Created
August 12, 2011 07:02
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Two operation updates in Google's Appengine using two, 2-phase locking transactions
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| The goal is to insert a new event and update the counter for it and have both | |
| operations fail or both succeed. The solution below places each of these | |
| 2 operations is in a 2 two-phase locking transaction which must attempt to | |
| both succeed or fail, and so the pattern of | |
| lock1 operation1 (commit1 and lock2 operation2 commit2) or (rollback1) | |
| is applied, with an additional pattern of first operation successfully committed | |
| with the second retried if it fails. | |
| private Key insert(Event event) { | |
| PersistenceManager pmRT = null; | |
| PersistenceManager pm = null; | |
| Transaction txRT = null; | |
| Transaction tx = null; | |
| final List<Boolean> insertSucceeded = new ArrayList<Boolean>(); | |
| final List<Boolean> counterUpdateSucceeded = new ArrayList<Boolean>(); | |
| try { | |
| pmRT = TransactionsRequiredPMF.get().getPersistenceManager(); | |
| pm = PMF.get().getPersistenceManager(); | |
| txRT = pmRT.currentTransaction(); | |
| tx = pm.currentTransaction(); | |
| txRT.setSynchronization(new javax.transaction.Synchronization() { | |
| public void beforeCompletion() { | |
| } | |
| public void afterCompletion(int status) { | |
| switch (status) { | |
| case javax.transaction.Status.STATUS_ROLLEDBACK : | |
| break; | |
| case javax.transaction.Status.STATUS_COMMITTED: | |
| log.info("insert succeeded"); | |
| insertSucceeded.add(Boolean.TRUE); | |
| break; | |
| } | |
| } | |
| }); | |
| tx.setSynchronization(new javax.transaction.Synchronization() { | |
| public void beforeCompletion() { | |
| } | |
| public void afterCompletion(int status) { | |
| switch (status) { | |
| case javax.transaction.Status.STATUS_ROLLEDBACK : | |
| if (!insertSucceeded.isEmpty() && insertSucceeded.get(0)) { | |
| log.severe("insert succeeded, but update did not"); | |
| startIncrementEventAndVenueCountersTasks(); | |
| } | |
| break; | |
| case javax.transaction.Status.STATUS_COMMITTED: | |
| counterUpdateSucceeded.add(Boolean.TRUE); | |
| break; | |
| } | |
| } | |
| }); | |
| event.setKey(); | |
| // this connection insists on transactional, so entities within scope | |
| // are immediately attached. set any keys before beginning the transaction | |
| txRT.begin(); | |
| pmRT.makePersistent(event); | |
| FetchAiderUtility.useGetters(event); | |
| pmRT.flush(); | |
| txRT.commit(); | |
| // if insert succeeded, update counters | |
| if (!insertSucceeded.isEmpty() && insertSucceeded.get(0)) { | |
| tx.begin(); | |
| log.info("increment counters"); | |
| EventCounterDAO.incrementNumberOfEntities(); | |
| VenueCounterDAO.incrementNumberOfEntities(); | |
| tx.commit(); | |
| } else { | |
| log.info("we will not commit transaction 2"); | |
| } | |
| } finally { | |
| if ((txRT != null) && txRT.isActive()) { | |
| txRT.rollback(); | |
| if ((tx != null) && tx.isActive()) { | |
| tx.rollback(); | |
| } | |
| } | |
| if (pmRT != null) { | |
| pmRT.close(); | |
| } | |
| if (pm != null) { | |
| pm.close(); | |
| } | |
| } | |
| return event.getKey(); | |
| } | |
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