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  1. Java Driver
  2. JAVA-680

Secondary reads on lists of mongos fail until first write

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      The fix for JAVA-656 introduced a regression. In the case where you have use the Mongo constructor that takes a list of ServerAddress, or a MongoURI with a list of server addresses, _masterPortPool starts as null. Before this change, the checkMaster call would block until an initial master was found. Now that it no longer does, there is a race between the background thread and application threads. If an application only does non-primary reads, and the list of servers is a list of mongos, until a write is done, then ensureMaster will never be called and you will keep getting this exception:

      10-25-2012 17:01:00 [Thread-427] ERROR com.xgen.svc.mms.svc.ping.BatchPrePopulationSvc [run:136] - Rare case where master=null, probably all servers are down
com.mongodb.MongoException: Rare case where master=null, probably all servers are down
	at com.mongodb.DBTCPConnector$MyPort.get(DBTCPConnector.java:434) ~[mongo.jar:na]
	at com.mongodb.DBTCPConnector.innerCall(DBTCPConnector.java:276) ~[mongo.jar:na]
	at com.mongodb.DBTCPConnector.call(DBTCPConnector.java:256) ~[mongo.jar:na]
	at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:289) ~[mongo.jar:na]
	at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:274) ~[mongo.jar:na]
	at com.mongodb.DBCursor._check(DBCursor.java:368) ~[mongo.jar:na]
	at com.mongodb.DBCursor._hasNext(DBCursor.java:459) ~[mongo.jar:na]
	at com.mongodb.DBCursor.hasNext(DBCursor.java:484) ~[mongo.jar:na]

      This test demonstrates the problem. 

      public class JAVA656Test {
    public static void main(String[] args) throws UnknownHostException {
        Mongo mongo = new Mongo(Arrays.asList(new ServerAddress("localhost:30999")));
        mongo.setReadPreference(ReadPreference.secondary());
        DBCollection collection = mongo.getDB("test").getCollection("JAVA656Test");
        for (int i = 0; i < 5; i++) {
            try {
                collection.findOne();
            } catch (MongoException e) {
                System.out.println(e.getMessage());
            }
        }

        System.out.println("Inserting");
        collection.insert(new BasicDBObject());

        System.out.println("Trying again");
        collection.findOne();
        System.out.println("Done");
    }
}

      Output is:

      Rare case where master=null, probably all servers are down
Rare case where master=null, probably all servers are down
Rare case where master=null, probably all servers are down
Rare case where master=null, probably all servers are down
Rare case where master=null, probably all servers are down
Inserting
Trying again
Done

            Assignee:
            Unassigned Unassigned
            Reporter:
            jeff.yemin@mongodb.com Jeffrey Yemin
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              Created:
              Updated:
              Resolved: