4.2.2.1 Executing ClusterJ Applications and Sessions

In this section, we discuss how to start ClusterJ applications and the ClusterJ application environment.

Executing a ClusterJ application. All of the ClusterJ jar files are normally found in share/mysql/java/ in the MySQL installation directory. When executing a ClusterJ application, you must set the classpath to point to these files. In addition, you must set java.library.path variable to point to the directory containing the Cluster ndbclient library, normally found in lib/mysql (also in the MySQL installation directory). Thus you might execute a ClusterJ program MyClusterJApp in a manner similar to what is shown here:

$> java -classpath /usr/local/mysql/share/mysql/java/clusterj.jar \
              -Djava.library.path=/usr/local/mysql/lib MyClusterJApp

Note

The precise locations of the ClusterJ jar files and of libndbclient depend on how the NDB Cluster software was installed. See Installation Layouts, for more information.

ClusterJ encourages you to use different jar files at compile time and runtime. This is to remove the ability of applications to access implementation artifacts accidentally. ClusterJ is intended to be independent of the NDB Cluster software version, whereas the ndbclient layer is version-specific. This makes it possible to maintain a stable API, so that applications written against it using a given NDB Cluster version continue to run following an upgrade of the cluster to a new version.

Getting the SessionFactory and getting a Session. SessionFactory is the source of all ClusterJ sessions that use a given NDB Cluster. Usually, there is only a single SessionFactory per NDB Cluster, per Java Virtual Machine.

SessionFactory can be configured by setting one or more properties. The preferred way to do this is by putting these in a properties file, like this:

com.mysql.clusterj.connectstring=localhost:1186
com.mysql.clusterj.database=mydb

The name of the properties file is arbitrary; however, by convention, such files are named with a .properties extension. For ClusterJ applications, it is customary to name the file clusterj.properties.

After editing and saving the file, you can load its contents into an instance of Properties, as shown here:

    
File propsFile = new File("clusterj.properties");
InputStream inStream = new FileInputStream(propsFile);
Properties props = new Properties();
props.load(inStream);

It is also possible to set these properties directly, without the use of a properties file:

Properties props = new Properties();

props.put("com.mysql.clusterj.connectstring", "localhost:1186");
props.put("com.mysql.clusterj.database", "mydb");

Once the properties have been set and loaded (using either of the techniques just shown), you can obtain a SessionFactory, and then from that a Session instance. For this, you use the SessionFactory's getSession() method, as shown here:

SessionFactory factory = ClusterJHelper.getSessionFactory(props);

Session session = factory.getSession();

It is usually sufficient to set and load the com.mysql.clusterj.connectstring and com.mysql.clusterj.database properties (and these properties, along with com.mysql.clusterj.max.transactions, cannot be changed after starting the SessionFactory). For a complete list of available SessionFactory properties and usual values, see com.mysql.clusterj.Constants.

Note

Session instances must not be shared among threads. Each thread in your application should use its own instance of Session.

For com.mysql.clusterj.connectstring, we use the default NDB Cluster connection string localhost:1186 (see NDB Cluster Connection Strings, for more information). For the value of com.mysql.clusterj.database, we use mydb in this example, but this value can be the name of any database containing NDB tables. For a listing of all SessionFactory properties that can be set in this manner, see com.mysql.clusterj.Constants.

Error Handling and Reconnection. Errors that occur while using ClusterJ should be handled by the application with a common error handler. The handler needs to be able to detect and distinguish among three types of errors, and handle them accordingly:

Normal errors: These are errors at the application level (for example, those to deal with duplicate key, foreign key constraint, or timeout). They should be handled in application-specific ways, and, if resolved, the application can continue with the transaction.
Unexpected errors: These are failures to work with the cluster that cannot be accounted for by the conditions of the application, but are nonfatal. The application should close the ClusterJ session and reopen a new one.
Connectivity errors: These are errors like error 4009 and 4010, which indicate a network outage. There are two possible scenarios, depending on whether the automatic reconnection feature (available for NDB Cluster 7.5.7 and later) has been enabled:
- Automatic reconnection is enabled : The feature is enabled when the connection property com.mysql.clusterj.connection.reconnect.timeout has been set to a positive number, which specifies a reconnection timeout in seconds.
  When ClusterJ detects a disconnect with the NDB Cluster, it changes the State of the SessionFactory from OPEN to RECONNECTING; the SessionFactory then waits for the application to close all the sessions, and then attempts to reconnect the application to the NDB Cluster by closing all connections in the connection pool and recreating the pool using the original pool properties. After reestablishing all the connections, the State of the SessionFactory becomes OPEN again, and the application can now obtain sessions.
  The SessionFactory.getState() method returns the State of the SessionFactory, which is one of OPEN, RECONNECTING, or CLOSED. Trying to obtain a session when the State is not OPEN results in a ClusterJUserException, with the message Session factory is not open.
  If the application does not close all sessions by the end of the timeout period specified with com.mysql.clusterj.connection.reconnect.timeout, the SessionFactory closes any open sessions forcibly (which might result in loss of resources), and then attempts reconnection.
- Automatic reconnection is not enabled: This is when the connection property com.mysql.clusterj.connection.reconnect.timeout has not been set, or it has been set to zero (this is also the case for older NDB Cluster releases that do not support the automatic reconnection feature).
  ClusterJ does not attempt to reconnect to the NDB Cluster once the connection is lost. The application should close all sessions and then restart the SessionFactory. The restarting of the SessionFactory can be an automatic application function or a manual intervention. In either case, the code should wait until all sessions have been closed (that is, the public method getConnectionPoolSessionCounts() in the SessionFactory interface returns zeros for all pooled connections). Then the SessionFactory can be closed and reopened, and the application can obtain sessions again.
Instead of enabling the feature and waiting for ClusterJ to detect a disconnection and attempt a reconnection, you can also have the application itself initiate the reconnection process upon the detection of a connection error by calling the SessionFactory.reconnect(int timeout) method: that triggers the reconnection process described above, but uses the timeout argument of the reconnect() method as the time limit for having all open sessions closed.

Logging. ClusterJ uses Java logging. Here are some default settings for the ClusterJ logging, which are specified in the logging.properties file and can be modified there:

Logging level is set at INFO for all classes.
Using java.util.logging.FileHandler as the handler.
Default level for java.util.logging.FileHandler is set at FINEST
Using java.util.logging.SimpleFormatter as the formatter for the handler.
Log files are put inside the target directory under the current working directory, and file names are, generally, in the pattern of logNum, where Num is a unique number for resolving file name conflicts (see the Java documentation for java.util.logging.FileHandler for details).

The logging.properties file is located by default in the current working directory, but the location can be changed by specifying the system property java.util.logging.config.file when you start Java.