In this recipe, we will demonstrate how to connect to a replica set using a Java client and execute queries and insert data using the Java client for MongoDB. We will also see how the client would automatically failover to another member in the replica set should a primary member goes down.
We first need to take a look at the Connecting to a single node from a Java client recipe, as it contains all the prerequisites and steps to set up Maven and other dependencies. As we are dealing with a Java client for replica sets, a replica set must be up and running. Refer to the Starting multiple instances as part of a replica set recipe for details on how to start the replica set.
Let's take a look at the steps in detail:
First, we need to write/copy the following piece of code (this Java class is also available for download from the book's site):
package com.packtpub.mongo.cookbook; import com.mongodb.BasicDBObject; import com.mongodb.DB; import com.mongodb.DBCollection; import com.mongodb.DBObject; import com.mongodb.MongoClient; import com.mongodb.ServerAddress; import java.util.Arrays; /** * */ public class ReplicaSetMongoClient { /** * Main method for the test client connecting to the replica set. * @param args */ public static final void main(String[] args) throws Exception { MongoClient client = new MongoClient( Arrays.asList( new ServerAddress("localhost", 27000), new ServerAddress("localhost", 27001), new ServerAddress("localhost", 27002) ) ); DB testDB = client.getDB("test"); System.out.println("Dropping replTest collection"); DBCollection collection = testDB.getCollection("replTest"); collection.drop(); DBObject object = new BasicDBObject("_id", 1).append("value", "abc"); System.out.println("Adding a test document to replica set"); collection.insert(object); System.out.println("Retrieving document from the collection, this one comes from primary node"); DBObject doc = collection.findOne(); showDocumentDetails(doc); System.out.println("Now Retrieving documents in a loop from the collection."); System.out.println("Stop the primary instance manually after few iterations"); for(int i = 0 ; i < 20; i++) { try { doc = collection.findOne(); showDocumentDetails(doc); } catch (Exception e) { //Ignoring or log a message } Thread.sleep(5000); } } /** * * @param obj */ private static void showDocumentDetails(DBObject obj) { System.out.printf("_id: %d, value is %s\n", obj.get("_id"), obj.get("value")); } }Connect to any of the nodes in the replica set, say to
localhost:27000, and, from the shell, executers.status(). Take a note of the primary instance in the replica set and connect to it from the shell iflocalhost:27000is not a primary node. Now, switch to the admin database as follows:repSetTest:PRIMARY>use adminNow, execute the preceding program from the operating system shell as follows:
$ mvn compile exec:java -Dexec.mainClass=com.packtpub.mongo.cookbook.ReplicaSetMongoClientShut down the primary instance by executing the following command on the Mongo shell connected to the primary node:
repSetTest:PRIMARY> db.shutdownServer()Watch the output on the console where the
com.packtpub.mongo.cookbook.ReplicaSetMongoClientclass is executed using Maven.
An interesting thing to observe is how we instantiate a MongoClient instance. It is done as follows:
MongoClient client = new MongoClient(Arrays.asList(new ServerAddress("localhost", 27000),new ServerAddress("localhost", 27001),new ServerAddress("localhost", 27002)));The constructor takes a list of com.mongodb.ServerAddress. This class has a lot of overloaded constructors, but we chose to use the one that takes the hostname and port number. Here. we provided all the server details in a replica set as a list. We haven't mentioned what the primary node is and what the secondary nodes are. The MongoClient class is intelligent enough to figure this out and connect to the appropriate instance. The list of servers provided is called the seed list. It need not contain an entire set of servers in a replica set, though the objective is to provide as much as we can. The MongoClient class will figure out all the server details from the provided subset. For example, if the replica set is of five nodes but we provide only three servers, it still works fine. On connecting with the provided replica set servers, the client will query them to get the replica set metadata and figure out the rest of the provided servers in the replica set. In the preceding case, we instantiated the client with three instances in the replica set. If the replica set has five members, instantiating the client with just three of them as we did earlier is still good enough, and the remaining two instances will be automatically discovered.
Next, we will start the client from the command prompt using Maven. Once the client is running in the loop to find one document, we will bring down the primary instance. We will see something like the following output on the console:
_id: 1, value is abc Now, retrieving documents in a loop from the collection. Stop the primary instance manually after a few iterations: _id: 1, value is abc _id: 1, value is abc Nov 03, 2013 5:21:57 PM com.mongodb.ConnectionStatus$UpdatableNode update WARNING: Server seen down: Amol-PC/192.168.1.171:27002 java.net.SocketException: Software caused connection abort: recv failed at java.net.SocketInputStream.socketRead0(Native Method) at java.net.SocketInputStream.read(SocketInputStream.java:150) … WARNING: Primary switching from Amol-PC/192.168.1.171:27002 to Amol-PC/192.168.1.171:27001 _id: 1, value is abc
As we can see, the query in the loop was interrupted when the primary node went down. The client, however, switched to the new primary node seamlessly, well, nearly seamlessly, as the client might have to catch an exception and retry the operation after a predetermined interval has elapsed.