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POhow to avoid race condition when using Scala's Actor
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  1. POhow to avoid race condition when using Scala's Actor
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    copied!<p>I am writing a piece of code that would populate a mongoDB collection when the buffer (list) grow to a certain size. </p> <pre><code>import scala.actors.Actor import com.mongodb.casbah.Imports._ import scala.collection.mutable.ListBuffer class PopulateDB extends Actor { val buffer = new ListBuffer[DBObject] val mongoConn = MongoConnection() val mongoCol = mongoConn("casbah_test")("logs") def add(info: DBObject = null) { if (info != null) buffer += info if (buffer.size &gt; 0 &amp;&amp; (info == null || buffer.length &gt;= 1000)) { mongoCol.insert(buffer.toList) buffer.clear println("adding a batch") } } def act() { loop { react { case info: DBObject =&gt; add(info) case msg if msg == "closeConnection" =&gt; println("Close connection") add() mongoConn.close } } } } </code></pre> <p>However, when I run the following code, scala will occasionally throw a "ConcurrentModificationException" on the "mongoCol.insert(buffer.toList)" line. I am pretty sure it has something to do with "mongoCol.insert". I am wondering if there is anything fundamentally wrong with the code. Or should I use something like the "atomic {...}" from Akka to avoid the issue.</p> <p>Here's the complete stack trace:</p> <pre><code>PopulateDB@7e859a68: caught java.util.ConcurrentModificationException java.util.ConcurrentModificationException at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:373) at java.util.LinkedHashMap$EntryIterator.next(LinkedHashMap.java:392) at java.util.LinkedHashMap$EntryIterator.next(LinkedHashMap.java:391) at org.bson.BSONEncoder.putObject(BSONEncoder.java:113) at org.bson.BSONEncoder.putObject(BSONEncoder.java:67) at com.mongodb.DBApiLayer$MyCollection.insert(DBApiLayer.java:215) at com.mongodb.DBApiLayer$MyCollection.insert(DBApiLayer.java:180) at com.mongodb.DBCollection.insert(DBCollection.java:85) at com.mongodb.casbah.MongoCollectionBase$class.insert(MongoCollection.scala:561) at com.mongodb.casbah.MongoCollection.insert(MongoCollection.scala:864) at PopulateDB.add(PopulateDB.scala:14) at PopulateDB$$anonfun$act$1$$anonfun$apply$1.apply(PopulateDB.scala:26) at PopulateDB$$anonfun$act$1$$anonfun$apply$1.apply(PopulateDB.scala:25) at scala.actors.ReactorTask.run(ReactorTask.scala:34) at scala.actors.Reactor$class.resumeReceiver(Reactor.scala:129) at PopulateDB.scala$actors$ReplyReactor$$super$resumeReceiver(PopulateDB.scala:5) at scala.actors.ReplyReactor$class.resumeReceiver(ReplyReactor.scala:69) at PopulateDB.resumeReceiver(PopulateDB.scala:5) at scala.actors.Actor$class.searchMailbox(Actor.scala:478) at PopulateDB.searchMailbox(PopulateDB.scala:5) at scala.actors.Reactor$$anonfun$startSearch$1$$anonfun$apply$mcV$sp$1.apply(Reactor.scala:114) at scala.actors.Reactor$$anonfun$startSearch$1$$anonfun$apply$mcV$sp$1.apply(Reactor.scala:114) at scala.actors.ReactorTask.run(ReactorTask.scala:36) at scala.concurrent.forkjoin.ForkJoinPool$AdaptedRunnable.exec(ForkJoinPool.java:611) at scala.concurrent.forkjoin.ForkJoinTask.quietlyExec(ForkJoinTask.java:422) at scala.concurrent.forkjoin.ForkJoinWorkerThread.mainLoop(ForkJoinWorkerThread.java:340) at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:325) </code></pre> <p>Thanks, Derek</p>
 

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