Kudu transaction semantics

This is a brief introduction to Kudu’s transaction and consistency semantics. Kudu's core philosophy is to provide transactions with simple, strong semantics, without sacrificing performance or the ability to tune to different requirements. Kudu’s transactional semantics and architecture are inspired by state-of-the-art systems such as Spanner and Calvin. For an in-depth technical exposition of what is mentioned here, see the Technical Report: HybridTime - Accessible Global Consistency with High Clock Uncertainty.

Kudu currently allows the following operations:

• Scans are read operations that can traverse multiple tablets and read information with different levels of consistency or correctness guarantees. Scans can also perform time-travel reads. That is, you can set a scan timestamp from the past and get back results that reflect the state of the storage engine at that point in time.

• Write operations are sets of rows to be inserted, updated, or deleted in the storage engine, in a single tablet with multiple replicas. Write operations do not have separate "read sets", that is, they do not scan existing data before performing the write. Each write is only concerned with the previous state of the rows that are about to change. Writes are not "committed" explicitly by the user. Instead, they are committed automatically by the system, after completion.

While Kudu is designed to eventually be fully ACID (Atomic, Consistent, Isolated, Durable), multi-tablet transactions have not yet been implemented. As such, the following discussion focuses on single-tablet write operations, and only briefly touches multi-tablet reads.