Hi, I had a similar issue (also with ID generation), and I would be also interested in a solution, or how synchronizations should be done to avoid deadlocks like this in the transaction.
Regards, --- balazs On 11/2/11 8:55 AM, Cres wrote: > (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 _______________________________________________ Neo4j mailing list [email protected] https://lists.neo4j.org/mailman/listinfo/user
