Lecture 025

Transaction (ACID Properties): atomic operation (resilient to power failure)

• Atomicity: all or nothing (commit, abort/rollback)

• Consistency: no break invariants

• Isolation: concurrent transactions do not interfere with each other

• Durability: once we declare committed, it must exist

Rollback Siturations:

• conflict: two transactions try to update the same file, we can only commit one of them, the other must be rolled back

• explicit software rollback: condition check failed

• system crash: roll back to last stable state

Write Ahead Log (WAL)

Write Ahead Log (WAL): sequential log of intended changes, [task1, task2, ..., commit, task3, task4, ..., commit, ...]

• before doing any write, write the intended change to a contiguous log (append-only)

• make sure log can be applied multiple times (idempotent, no "increment" operation, only "set to value")

• write backs (resume) can be applied asynchronously

Journaling: only apply WAL to metadata (most OS). Logging: apply WLA to metadata and data. (ext4 support both, but default to journaling)