I'm currently working on a project using the MongoDB Java API. I have been working on this project for a while, but have recently come across an issue that I cannot resolve. I am trying to make a database system that is fault tolerant. To simulate a database crashing, I have my program connect to a Mongodb server that I have made, execute a simple read or write, and then shut down the database server. I had originally thought that this would cause certain methods that I am calling to throw a MongoException
that I could catch and then recover from the database crash. However, I am getting a strange stack trace that says I am throwing an EOFException, among other things. Below is the stack trace itself.
Mar 04, 2013 8:06:15 PM com.mongodb.DBPortPool gotError
WARNING: emptying DBPortPool to polaris.cs.wcu.edu/152.30.5.5:12345 b/c of error
java.io.EOFException
at org.bson.io.Bits.readFully(Bits.java:48)
at org.bson.io.Bits.readFully(Bits.java:33)
at org.bson.io.Bits.readFully(Bits.java:28)
at com.mongodb.Response.<init>(Response.java:40)
at com.mongodb.DBPort.go(DBPort.java:124)
at com.mongodb.DBPort.call(DBPort.java:74)
at com.mongodb.DBTCPConnector.innerCall(DBTCPConnector.java:282)
at com.mongodb.DBTCPConnector.call(DBTCPConnector.java:256)
at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:289)
at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:274)
at com.mongodb.DBCursor._check(DBCursor.java:368)
at com.mongodb.DBCursor._hasNext(DBCursor.java:459)
at com.mongodb.DBCursor.hasNext(DBCursor.java:484)
at edu.wcu.cs.capstone.view.AbstractViewEngine.getView(AbstractViewEngine.java:57)
at edu.wcu.cs.capstone.transaction.ServerTransactionManager.getView(ServerTransactionManager.java:52)
at edu.wcu.cs.capstone.transaction.ServerTransactionManager.run(ServerTransactionManager.java:183)
at java.lang.Thread.run(Thread.java:722)
Caught exception
Mar 04, 2013 8:06:15 PM com.mongodb.DBPortPool gotError
WARNING: emptying DBPortPool to polaris.cs.wcu.edu/152.30.5.5:12345 b/c of error
java.io.IOException: couldn't connect to [polaris.cs.wcu.edu/152.30.5.5:12345] bc:java.net.ConnectException: Connec
at com.mongodb.DBPort._open(DBPort.java:214)
at com.mongodb.DBPort.go(DBPort.java:107)
at com.mongodb.DBPort.call(DBPort.java:74)
at com.mongodb.DBTCPConnector.innerCall(DBTCPConnector.java:282)
at com.mongodb.DBTCPConnector.call(DBTCPConnector.java:256)
at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:289)
at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:274)
at com.mongodb.DBCursor._check(DBCursor.java:368)
at com.mongodb.DBCursor._hasNext(DBCursor.java:459)
at com.mongodb.DBCursor.hasNext(DBCursor.java:484)
at edu.wcu.cs.capstone.view.AbstractViewEngine.getView(AbstractViewEngine.java:61)
at edu.wcu.cs.capstone.transaction.ServerTransactionManager.getView(ServerTransactionManager.java:52)
at edu.wcu.cs.capstone.transaction.ServerTransactionManager.run(ServerTransactionManager.java:183)
at java.lang.Thread.run(Thread.java:722)
DB is down.
Exception in thread "Thread-3" java.lang.NullPointerException
at edu.wcu.cs.capstone.transaction.ServerTransactionManager.run(ServerTransactionManager.java:184)
at java.lang.Thread.run(Thread.java:722)
The Caught Exception
and DB is down.
are print statements I am using to verify I am catching certain exceptions. Here is the relevant code:
public View getView(Mongo mongo, Query query) throws MongoException,
EOFException {
String connected = "";
try {
connected = mongo.getConnectPoint();
} catch (Exception e) {
throw new MongoException("Error.");
}
System.out.println("Connected: " + connected);
DB db = mongo.getDB(query.getServer());
List<DBObject> viewList = new ArrayList<DBObject>();
DBCollection collection = db.getCollection(query.getCollection());
DBCursor cursor = collection.find(query.getQuery(), excludeID);
try {
cursor.hasNext();
} catch (Exception e) {
System.out.println("Caught exception");
}
while (cursor.hasNext()) {
viewList.add(cursor.next());
}
return new View(viewList);
}
As you can see, the error is occurring when I call cursor.hasNext()
. I am also actually still catching the exception that is being thrown because of the Caught exception
. However, I am still getting a stack trace as if it was not being caught. I am suspicious that this has something to do with the DBPortPoolgotError()
method, but I have looked at the code for this method, and cannot determine what it is actually doing or even how it is being called. (GrepCode link)
As stated above, I thought the behavior for this type of code would have been to throw a MongoException
when a call on that specific Mongo
object failed because the database was no longer active. Any help that anyone could provide would be greatly appreciated!