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Two-Phase Commit (2PC)

A protocol guaranteeing atomicity across multiple resources, involving a prepare and commit phase.
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The statement of the theorem

Let C={Coordinator,Participant1,…,Participantk}\mathcal{C} = \{\text{Coordinator}, \text{Participant}_1, \dots, \text{Participant}_k\} be the set of nodes. The Two-Phase Commit protocol proceeds through two phases:\n\n**Phase 1: Prepare (Voting)**\n1. Coordinator→PREPAREParticipanti\text{Coordinator} \xrightarrow{\text{PREPARE}} \text{Participant}_i. \n2. Participanti\text{Participant}_i writes all necessary changes to stable storage (undo/redo logs) and replies VOTEYES\text{VOTE}_{YES} if successful, or VOTENO\text{VOTE}_{NO} otherwise.\n\n**Phase 2: Commit/Abort (Decision)**\n1. If Coordinator\text{Coordinator} receives VOTEYES\text{VOTE}_{YES} from all Participanti\text{Participant}_i: Coordinator→COMMITParticipanti\text{Coordinator} \xrightarrow{\text{COMMIT}} \text{Participant}_i. Participanti\text{Participant}_i makes changes permanent.\n2. Otherwise: Coordinator→ABORTParticipanti\text{Coordinator} \xrightarrow{\text{ABORT}} \text{Participant}_i. Participanti\text{Participant}_i rolls back changes.
Source: Wikipedia