Embedded Derby in OSGi, creating multiple connection using connection pool

Question

I want to create instances of a class which will have access to the underlying Embedded derby database and pass this class to each bundle binding to my database bundle using declarative services.

I have seen in the derby documentation that sharing one connection for multiple threads has many pitfalls. So I was thinking to create a connection for each instance of the class I am creating. Since I only want a very simple way to just create multiple connections and manage them, using "MiniConnectionPoolManager" here seems like a good option. The sample code for derby is shown below:

org.apache.derby.jdbc.EmbeddedConnectionPoolDataSource dataSource = new org.apache.derby.jdbc.EmbeddedConnectionPoolDataSource();
dataSource.setDatabaseName("c:/temp/testDB");
dataSource.setCreateDatabase("create");
MiniConnectionPoolManager poolMgr = new MiniConnectionPoolManager(dataSource, maxConnections);
...
Connection connection = poolMgr.getConnection();
...
connection.close();

But the documentation does not cover many things plus I am a beginner in using Database. My questions are:

1. When I am creating a new class that will need database connection to perform insert,update & other actions. Shall I pass the 'poolMgr' and call poolMgr.getConnection() from the newly created class?

2. When should I close this connection? I don't know for how long the bundle (user) will use the new class so shall I save the newly created connection in a private global variable and force the user to execute unregister class where I could then close the connection? Or shall I just close all connections when my database bundle is being deactivated.

Other suggestions are also appreciated to manage different classes accessing one database. Thank you in advance.

Edit:

The main class in my database bundle is always active as long as the application is running. It is the bundles requesting for an instance of a new class(performing database operation) that come and go. And also since it will be deployed in embedded system, I can only use small footprint applications.

Answer 1

You should get a connection from a connection pool when you need it and close the connection as soon as you can. It is the job of the connection pool to re-use connections, not yours.

In other words: Do not keep a connection alive until your consumer bundle is deactivated.

Connection pools normally implement DataSource interface, you should use the pools via it. In that case you can replace the pool implementation easily without changing your code. Eg:

@Component
public class MyComponent {

    // Connection pool based DataSource
    @Reference
    DataSource dataSource;

    public void myFunction() {
        try (Connection c = dataSource.getConnection()) {
            // Database operations
        } catch (SQLException e) {
            // TODO
        }
    }
}

When you find yourself repeating the same code many times (getting connection, catching SQLException), you can write a simple component that accepts functional interfaces. Eg:

@Component
@Service
public class SQLHelper {

    @Reference // This is a connection pool DataSource
    private DataSource dataSource;

    public <R> R execute(Callback<R> callback) {
        try (Connection c = dataSource.getConnection()) {
            return callback.call(c);
        } catch (SQLException e) {
            throw new UncheckedSQLException(e);
        }
    }
}

Your functional interface would look like this:

public interface Callback<R> {

    R call(Connection connection);

}

And you would use it like this:

sqlHelper.execute((Connection c) -> {
    // Do some stuff with the connection
});

Using transactions

If you want to use atomic transactions, I suggest that you should use org.apache.derby.jdbc.EmbeddedXADataSource together with org.apache.commons.dbcp.managed.BasicManagedDataSource from commons-dbcp. After that, you can handle transactions via JTA.

It is hard to use the JTA API directly. You should choose a library that helps you propagating transactions.

A small guide based on Declarative Services:

• Install derby jar into your OSGi container
• Install pax-derby bundle as well! By doing that, you will have a DataSourceFactory OSGi service
• Install everit-dsf-bundle with its dependencies! You will see two new DS components. Create a configuration for the one called XADataSource via the webconsole! All configuration options have descriptions.
• Install a JTA Transaction Manager into the OSGi container! You have several choices. I
• Install everit-commons-dbcp-component with its dependencies! You will see two new DS components. Configure the Managed one in the webconsole and set the previously created XADataSource as the target! The transactional pool will take care of providing the same connection if you request-and-close connections whitin the scope of the same transaction. normally use Aries Transaction Manager that embeds Geronimo TM.
• Install everit-transaction-helper to your OSGi container! You will see a new OSGi service with the interface TransactionHelper (that is provided by a configurable DS component).

Now you have everything to write your code. Your component would similar to the following:

@Component
@Service
public class MyComponent {

    @Reference
    private DataSource dataSource;

    @Reference
    private TransactionHelper th;

    public void myFunction() {
        th.required(() -> {
            try (Connection c = dataSource.getConnection()) {
                // My SQL statements
            } catch (SQLException e) {
                // TODO
            }
        }
    }
 }

In case you do not need transaction handling, you can:

• use the standard EmbeddedDataSource
• use any non-transactional connection pool
• skip the installation of the TransactionManager and TransactionHelper bundles
• skip the usage of TransactionHelper from the code

A more complex guide (that also takes care of schema creation and uses OO based queries) is available at http://cookbook.everit.org/persistence/index.html .

Update

You do not have to get a connection for every SQL statement. You should get a connection, execute as many SQL statements that you can within a "moment" and than call close on the connection.

• If you have to run three SQL statements right behind each other, you should request a connection, execute the three SQL statements and than call close on the connection
• If you close the requested connection within the same function you requested it from the pool, you probably do things right. You might call other functions passing the connection as a parameter, but they should only use it to run SQL statements and than return.
• You should not keep alive a connection and wait for another user action. That is the job of the connection pool. When you call close on a connection that is provided by a pool, the connection is not closed physically, but only retrieved to the pool.
• You should keep the connection object in a local variable. If you use a member variable for your connection object, you should suspect that something is wrong with your code (the only exception is if you pass the Connection to an object that lives for a very short time and that object holds the connection in a member variable to have cleaner code).
• Please note that if you use Java 6 or earlier, you should close the connection in a finally block to avoid unclosed connections.

MiniConnectionPoolManager might be a great solution for embedded devices as it is really "mini". The only issue is that it does not implement the DataSource interface so your business code shuold directly use the MiniCPM classes. By doing that, it will be much harder to switch to other Connection pool if you find a bug or you need a more complex pool later.

If you decide to use MiniCPM, I suggest that you should write a component that implements DataSource and delegates the getConnection() function to a MiniCPM instance. Eg:

@Component
@Service
public class MiniCPMDataSourceComponent implements DataSource {

    @Reference
    protected ConnectionPoolDataSource cpDataSource;

    private MiniConnectionPoolManager wrapped;

    @Activate
    public void activate() {
        this.wrapped = new MiniConnectionPoolManager(cpDataSource);
    }

    @Override
    public Connection getConnection() {
        return wrapped.getConnection();
    }

    @Override
    public Connection getConnection(String user, String password) {
        throw new UnsupportedOperationException();
    }

    @Deactivate
    public void deactivate() {
        wrapped.dispose();
    }
}

You can decorate this component with configuration possibilities like the max connection number and timeout (that is supported by MiniCPM). If you use the service that is provided by this component, you will be able to switch the connection pool without changing your business code. Also, your business bundle will not be wired directly to MiniCPM.