(Sorry for re-opening this thread, I hadn't yet subscribed to the mailing list when I opened it the previous time, so I had to re-start it..)
Hi, I'm using Neo4J version 1.4.1. I've tried following the design guide ( http://wiki.neo4j.org/content/Design_Guide#Make_use_of_Factories ) writing a short program, but when I try to run it, it goes into a deadlock and the application freezes. Basically what my program does is to create a new embedded graph database, initalize a factory node, then it creates two threads where each of them is tasked with creating two nodes, giving them an ID (according to the pattern shown in the design guide), and that's about it. Once I run it, one thread end up on WAIT state, while the other is on MONITOR. Showing the threads dump, I can see that the MONITOR-state thread is waiting for the other thread to release the ID generation method's lock ("_lock"), and the WAIT-state thread is waiting inside Neo4J's RWLock class, trying to acquire a write lock, but for some reason it's waiting (line 326). It got there from a call in the ID generation method which tries to update the factory node's ID sequence property, as shown in the design guide. I'm appending the code here. I'm aware that this program misses some things (such as shutting down the DB once i'm done, adding the relationship between factory node and created node, and for the createNode() method to actually return a node) - i've left out everything I could to have the least amount of code which makes this problem occur. The code: public class MultiThreadingBasicTest { private final GraphDatabaseService _graphDB; private final Object _lock; private final Node _factoryNode; public static void main(String[] args) { MultiThreadingBasicTest test = new MultiThreadingBasicTest(); test.run(); } public MultiThreadingBasicTest() { _graphDB = new EmbeddedGraphDatabase("/MTBT-graphDB"); _lock = new Object(); _factoryNode = createFactoryNode(); } public void run() { Thread[] threads = new Thread[2]; for (int i=0; i < threads.length; i++) { threads[i] = new Thread(new Runnable() { @Override public void run() { Transaction tx = _graphDB.beginTx(); try { for (int i=0; i < 2; i++) { createNode(); System.out.println("echo from thread " + Thread.currentThread().getName()); } tx.success(); } finally { tx.finish(); } } }); } System.out.println("running the threads now"); for (Thread thread : threads) { thread.start(); } } private Node createFactoryNode() { Node factoryNode = null; Transaction tx = _graphDB.beginTx(); try { factoryNode = _graphDB.createNode(); tx.success(); } finally { tx.finish(); } return factoryNode; } private void createNode() { long id = generateId(); // Node node = _graphDB.createNode(); // // node.setProperty("idseq", id); } private long generateId() { synchronized (_lock) { Long id; if (_factoryNode.hasProperty("idseq")) { id = (Long) _factoryNode.getProperty("idseq"); } else { id = 1L; } _factoryNode.setProperty("idseq", id+1); return id; } } } (One last thing - I kept the line: System.out.println("echo from thread " + Thread.currentThread().getName()); since if I remove it, the deadlock actually does not occur - at least not on my machine. However this is just a matter of race conditions, and if you use a higher number of threads / created-nodes-per-thread, then the problem will occur even without this line. I decided to keep it since I figured a case with two threads each creating two nodes is as simply as this can get). Thank you for your help, Ran. -- View this message in context: http://neo4j-community-discussions.438527.n3.nabble.com/Node-Id-generation-deadlock-tp3473118p3473118.html Sent from the Neo4j Community Discussions mailing list archive at Nabble.com. _______________________________________________ Neo4j mailing list [email protected] https://lists.neo4j.org/mailman/listinfo/user
