared log abstraction along the lines of the Corfu project from Microsoft. It is distributed and replicated, with the scheduler layer reading from their local copy. Stalled scheduler nodes do not block writes in the system.
I mistyped, I meant to say that a stalled sequencer stalls all schedulers. It's true that Corfu has impressive per-sequencer throughput (by moving most of the work onto other nodes), but you have to move to multiple sequencers to get to Spanner scale.
So to get back to Spanner's availability (if you need it), you need the Calvin sequencer Paxos groups to span data centers. Since you're not exploiting the commutativity structure of transactions you need either a leader-based consensus implementation, which will have latency stalls when a leader becomes unavailable (amplified by the all-to-all communication), or you can use old-school 2 RTT Paxos, and your end latency ends up the same as Spanner.
Lest it seem like I'm not actually a fan of the log-based approach, let me point out a way in which Calvin crushes Spanner: write contention. So long as the Calvin transactions can encode the application logic, they can support extremely high write rates on contended objects in the database. Spanner's 2PL, on the other hand, has single-object object update rates visible to the naked eye.
Corfu looks interesting. I need a distributed, consistent, persistent log for a project. Corfu is quite complex (three different RAM-hungry Java components that each wants to be distributed), however. Do you know of any about other, more lightweight, but still decently scalable alternatives? I was thinking about Etcd, but apparently it's not designed for large amounts of data.
