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How to Create an External Connection Pool

IMPORTANT: THIS IS A BETA VERSION

This page is under active development and may contain unstable or incomplete features. Use it at your own risk.

Overview

Etendo uses connection pools to reuse existing connections/prepared statements, avoiding the cost of initiating a connection, parsing SQL etc. By default, Etendo uses two different connection pools:

  • Hibernate default connection pool for DAL-related queries
  • Apache DBCP for the connections provided by the ConnectionProviderImpl.

Etendo provides a module in modules_core that implements the Tomcat JDBC Connection Pool, but it is possible for developers to publish a module and implement their own connection pool.

ExternalConnectionPool abstract class

In order to implement an external connection pool, it is necessary to subclass the ExternalConnectionPool abstract class. This class has three methods:

/**
     * 
     * @param externalConnectionPoolClassName
     *          The full class name of the external connection pool
     * @return An instance of the external connection pool
     */
public final synchronized static ExternalConnectionPool getInstance(
    String externalConnectionPoolClassName) throws InstantiationException,
    IllegalAccessException, ClassNotFoundException;

The getInstance method is in charge of instantiating the external connection pool. The name of the class that implements the connection pool is passed as a parameter, its value is taken from the db.externalPoolClassName property of gradle.properties

/**
    * @return A Connection from the external connection pool
    */
public abstract Connection getConnection()

This is the main method that should be overwritten by all external connection providers. It takes no arguments, and it should return a connection that has been borrowed from the pool.

/**
     * If the external connection pool supports interceptors this method should be overwritten
     * 
     * @param interceptors
     *          List of PoolInterceptorProvider comprised of all the interceptors injected with Weld
     */
public void loadInterceptors(List<PoolInterceptorProvider> interceptors)

This method should be overwritten if the connection pool supports interceptors and allows custom interceptors to be instantiated using dependency injection. A list of PoolInterceptorProvider will be passed as arguments, containing the full class name of the interceptors.

Design Considerations

  • Include a configuration file so that users can customize the external connection pool to meet their needs.
  • If the pool needs to execute some initialization code, place it in the getConnection method and make sure that is only executed the first time that method is invoked.

  • For all connection returned by the pool:

    • SessionInfo.setDBSessionInfo(Connection conn, String rdbms) must be executed.
    • The sql query included in the bbdd.sessionConfig property must be executed using that connection.

  • Take into account that these two operations must be executed on the returned connection only when the connection is actually created, not every time it is returned from the pool.

This work is a derivative of How to Create an External Connection Pool by Openbravo Wiki, used under CC BY-SA 2.5 ES. This work is licensed under CC BY-SA 2.5 by Etendo.