Todd's MySQL Blog

Tiny tidbits of trivia from Todd

Archive for the month “August, 2010”

Connector/J ping (there will be a test on this)

MySQL Connector/J has a useful feature which executes a lightweight ping against a server (or, in the case of load-balanced connections, all active pooled internal connections that are retained) to validate the connection. As you might guess, this feature is useful for Java apps which use connection pools, so that the pool can validate the connection. Depending on your connection pool and configuration, this can be done at different times:

  • before the pool returns a connection to the application
  • when the application returns a connection to the pool
  • during periodic checks of idle connections

So, if you want to use this magic light-weight ping process, here’s how you do it:

Specifying a “validation query” in your connection pool that starts with “/* ping */” _exactly_ will cause the driver to instead send a ping to the server and return a fake result set (much lighter weight), and when using a ReplicationConnection or a LoadBalancedConnection, will send the ping across all active connections.

So says the relevant note in the changelogs.

Now for the test.  Which of the following Strings match the condition above?

  • sql = “/* PING */ SELECT 1″;
  • sql = “SELECT 1 /* ping*/”;
  • sql = “/*ping*/ SELECT 1″;
  • sql = ” /* ping */ SELECT 1″;
  • sql = “/*to ping or not to ping*/ SELECT 1″;

If you said “none of the above”, you pass the test.  The test is sensitive to whitespace, capitalization, and placement.  It’s this way for efficiency, as this test is done for every Statement that is executed:

protected static final String PING_MARKER = "/* ping */";
if (sql.charAt(0) == '/') {
if (sql.startsWith(PING_MARKER)) {

All of these statements will issue normal SELECT statements and not get transformed into the lightweight ping.  More problematic, this means that loadbalanced connections only execute one statement against one physical connection in the internal pool, rather than validating the connection for each underlying physical connection.  So the non-active physical connections sit stale, and stale connections die, and then Connector/J re-balances and selects that stale dead connection, and suddenly you have an Exception pushed up to the application (loadBalanceValidateConnectionOnSwapServer can save you here).

If your Connector/J deployment uses a connection pool which allows you to specify a validation query, check it, and make sure that it starts with “/* ping */” – exactly.  This is especially true if you are using load-balancing or replication-aware features of Connector/J – it should help keep alive connections which otherwise will go stale and die, causing problems later.

Managing load-balanced Connector/J deployments

Connector/J has long provided an effective means to distribute read/write load across multiple MySQL server instances for Cluster or master-master replication deployments, but until version 5.1.13, managing such deployments frequently required a service outage to redeploy a new configuration.  Given that ease of scaling out by adding additional MySQL Cluster (server) instances is a key element in that product offering, which is also naturally targeted at deployments with very strict availability requirements, we had to add support for online changes of this nature.  It’s also critical for online upgrades – the other option is to take a MySQL Cluster server instance down hard, which loses any in-process transactions and generates application exceptions, if any application is trying to use that particular server instance.

Mark Matthews and I first presented this at the MySQL Conference and Expo 2010, but the live demo we had planned didn’t work out (buggy video adapter, not product, really!).  This is my belated attempt to rectify that situation by walking through configuration, deployment and management of load-balancing configurations.

First, let’s talk about the configuration properties associated with this new functionality:

  • loadBalanceConnectionGroup – This is somewhat tricky to explain – imagine a scenario where you have an application which pulls JDBC connections from three different sources.  They may be dedicated resources for specific components – one for your JMS, one for your web app, etc.  This allows you to manage these JDBC sources within a single class-loader in any combination you choose.  If you have different servers for JMS than for your web app, you would define two different connection groups.  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, though – if you don’t define a name (string) for loadBalanceConnectionGroup, you cannot manage the connections.
  • loadBalanceEnableJMX – The ability to manage the connections is exposed when you define a loadBalanceConnectionGroup, but if you want to manage this externally, you will want to enable JMX by setting this property to true.  This enables a JMX implementation which exposes the management and monitoring operations of a connection group.  To expose this externally, you need to start your application with the JVM flag.  You can then perform connect and perform operations using a JMX client such as jconsole.

Now that you have the properties set, what operations can you perform?  First, there are a few monitoring details exposed:

  • 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 are also exposed:

  • Add host
  • Remove host

The JMX interface (com.mysql.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 will return the names of all connection groups defined in that class-loader.

So, what does this look like when running?  Let’s take a look at the long-delayed demo!

Here’s the code for my test application:

public class ToddTest {

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) {

I start this (with flag), then start jconsole:

There's the testsuite.ToddTest main class listed there I select that and navigate to the com.mysql.jdbc.jmx.LoadBalanceConnectionGroupManager bean:

Great, let's look at the methods exposed:

Let's get the number of total physical connections established:

So, 27 physical connections have been established.  Now, let's see what the currently-configured hosts are:

So the connections are being balanced against instances running on ports 3306 and 3310.  Now, I've started up an instance on port 3309, and I want to tell Connector/J to start using it.  I do this with the addHost() method:

Adding port 3309 instance

That worked, and here's how I can tell - the queries start showing up in SHOW PROCESSLIST output on port 3309:

PROCESSLIST output on port 3309

Not quite as riveting as a live demo, I'm sure, but the instructions and code necessary to reproduce this quickly on your own local machine are above.  Hopefully this will prove useful to users who want to scale out, maintain and reconfigure Connector/J-driven applications without downtime.

Connector/J load-balancing for auto-commit-enabled deployments

In a previous blog post, I wrote about Connector/J’s failover policies, I described three triggers which cause Connector/J to re-balance (potentially selecting a new physical connection to another host):

  1. At transaction boundaries (transactions are explicitly committed or rolled back)
  2. A communication exception (SQL State starting with “08″) is encountered
  3. When a SQLException matches conditions defined by user, using the extension points defined by the loadBalanceSQLStateFailover, loadBalanceSQLExceptionSubclassFailover or loadBalanceExceptionChecker properties.

Those conditions fit most needs very well, but there are situations where people are running with auto-commit enabled (no explicit transaction commit or rollback), and the end result is that Connector/J never re-balances, and keeps chugging away using the same physical connection.  That’s a problem – particularly when load-balancing is being used under-the-hood to distribute read-only load across multiple slaves.  And as of today, there is new functionality which allows Connector/J to be configured to re-balance after a certain number of statements when auto-commit is enabled.  This functionality is dependent upon the following new properties:

  • loadBalanceAutoCommitStatementThreshold – defines the number of matching statements which will trigger the driver to (potentially) swap physical server connections.  The default value (0) retains the previously-established behavior that connections with auto-commit enabled are never balanced.
  • loadBalanceAutoCommitStatementRegex – the regular expression against which statements must match.  The default value (blank) matches all statements.

So, for example, using the following properties will cause Connector/J to re-balance after every third statement which contains the string “test”:



Why does loadBalanceAutoCommitStatementRegex exist?  Maybe your application uses temporary tables.  Maybe it uses server-side variables.  Maybe it uses some other connection state that wouldn’t be good to lose by letting the driver arbitrarily swap physical connections before you are done with it (this post explores some of these scenarios).  This allows you to identify a trigger statement which is only executed at times where it is known to be safe to swap physical connections.  Generally, you likely won’t need to use it, but it may come in handy if you rely on server session-side state.

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