Neo4j:通过Java API或Cypher显式悲观锁定

Question

有没有办法通过Neo4J、Java API或Cypher手动获取某组节点上的写锁？

有examples in documentation，但仅适用于嵌入式Neo4j版本。标准的Java Transaction接口不包含这样的方法：https://neo4j.com/docs/api/java-driver/current/org/neo4j/driver/v1/Transaction.html

另外，我也找不到通过Cypher来实现的方法。

Answer 1

您可以通过写入节点来获取写锁，例如通过设置或移除属性。我认为这在删除不存在的属性时也是有效的。

如果安装了APOC Procedures，则可以调用apoc.lock.nodes()过程，向其传递要锁定的节点列表。

Answer 2

为了扩展@InverseFalcon的答案，这里有一个通过写入节点实现写锁的示例：

@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);
}

该测试创建一个标签为"Lock“的节点。它并行启动两个任务，试图获取节点上的写锁。一旦获得锁，每个任务在事务中等待5秒(模拟工作负载)。调试输出为：

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

在日志中，您可以看到就在另一个任务完成事务时，另一个任务正在获取锁。