Querying tag based replica set

Question

I have configured my replica set with tag fields as given in doc example:

{
    "_id" : "rs0",
    "version" : 2,
    "members" : [
             {
                     "_id" : 0,
                     "host" : "mongodb0.example.net:27017",
                     "tags" : {
                             "dc": "east",
                             "use": "production"
                     }
             },
             {
                     "_id" : 1,
                     "host" : "mongodb1.example.net:27017",
                     "tags" : {
                             "dc": "east",
                             "use": "reporting"
                     }
             },
             {
                     "_id" : 2,
                     "host" : "mongodb2.example.net:27017",
                     "tags" : {
                             "use": "production"
                     }
             }
     ]
}

Now my question is if I send read queries with read concern like "nearest","primarypreferred" will it be able to pick this tags automaticaly, or i should pass the "dc" name along with my find query.If anyone can help me regarding the same.

Answer 1

If you want to set tags with your read preference (whatever it is except primary), you have to set it manually, something such as:

DBCollection collection = ...    

ReadPreference dcReadPref =
     ReadPreference.nearest(new BasicDBObject("dc", "east"));

collection.findOne(new BasicDBObject("field", "value"), null, dcReadPref);

Unfortunately, the find() method doesn't have a variance including ReadPreference as argument (at least couldn't find such in javadoc) . You will have to do something like this:

DBCollection collection = ...
ReadPreference defaultReadPref = collection.getReadPreference();
collection.setReadPreference(dcReadPref);
...
collection.find(new BasicDBObject("field", "value"));
...
collection.setReadPreference(defaultReadPref); // when done restore default

You can set read preference (w/wo tags) at driver, database, collection or operation level.

To also include your environment in tags set:

String env = ... / get it from somewhere

DBObject dcTag = new BasicDBObject("dc", "east"):
DBObject envTag = new BasicDBObject("use", env);

ReadPreference pref = ReadPreference.primaryPreferred(dcTag, envTag);