Add locking module: optimistic vs pessimistic locking on Hibernate 7.4.14.Final

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YPgGH6aMbSLANL3E2HHmS6

locking/output/01-lost-update.txt

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+start balance 100; transaction A deposits 10, transaction B deposits 20
+both read 100 before either writes; A commits first, then B
+final balance: 120  (a correct result would be 130)
+
+update statements Hibernate sent:
+update Account set balance=?,owner=? where id=?
+update Account set balance=?,owner=? where id=?

locking/output/02-version-stops-it.txt

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+same race, entity has @Version
+A: committed. B: failed
+B exception:   jakarta.persistence.OptimisticLockException
+caused by:     org.hibernate.StaleObjectStateException
+final balance: 110, version: 1
+
+update statements Hibernate sent:
+update VersionedAccount set balance=?,owner=?,version=? where id=? and version=?
+update VersionedAccount set balance=?,owner=?,version=? where id=? and version=?

locking/output/03-retry-loop.txt

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+6 threads x 5 deposits of 1, each retried on OptimisticLockException
+final balance: 30 (expected 30)
+version:       30
+retries happened: true
+retry count in this run: 27  (varies from run to run)

locking/output/04-pessimistic-write.txt

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+A locks row 1 with PESSIMISTIC_WRITE, holds it 600 ms, deposits 10
+B asks for the same lock, then deposits 20
+B waited at least 400 ms for the lock: true
+final balance: 130  (correct: 130)
+
+select statements Hibernate sent (one per transaction):
+select a1_0.id,a1_0.balance,a1_0.owner from Account a1_0 where a1_0.id=? for update
+select a1_0.id,a1_0.balance,a1_0.owner from Account a1_0 where a1_0.id=? for update

locking/output/05-no-wait.txt

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+A holds the lock; B asks with Timeouts.NO_WAIT
+B exception: org.hibernate.exception.LockTimeoutException
+    caused by org.h2.jdbc.JdbcSQLTimeoutException
+      H2 message: Timeout trying to lock table "ACCOUNT"; SQL statement:
+    caused by org.h2.mvstore.MVStoreException
+
+select statements B sent:
+select a1_0.id,a1_0.balance,a1_0.owner from Account a1_0 where a1_0.id=? for update nowait

locking/output/06-skip-locked.txt

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+5 tickets, all NEW. Worker A claims 2 and keeps its transaction open.
+Worker B claims 2 with SKIP LOCKED.
+A got tickets: [1, 2]
+B got tickets: [3, 4]
+
+select statements B sent:
+select t1_0.id,t1_0.claimedBy,t1_0.status from Ticket t1_0 where t1_0.status='NEW' order by t1_0.id fetch first ? rows only for update skip locked

locking/output/07-deadlock.txt

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+opposite lock order (A: 1 then 2, B: 2 then 1)
+transactions that failed: 1
+  jakarta.persistence.PessimisticLockException
+    caused by org.hibernate.exception.LockAcquisitionException
+    caused by org.h2.jdbc.JdbcSQLTransactionRollbackException
+      H2 message: Deadlock detected. The current transaction was rolled back. Details: "ACCOUNT"; SQL statement:
+    caused by org.h2.mvstore.MVStoreException
+
+same lock order for both (always the lower id first)
+transactions that failed: 0