Neo4j: explicit pessimistic locking via Java API or Cypher

Question

Is there a way to manually acquire write lock on a certain set of nodes via Neo4J Java API or Cypher?

There are examples in documentation, but only for embedded Neo4j version. Standard Java Transaction interface does not contain such methods: https://neo4j.com/docs/api/java-driver/current/org/neo4j/driver/v1/Transaction.html

Also I can't find a way to do it via Cypher.

Did you solve your question? Maybe you can share some code please? — Steffen Harbich, Oct 15 '19 at 14:32

score 3 · Answer 1 · answered Apr 25 '18 at 01:54

3

You can take a write lock by writing to a node, such as by setting or removing a property. I think this also works when removing non-existent properties.

If you have APOC Procedures installed, you can call the apoc.lock.nodes() procedure, passing it a list of the nodes to lock.

answered Apr 25 '18 at 01:54

InverseFalcon

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It works for me, thank you. The tricky part is that you need to do smth like transaction.run(...).consume() to block the thread, because of the lazy transaction API – silent-box Apr 26 '18 at 14:30
@silent-box So you changed a node to acquire the lock? – Steffen Harbich Oct 15 '19 at 14:27

score 1 · Answer 2 · answered Oct 26 '19 at 08:07

To extend the answer of @InverseFalcon, here is an example for the write lock by writing to a node:

@Test
public void testPessimisticLocking() throws InterruptedException {
    txn.execute(status -> {
        session.query("CREATE (n:Lock {uuid:'test'})", Map.of(), false);
        return null;
    });

    ExecutorService executor = Executors.newFixedThreadPool(2);
    Runnable task = () -> {
        LOG.debug("Starting task");

        txn.execute(status -> {
            long time = System.nanoTime();
            LOG.debug("Locking node with time={}", time);
            session.query("MATCH (n:Lock {uuid:'test'}) SET n.time = {0}", Map.of("0", time), false);

            LOG.debug("Query done, waiting some time...");
            try {
                Thread.sleep(5000);
            }
            catch (InterruptedException e) {
                LOG.warn("Interrupted", e);
            }
            LOG.debug("Waiting done");

            return null;
        });

        LOG.debug("Finished task");
    };

    for (int i = 0; i < 2; i++) {
        executor.execute(task);
    }

    executor.shutdown();
    executor.awaitTermination(20, TimeUnit.MINUTES);
}

The test creates a node of label "Lock". It starts two tasks in parallel that try to acquire the write lock on the node. Each task is waiting 5sec within the transaction once it has acquired the lock (simulates workload). Debug output is:

2019-10-26 09:47:09,502 [pool-3-thread-1] DEBUG - Starting task
2019-10-26 09:47:09,508 [pool-3-thread-1] DEBUG - Locking node with time=82297334790500
2019-10-26 09:47:09,513 [pool-3-thread-2] DEBUG - Starting task
2019-10-26 09:47:09,515 [pool-3-thread-2] DEBUG - Locking node with time=82297342219000
2019-10-26 09:47:09,605 [pool-3-thread-2] DEBUG - Query done, waiting some time...
2019-10-26 09:47:14,627 [pool-3-thread-2] DEBUG - Waiting done
2019-10-26 09:47:14,643 [pool-3-thread-1] DEBUG - Query done, waiting some time...
2019-10-26 09:47:14,645 [pool-3-thread-2] DEBUG - Finished task
2019-10-26 09:47:19,643 [pool-3-thread-1] DEBUG - Waiting done
2019-10-26 09:47:19,841 [pool-3-thread-1] DEBUG - Finished task

In the log you can see that one task is acquiring the lock just when the other task finished the transaction.

Neo4j: explicit pessimistic locking via Java API or Cypher

2 Answers2