[OAK-566] MongoMK throws exception when adding nodes concurrently

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 0.7
• Component/s: mongomk
• Assignee: Unassigned
• Reporter: Damien Obrist
• Created: 17/Jan/13 13:21
• Updated: 21/May/13 11:26
• Resolved: 24/Jan/13 14:47

Description

It seems that there is a problem in the MongoDB microkernel when lots of threads try to commit new nodes concurrently. I have attached a JUnit test which reproduces the error. The test creates 16 microkernels, each committing two nodes in a separate thread. Some threads will throw an exception saying that the node they're trying to commit already exists:

java.lang.RuntimeException: There's already a child node with name 'node6'
at org.apache.jackrabbit.mongomk.impl.instruction.CommitCommandInstructionVisitor.visit(CommitCommandInstructionVisitor.java:97)
at org.apache.jackrabbit.mongomk.impl.instruction.AddNodeInstructionImpl.accept(AddNodeInstructionImpl.java:40)
at org.apache.jackrabbit.mongomk.impl.command.CommitCommandNew.createMongoNodes(CommitCommandNew.java:155)
at org.apache.jackrabbit.mongomk.impl.command.CommitCommandNew.execute(CommitCommandNew.java:97)
at org.apache.jackrabbit.mongomk.impl.command.CommitCommandNew.execute(CommitCommandNew.java:1)
at org.apache.jackrabbit.mongomk.impl.command.DefaultCommandExecutor.execute(DefaultCommandExecutor.java:38)
at org.apache.jackrabbit.mongomk.impl.MongoNodeStore.commit(MongoNodeStore.java:110)
at org.apache.jackrabbit.mongomk.impl.MongoMicroKernel.commit(MongoMicroKernel.java:112)
... 7 more

This cannot be the case however, as the nodes added by the different threads do not overlap / conflict. No exceptions are thrown if setting the number of threads to some lower number, but I have observed that in this case the problem still exists when trying to add a larger number of nodes (more than just 2).

Issue Links

• relates to: OAK-560 MongoMK.commit() not atomic