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Connectors and APIs Manual  /  ...  /  Configuring Load Balancing with Connector/J

3.8.3 Configuring Load Balancing with Connector/J

Connector/J has long provided an effective means to distribute read/write load across multiple MySQL server instances for Cluster or source-source replication deployments. You can dynamically configure load-balanced connections, with no service outage. In-process transactions are not lost, and no application exceptions are generated if any application is trying to use that particular server instance.

The load balancing is configured at the initial setup stage of the server connection by the following connection URL, which has a similar format as the general JDBC URL for MySQL connection, but a specialized scheme:

jdbc:mysql:loadbalance://[host1][:port],[host2][:port][,[host3][:port]]...[/[database]] »

There are two configuration properties associated with this functionality:

  • loadBalanceConnectionGroup – This provides the ability to group connections from different sources. This allows you to manage these JDBC sources within a single class loader in any combination you choose. If they use the same configuration, and you want to manage them as a logical single group, give them the same name. This is the key property for management: if you do not define a name (string) for loadBalanceConnectionGroup, you cannot manage the connections. All load-balanced connections sharing the same loadBalanceConnectionGroup value, regardless of how the application creates them, will be managed together.

  • ha.enableJMX – The ability to manage the connections is exposed when you define a loadBalanceConnectionGroup; but if you want to manage this externally, enable JMX by setting this property to true. This enables a JMX implementation, which exposes the management and monitoring operations of a connection group. Further, start your application with the JVM flag. You can then perform connect and perform operations using a JMX client such as jconsole.

Once a connection has been made using the correct connection properties, a number of monitoring properties are available:

  • Current active host count.

  • Current active physical connection count.

  • Current active logical connection count.

  • Total logical connections created.

  • Total transaction count.

The following management operations can also be performed:

  • Add host.

  • Remove host.

The JMX interface, com.mysql.cj.jdbc.jmx.LoadBalanceConnectionGroupManagerMBean, has the following methods:

  • int getActiveHostCount(String group);

  • int getTotalHostCount(String group);

  • long getTotalLogicalConnectionCount(String group);

  • long getActiveLogicalConnectionCount(String group);

  • long getActivePhysicalConnectionCount(String group);

  • long getTotalPhysicalConnectionCount(String group);

  • long getTotalTransactionCount(String group);

  • void removeHost(String group, String host) throws SQLException;

  • void stopNewConnectionsToHost(String group, String host) throws SQLException;

  • void addHost(String group, String host, boolean forExisting);

  • String getActiveHostsList(String group);

  • String getRegisteredConnectionGroups();

The getRegisteredConnectionGroups() method returns the names of all connection groups defined in that class loader.

You can test this setup with the following code:

public class Test {
    private static String URL = "jdbc:mysql:loadbalance://" +
        "localhost:3306,localhost:3310/test?" +
    public static void main(String[] args) throws Exception {
        new Thread(new Repeater()).start();
        new Thread(new Repeater()).start();
        new Thread(new Repeater()).start();
    static Connection getNewConnection() throws SQLException, ClassNotFoundException {
        return DriverManager.getConnection(URL, "root", "");
    static void executeSimpleTransaction(Connection c, int conn, int trans){
        try {
            Statement s = c.createStatement();
            s.executeQuery("SELECT SLEEP(1) /* Connection: " + conn + ", transaction: " + trans + " */");
        } catch (SQLException e) {
    public static class Repeater implements Runnable {
        public void run() {
            for(int i=0; i < 100; i++){
                try {
                    Connection c = getNewConnection();
                    for(int j=0; j < 10; j++){
                        executeSimpleTransaction(c, i, j);
                        Thread.sleep(Math.round(100 * Math.random()));
                } catch (Exception e) {

After compiling, the application can be started with the flag, to enable remote management. jconsole can then be started. The Test main class will be listed by jconsole. Select this and click Connect. You can then navigate to the com.mysql.cj.jdbc.jmx.LoadBalanceConnectionGroupManager bean. At this point, you can click on various operations and examine the returned result.

If you now had an additional instance of MySQL running on port 3309, you could ensure that Connector/J starts using it by using the addHost(), which is exposed in jconsole. Note that these operations can be performed dynamically without having to stop the application running.

For further information on the combination of load balancing and failover, see Section 3.8.5, “Advanced Load-balancing and Failover Configuration”.

Configuring Load Balancing with DNS SRV

See Section 3.5.14, “Support for DNS SRV Records” for details.