src/org/objectweb/cjdbc/controller/loadbalancer/paralleldb/ParallelDB.java

説明を見る。

package org.objectweb.cjdbc.controller.loadbalancer.paralleldb;

import java.sql.Connection;
import java.sql.SQLException;
import java.util.Hashtable;

import org.objectweb.cjdbc.common.exceptions.BadConnectionException;
import org.objectweb.cjdbc.common.exceptions.UnreachableBackendException;
import org.objectweb.cjdbc.common.i18n.Translate;
import org.objectweb.cjdbc.common.sql.AbstractRequest;
import org.objectweb.cjdbc.common.sql.AbstractWriteRequest;
import org.objectweb.cjdbc.common.sql.ParsingGranularities;
import org.objectweb.cjdbc.common.sql.SelectRequest;
import org.objectweb.cjdbc.common.sql.StoredProcedure;
import org.objectweb.cjdbc.common.sql.UnknownRequest;
import org.objectweb.cjdbc.common.xml.DatabasesXmlTags;
import org.objectweb.cjdbc.controller.backend.DatabaseBackend;
import org.objectweb.cjdbc.controller.cache.metadata.MetadataCache;
import org.objectweb.cjdbc.controller.connection.AbstractConnectionManager;
import org.objectweb.cjdbc.controller.loadbalancer.AbstractLoadBalancer;
import org.objectweb.cjdbc.controller.loadbalancer.AllBackendsFailedException;
import org.objectweb.cjdbc.controller.requestmanager.RAIDbLevels;
import org.objectweb.cjdbc.controller.requestmanager.TransactionMarkerMetaData;
import org.objectweb.cjdbc.controller.virtualdatabase.ControllerResultSet;
import org.objectweb.cjdbc.controller.virtualdatabase.VirtualDatabase;

public abstract class ParallelDB extends AbstractLoadBalancer
{
  //transaction id -> DatabaseBackend
  private Hashtable backendPerTransactionId;

  public ParallelDB(VirtualDatabase vdb) throws SQLException
  {
    super(vdb, RAIDbLevels.SingleDB, ParsingGranularities.NO_PARSING);
  }

  public ControllerResultSet execReadRequest(SelectRequest request,
      MetadataCache metadataCache) throws SQLException
  {
    DatabaseBackend backend;
    if (request.isAutoCommit())
      backend = chooseBackendForReadRequest(request);
    else
      backend = (DatabaseBackend) backendPerTransactionId.get(new Long(request
          .getTransactionId()));

    if (backend == null)
      throw new SQLException(Translate.get(
          "loadbalancer.execute.no.backend.found", request.getSQLShortForm(vdb
              .getSQLShortFormLength())));

    ControllerResultSet rs = null;
    // Execute the request on the chosen backend
    try
    {
      rs = executeReadRequestOnBackend(request, backend, metadataCache);
    }
    catch (UnreachableBackendException urbe)
    {
      // Try to execute query on different backend
      return execReadRequest(request, metadataCache);
    }
    catch (SQLException se)
    {
      String msg = Translate.get("loadbalancer.request.failed", new String[]{
          String.valueOf(request.getId()), se.getMessage()});
      if (logger.isInfoEnabled())
        logger.info(msg);
      throw new SQLException(msg);
    }
    catch (RuntimeException e)
    {
      String msg = Translate.get("loadbalancer.request.failed.on.backend",
          new String[]{request.getSQLShortForm(vdb.getSQLShortFormLength()),
              backend.getName(), e.getMessage()});
      logger.error(msg, e);
      throw new SQLException(msg);
    }

    return rs;
  }

  public ControllerResultSet execReadOnlyReadStoredProcedure(
      StoredProcedure proc, MetadataCache metadataCache) throws SQLException
  {
    return execReadStoredProcedure(proc, metadataCache);
  }

  public ControllerResultSet execReadStoredProcedure(StoredProcedure proc,
      MetadataCache metadataCache) throws SQLException
  {
    DatabaseBackend backend;
    if (proc.isAutoCommit())
      backend = chooseBackendForReadRequest(proc);
    else
      backend = (DatabaseBackend) backendPerTransactionId.get(new Long(proc
          .getTransactionId()));

    if (backend == null)
      throw new SQLException(Translate.get(
          "loadbalancer.storedprocedure.no.backend.found", proc
              .getSQLShortForm(vdb.getSQLShortFormLength())));

    ControllerResultSet rs = null;
    // Execute the request on the chosen backend
    try
    {
      rs = executeReadStoredProcedureOnBackend(proc, backend, metadataCache);
    }
    catch (UnreachableBackendException urbe)
    {
      // Try to execute query on different backend
      return execReadStoredProcedure(proc, metadataCache);
    }
    catch (SQLException se)
    {
      String msg = Translate.get("loadbalancer.storedprocedure.failed",
          new String[]{String.valueOf(proc.getId()), se.getMessage()});
      if (logger.isInfoEnabled())
        logger.info(msg);
      throw new SQLException(msg);
    }
    catch (RuntimeException e)
    {
      String msg = Translate.get(
          "loadbalancer.storedprocedure.failed.on.backend", new String[]{
              proc.getSQLShortForm(vdb.getSQLShortFormLength()),
              backend.getName(), e.getMessage()});
      logger.error(msg, e);
      throw new SQLException(msg);
    }

