mwhittaker / occ

Optimistic Concurrency Control

Optimistic Concurrency Control

Transactions executed with optimistic concurrency control are divided into three phases:

1. a read phase during which a transaction reads and writes values to a local cache,
2. a validation phase during which a transaction checks to see if its execution is consistent with a serialization of concurrently executing transactions, and
3. a write phase during which a transaction copies its cached writes into the database (provided it passed validation).

A transaction is assigned a timestamp when it enters its validation phase. The logic to validate transaction j is as follows:

• for all transactions i < j:
  • if the write set of i overlaps the read set of j:
    • transaction i must finish its write phase before transaction j starts its read phase
  • elif the write set of i overlaps the write set of j:
    • transaction i must finish its write phase before transaction j starts its write phase
  • else:
    • the two transactions can overlap arbitrarily

This repo includes a toy implementation of the OCC algorithms presented in [1].

• [1]: Kung, Hsiang-Tsung, and John T. Robinson. "On optimistic methods for concurrency control." ACM Transactions on Database Systems (TODS) 6.2 (1981): 213-226.