JDBC Connection Failover
If a client application attempts to connect to a host in the Vertica Analytic Database cluster that is down, the connection attempt fails when using the default connection configuration. This failure usually returns an error to the user. The user must either wait until the host recovers and retry the connection or manually edit the connection settings to choose another host.
Due to Vertica Analytic Database's distributed architecture, you usually do not care which database host handles a client application's connection. You can use the client driver's connection failover feature to prevent the user from getting connection errors when the host specified in the connection settings is unreachable. It gives you two ways to let the client driver automatically attempt to connect to a different host if the one specified in the connection parameters is unreachable:
- Configure your DNS server to return multiple IP addresses for a host name. When you use this host name in the connection settings, the client attempts to connect to the first IP address from the DNS lookup. If the host at that IP address is unreachable, the client tries to connect to the second IP, and so on until it either manages to connect to a host or it runs out of IP addresses.
- Supply a list of backup hosts for the client driver to try if the primary host you specify in the connection parameters is unreachable.
For both methods, the process of failover is transparent to the client application (other than specifying the list of backup hosts, if you choose to use the list method of failover). If the primary host is unreachable, the client driver automatically tries to connect to other hosts.
Failover only applies to the initial establishment of the client connection. If the connection breaks, the driver does not automatically try to reconnect to another host in the database.
Choosing a Failover Method
You usually choose to use one of the two failover methods. However, they do work together. If your DNS server returns multiple IP addresses and you supply a list of backup hosts, the client first tries all of the IPs returned by the DNS server, then the hosts in the backup list.
If a host name in the backup host list resolves to multiple IP addresses, the client does not try all of them. It just tries the first IP address in the list.
The DNS method of failover centralizes the configuration client failover. As you add new nodes to your Vertica Analytic Database cluster, you can choose to add them to the failover list by editing the DNS server settings. All client systems that use the DNS server to connect to Vertica Analytic Database automatically use connection failover without having to change any settings. However, this method does require administrative access to the DNS server that all clients use to connect to the Vertica Analytic Database cluster. This may not be possible in your organization.
Using the backup server list is easier than editing the DNS server settings. However, it decentralizes the failover feature. You may need to update the application settings on each client system if you make changes to your Vertica Analytic Database cluster.
Using DNS Failover
To use DNS failover, you need to change your DNS server's settings to map a single host name to multiple IP addresses of hosts in your Vertica Analytic Database cluster. You then have all client applications use this host name to connect to Vertica Analytic Database.
You can choose to have your DNS server return as many IP addresses for the host name as you want. In smaller clusters, you may choose to have it return the IP addresses of all of the hosts in your cluster. However, for larger clusters, you should consider choosing a subset of the hosts to return. Otherwise there can be a long delay as the client driver tries unsuccessfully to connect to each host in a database that is down.
Using the Backup Host List
To enable backup list-based connection failover, your client application has to specify at least one IP address or host name of a host in the BackupServerNode
parameter. The host name or IP can optionally be followed by a colon and a port number. If not supplied, the driver defaults to the standard Vertica port number (5433). To list multiple hosts, separate them by a comma.
The following example demonstrates setting the BackupServerNode
connection parameter to specify additional hosts for the connection attempt. The connection string intentionally has a non-existent node, so that the initial connection fails. The client driver has to resort to trying the backup hosts to establish a connection to Vertica.
import java.sql.*; import java.util.Properties; public class ConnectionFailoverExample { public static void main(String[] args) { // Assume using JDBC 4.0 driver on JVM 6+. No driver loading needed. Properties myProp = new Properties(); myProp.put("user", "dbadmin"); myProp.put("password", "vertica"); // Set two backup hosts to be used if connecting to the first host // fails. All of these hosts will be tried in order until the connection // succeeds or all of the connections fail. myProp.put("BackupServerNode", "VerticaHost02,VerticaHost03"); Connection conn; try { // The connection string is set to try to connect to a known // bnad host (in this case, a host that never existed). conn = DriverManager.getConnection( "jdbc:vertica://BadVerticaHost:5433/vmart", myProp); System.out.println("Connected!"); // Query system to table to see what node we are connected to. // Assume a single row in response set. Statement stmt = conn.createStatement(); ResultSet rs = stmt.executeQuery( "SELECT node_name FROM v_monitor.current_session;"); rs.next(); System.out.println("Connected to node " + rs.getString(1).trim()); // Done with connection. conn.close(); } catch (SQLException e) { // Catch-all for other exceptions e.printStackTrace(); } } }
When run, the example outputs output similar to the following on the system console:
Connected! Connected to node v_vmart_node0002
Notice that the connection was made to the first node in the backup list (node 2).
Notes
- When native connection load balancing is enabled, the additional servers specified in the BackupServerNode connection parameter are only used for the initial connection to a Vertica host. If host redirects the client to another host in the database cluster to handle its connection request, the second connection does not use the backup node list. This is rarely an issue, since native connection load balancing is aware of which nodes are currently up in the database. See Enabling Native Connection Load Balancing in JDBC for more information.