Java driver 2.3.

We have 3 mongo servers. Each has 48G with 24 CPUs. The servers are running with replica set.

[joseph.wang@lpsdb1.la2 ~]$ /usr/local/mongodb-linux-x86_64-1.6.3/bin/mongo localhost:4110
MongoDB shell version: 1.6.3
connecting to: localhost:4110/test
> rs.status()
{
"set" : "prod",
"date" : "Sat Dec 11 2010 13:04:28 GMT-0800 (PST)",
"myState" : 1,
"members" : [

{ "_id" : 0, "name" : "lpsdb1.la2.estalea.net:4110", "health" : 1, "state" : 1, "self" : true }

,

{ "_id" : 1, "name" : "mongo-prod-mem2.lps.la2.estalea.net:4110", "health" : 1, "state" : 2, "uptime" : 11095, "lastHeartbeat" : "Sat Dec 11 2010 13:04:27 GMT-0800 (PST)" }

,

{ "_id" : 2, "name" : "mongo-prod-mem3.lps.la2.estalea.net:4110", "health" : 1, "state" : 2, "uptime" : 11178, "lastHeartbeat" : "Sat Dec 11 2010 13:04:26 GMT-0800 (PST)" }

],
"ok" : 1
}

The connection pool was connecting to all three servers. We manually brought down two slaves to see if we can get query result (as part of fault tolerance testing).
There were no update/writes. Just query. When 1 slave was down, we'd no problem getting query result. When 2 slaves were down, we got no query result.
When a consumer query comes in, we fork out multiple threads, each responsible for fetch data from specific collection.

MongoConnection.java shows our singleton connection pool code.
BaseTableQueryEngine.java shows our query to one of our collections.

As you can see, we set slaveOk at the query level.

if (db != null) {
DBCollection coll = db.getCollection(currentCollection);
DBCursor cur = null;

cur = coll.find(dbQuery).addOption(
Bytes.QUERYOPTION_SLAVEOK);

DBObject dbObject = db.getLastError();
if (dbObject != null && dbObject.get("err") != null)

{ log.warn("BaseTableQueryEngine: Encounter error for query " + dbQuery.toString()); setSuccess(false); }

if (enable_debug)

{ log.debug("BaseTableQueryEngine: Run query " + dbQuery.toString()); log.debug("BaseTableQueryEngine: Found " + cur.count() + " in " + (System.currentTimeMillis() - fStart)); }

while (cur.hasNext()) {
BasicDBObject dbo = (BasicDBObject) cur.next();
BaseTableRow row = new BaseTableRow(dbo);
if (row.isValid())

{ tuples.add(row.getTuple()); }

long time = (timeout - (System.currentTimeMillis() - fStart));
if (time < 0)

{ break; }

}

if (enable_debug)

{ log.debug("BaseTableQueryEngine: tuples " + tuples.size() + " in time " + (System.currentTimeMillis() - fStart)); }

}