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There are 6 7 main classes in Flink Jdbc Driver: FlinkDriver, FlinkDataSource, FlinkConnection, FlinkStatement, FlinkResultSet, FlinkDatabaseMetaData and FlinkResultSetMetaData which implement jdbc interface Driver, DataSource, Connection, Statement, ResultSet, DatabaseMetaData and ResultSetMetaData.

1. FlinkDriver parses gateway address from url, and creates FlinkConnection
2. FlinkDataSource manages connection pool for flink jdbc, it will create specific count of connections and choose one for client directly.
3. FlinkConnection creates Executor according to gateway address. When the Connections is closed, it can close the connection with gateway by Executor
4. FlinkStatement can get Executor from FlinkConnection, and submit sql query to it. After query is executed, FlinkStatement can get StatementResult from Executor, and create FlinkResultSet
5. FlinkResultSet is an iterator, it gets results from StatementResult and return them to users
6. FlinkDatabaseMetaData provides meta data of catalogs, databases and tables
7. FlinkResultSetMetaData provides meta data of ResultSet such as columns

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There're Sessions and Operations in SqlGateway. SqlGateway will open a Session for each FlinkConnection , and then do multiple Operations in one Session. When users create a FlinkConnection by FlinkDriver with SqlGateway, it will open an exist or a new Session. Any time users want to issue SQL statements to the database, they require a FlinkStatement instance from FlinkConnection. Once users have a FlinkStatement , they can use issue a query. This will return a FlinkResultSet instance, which contains the entire result. Each operation such as the execution query(Flink job), fetching results in FlinkResultSet will be an Operation in the Session of SqlGateway .

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/* Jdbc Driver for flink sql gateway */
public class FlinkDriver implements Driver {
    /* Connect sql gateway with given url and open/create session with given priperties. */
	@Override
    public Connection connect(String url, Properties info) throws SQLException;
}

• Methods in FlinkDataSource

/* Jdbc DataSource manages connections for client, we can support more operations in it in the future. */
public class FlinkDataSource implements DataSource {
	/* The max count of connections which the data source holds. */
	private int maxActive;

	/* Set the url of connection. */
	public synchronized void setUrl(String url);

	/* Set the driver class name for the source. */
	public synchronized void setDriverClassName(String driverClassName); 

	/* Set the max active connection for the source. */
    public synchronized void setMaxActive(int maxActive);

    /* Get a connection from data source. */
	@Override
    public Connection getConnection() throws SQLException;
 	@Override 
    public Connection getConnection(String username, String password) throws SQLException;
}

• Methods in FlinkConnection 

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