Phenomenon G1a, or Aborted Read, occurs when a write performed by an aborted transaction is read by a committed transaction. G1a is legal under Read Uncommitted, but proscribed by Read Committed.

For example, imagine a pair of transactions T_i and T_j. T_i writes version x_i, and aborts. Transaction T_j reads x_i, and commits. Information from an aborted transaction has now leaked in to a committed transaction.

If writes are arbitrary transformations of values (rather than blind writes), this definition of G1a is insufficient. Imagine an aborted transaction T_i writes x_i, and a committed transaction T_j writes x_j = f(x_i). Finally, a committed transaction T_k reads x_j. Clearly, information from an aborted transaction is visible to a committed one, but G1a is technically not present. One could resolve this by treating writes as implicit reads: T_i and T_j would then be G1a. Jepsen opts to call this phenomenon dirty update.

Formally

G1a is defined in Adya’s thesis, section 3.2.2:

G1a: Aborted Reads. A history H exhibits phenomenon G1a if it contains an aborted transaction T_i and a committed transaction T_j such that T_j has read some object (maybe via a predicate) modified by T_i.

See Also

Aborted read is clearly analogous to the ANSI SQL anomaly P1 (Dirty Read). However, P1 has two interpretations: one allows non-Serializable histories, and the other rules out Serializable histories. Adya’s G1 captures the spirit of P1 in three general, precise phenomena: G1a (Aborted Read), G1b (Intermediate Read), and G1c (Cyclic Information Flow). Ruling out all three ensures Read Committed.