[WIP] feat: query-level load balancer plugin

[sergiyvamz]

Summary

Introduces a new qlb plugin. The main purpose of the new plugin is to provide a query-level load balancing among reader nodes.

• New plugin is based on readWriteSplitting plugin and handle setReadOnly() calls similar to original readWriteSplitting plugin.
• When a connection is read-only (i.e. after calling setReadOnly(true)), the new plugin switches reader connections on every call to createStatement(), preapreStatement(), or prepareCall().
• Connection transaction boundaries are respected and no reader connection switches are occurred while in transaction.

final Properties props = new Properties();
props.setProperty("user", "test_user");
props.setProperty("password", "test_password");
props.setProperty("wrapperDialect", "mysql");
props.setProperty("wrapperPlugins", "qlb");
props.setProperty("readerHostSelectorStrategy", "roundRobin"); // "random"
props.setProperty("singleWriterConnectionString", "true");

String url = "jdbc:aws-wrapper:mysql://test-instance.XYZ.us-west-2.rds.amazonaws.com,test-instance-2.XYZ.us-west-2.rds.amazonaws.com,test-instance-3.XYZ.us-east-1.rds.amazonaws.com/";

try (final Connection conn = DriverManager.getConnection(url, props)) {

  conn.setReadOnly(true);

  while (true) {
	try (final Statement myStmt = conn.createStatement();
		final ResultSet rs = myStmt.executeQuery("SELECT @@hostname")) {
	  if (rs.next()) {
		String id = rs.getString(1);
		LOGGER.info("host id: " + rs.getString(1));
	  }
	}
  }
}

Description

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