An ExternalDecider is given the final control over the decision of whether or not to commit a transaction, which allows two-phase commit to be integrated with a single non-transactional resource. shouldCommit will only be called if a InTxn has successfully called all of its before-commit handlers, acquired all necessary write locks, validated all of its reads and called all of its while-preparing handlers. The decider may then attempt a non-transactional operation whose outcome is uncertain, and based on the outcome may directly cause the transaction to commit or roll back.