Chardonnay: Fast and General Datacenter Transactions for On-Disk Databases

Distributed on-disk database systems could either use an expensive commit protocol like two-phase commit (2PC) to guarantee atomicity, and suffer from slow distributed transactions, or forgo 2PC, which lead to weaker semantics, limitations to the programming model, or constrained scalability, making the system less general. We argue this compromise is no longer necessary within modern datacenters. Low latency 2PC (∼150 µs on Azure for 2PC over Paxos) can be achieved using lowlatency storage for the relatively small transaction logs, fast RPCs, and careful protocol design. With fast 2PC, the data contention bottleneck for many transactions shifts from 2PC to reading the data itself from the relatively slow storage while holding transaction locks.

We present Chardonnay, a scalable, on-disk, multiversioned transactional key-value store optimized for single datacenter deployments with fast 2PC. Chardonnay has a general interface supporting point reads, scans, and writes within multi-step strictly serializable ACID transactions. The key mechanism underlying Chardonnay's design is strongly consistent snapshot reads on commodity hardware, using a novel lock-free read protocol. Chardonnay uses this protocol to cheaply determine the read-write sets of queries, enabling Chardonnay to transparently prefetch data needed for a transaction prior to the execution of the transaction and the acquisition of locks. This enables Chardonnay to achieve fast transactions by minimizing contention, and avoids aborts due to deadlocks by ordering lock requests.