[Solution] Mason: Scalable, Contiguous Sequencing for Building Consistent Services

Some recent services use a sequencer to simplify ordering operations on sharded data. The sequencer assigns each operation a multi-sequence number which explicitly orders the operation on each shard it accesses. Existing sequencers have two shortcomings. First, failures can result in some multi-sequence numbers never being assigned, exposing a non-contiguous multi-sequence, which requires complex scaffolding to handle. Second, existing implementations use single-machine sequencers, limiting service throughput to the ordering throughput of one machine.We make two contributions. First, we posit that sequencers should expose our new contiguous multi-sequence abstraction. Contiguity guarantees every sequence number is assigned an operation, simplifying the abstraction. Second, we design and implement MASON , the first system to expose the contiguous multi-sequence abstraction and the first to provide a scalable multi-sequence. MASON is thus an ideal building block for consistent, scalable services. Our evaluation shows MASON unlocks scalable throughput for two strongly-consistent services built on it.