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Atomic Commit and Two-Phase Commit (2PC)

In Chapter 7 we learned that the purpose of transaction atomicity is to provide simple semantics in the case where something goes wrong in the middle of making several writes. The outcome of a transaction is either a successful commit, in which case all of the transaction’s writes are made durable, or an abort, in which case all of the transaction’s writes are rolled back (i.e., undone or discarded).

Atomicity prevents failed transactions from littering the database with half-finished results and half-updated state. This is especially important for multi-object transactions (see “Single-Object and Multi-Object Operations” on page 228) and databases that maintain secondary indexes. Each secondary index is a separate data structure from the primary data—thus, if you modify some data, the corresponding change needs to also be made in the secondary index. Atomicity ensures that the secondary index stays consistent with the primary data (if the index became inconsistent with the primary data, it would not be very useful).

 
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