atomic commitment

it doesn’t allow disagreement between participants -> a transaction will not commit if even one of participants votes against it

2-phase commit

contains
- a leader/coordinator that holds the state, collects votes
- cohorts (other nodes) vote to accept/reject values that are proposed by the coordinator
2 phases
- prepare: the coordinator sends propose message to all cohorts and collects/persists their votes
- commit/abort

cohort failures

if one of cohorts is unavailable or fails during the propose phase, the coordinator will abort the transaction

coordinator failures

if one of cohorts doesn’t receive a commit/abort command from the coordinator, it should find out which decision was made by the coordinator
if the coordinator fails after phase 1 and before phase 2 (commit/abort), the system will be uncertainty -> blocking state

3-phase commit

fix coordinator failures of 2PC
3 phases
- prepare
- pre-commit: safety buffer. the coordinator sends result to cohorts and prepare them to commit (cohorts send ACK to the coordinator)
- commit/abort
if ACK is not sent to the coordinator from any cohort in phase 1 and 2, transaction will be aborted

failure point	Outcome
P1	coordinator sends abort
P2	coordinator sends abort
P3	coordiantor ignores and finishes (transaction is committed)

distributed transaction models

Calvin

Spanner

Percolator