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Asynchronously replicated primary-backup databases are commonly deployed toimprove availability and offload read-only transactions. To both applyreplicated writes from the primary and serve read-only transactions, thebackups implement a cloned concurrency control protocol. The protocol ensuresread-only transactions always return a snapshot of state that previouslyexisted on the primary. This compels the backup to exactly copy the commitorder resulting from the primary's concurrency control. Existing clonedconcurrency control protocols guarantee this by limiting the backup'sparallelism. As a result, the primary's concurrency control executes someworkloads with more parallelism than these protocols. In this paper, we provethat this parallelism gap leads to unbounded replication lag, where writes cantake arbitrarily long to replicate to the backup and which has led tocatastrophic failures in production systems. We then design C5, the firstcloned concurrency protocol to provide bounded replication lag. We implementtwo versions of C5: Our evaluation in MyRocks, a widely deployed database,demonstrates C5 provides bounded replication lag. Our evaluation in Cicada, arecent in-memory database, demonstrates C5 keeps up with even the fastest ofprimaries.

C5: Cloned Concurrency Control that Always Keeps Up