    return rs;
  }

  public int execWriteRequest(AbstractWriteRequest request)
      throws AllBackendsFailedException, SQLException
  {
    DatabaseBackend backend;
    if (request.isAutoCommit())
      backend = chooseBackendForWriteRequest(request);
    else
      backend = (DatabaseBackend) backendPerTransactionId.get(new Long(request
          .getTransactionId()));

    if (backend == null)
      throw new SQLException(Translate.get(
          "loadbalancer.execute.no.backend.found", request.getSQLShortForm(vdb
              .getSQLShortFormLength())));

    int result;
    // Execute the request on the chosen backend
    try
    {
      result = executeWriteRequestOnBackend(request, backend);
    }
    catch (UnreachableBackendException urbe)
    {
      // Try to execute query on different backend
      return execWriteRequest(request);
    }
    catch (SQLException se)
    {
      String msg = Translate.get("loadbalancer.request.failed", new String[]{
          String.valueOf(request.getId()), se.getMessage()});
      if (logger.isInfoEnabled())
        logger.info(msg);
      throw new SQLException(msg);
    }
    catch (RuntimeException e)
    {
      String msg = Translate.get("loadbalancer.request.failed.on.backend",
          new String[]{request.getSQLShortForm(vdb.getSQLShortFormLength()),
              backend.getName(), e.getMessage()});
      logger.error(msg, e);
      throw new SQLException(msg);
    }

    return result;
  }

  public ControllerResultSet execWriteRequestWithKeys(
      AbstractWriteRequest request, MetadataCache metadataCache)
      throws AllBackendsFailedException, SQLException
  {
    DatabaseBackend backend;
    if (request.isAutoCommit())
      backend = chooseBackendForWriteRequest(request);
    else
      backend = (DatabaseBackend) backendPerTransactionId.get(new Long(request
          .getTransactionId()));

    if (backend == null)
      throw new SQLException(Translate.get(
          "loadbalancer.execute.no.backend.found", request.getSQLShortForm(vdb
              .getSQLShortFormLength())));

    ControllerResultSet rs;
    // Execute the request on the chosen backend
    try
    {
      rs = executeWriteRequestWithKeysOnBackend(request, backend, metadataCache);
    }
    catch (UnreachableBackendException urbe)
    {
      // Try to execute query on different backend
      return execWriteRequestWithKeys(request, metadataCache);
    }
    catch (SQLException se)
    {
      String msg = Translate.get("loadbalancer.request.failed", new String[]{
          String.valueOf(request.getId()), se.getMessage()});
      if (logger.isInfoEnabled())
        logger.info(msg);
      throw new SQLException(msg);
    }
    catch (RuntimeException e)
    {
      String msg = Translate.get("loadbalancer.request.failed.on.backend",
          new String[]{request.getSQLShortForm(vdb.getSQLShortFormLength()),
              backend.getName(), e.getMessage()});
      logger.error(msg, e);
      throw new SQLException(msg);
    }

    return rs;
  }

  public int execWriteStoredProcedure(StoredProcedure proc) throws SQLException
  {
    DatabaseBackend backend;
    if (proc.isAutoCommit())
      backend = chooseBackendForReadRequest(proc);
    else
      backend = (DatabaseBackend) backendPerTransactionId.get(new Long(proc
          .getTransactionId()));

    if (backend == null)
      throw new SQLException(Translate.get(
          "loadbalancer.storedprocedure.no.backend.found", proc
              .getSQLShortForm(vdb.getSQLShortFormLength())));

    int result;
    // Execute the request on the chosen backend
    try
    {
      result = executeWriteStoredProcedureOnBackend(proc, backend);
    }
    catch (UnreachableBackendException urbe)
    {
      // Try to execute query on different backend
      return execWriteStoredProcedure(proc);
    }
    catch (SQLException se)
    {
      String msg = Translate.get("loadbalancer.storedprocedure.failed",
          new String[]{String.valueOf(proc.getId()), se.getMessage()});
      if (logger.isInfoEnabled())
        logger.info(msg);
      throw new SQLException(msg);
    }
    catch (RuntimeException e)
    {
      String msg = Translate.get(
          "loadbalancer.storedprocedure.failed.on.backend", new String[]{
              proc.getSQLShortForm(vdb.getSQLShortFormLength()),
              backend.getName(), e.getMessage()});
      logger.error(msg, e);
      throw new SQLException(msg);
    }

    return result;
  }

  private ControllerResultSet executeReadRequestOnBackend(
      SelectRequest request, DatabaseBackend backend,
      MetadataCache metadataCache) throws SQLException,
      UnreachableBackendException
  {
    // Ok, we have a backend, let's execute the request
    AbstractConnectionManager cm = backend.getConnectionManager(request
        .getLogin());

    // Sanity check
    if (cm == null)
    {
      String msg = Translate.get("loadbalancer.connectionmanager.not.found",
          new String[]{request.getLogin(), backend.getName()});
      logger.error(msg);
      throw new SQLException(msg);
    }

    // Execute the query
    if (request.isAutoCommit())
    {
      ControllerResultSet rs = null;
      boolean badConnection;
      do
      {
        badConnection = false;
        // Use a connection just for this request
        Connection c = null;
        try
        {
          c = cm.getConnection();
        }
        catch (UnreachableBackendException e1)
        {
          logger.error(Translate.get(
              "loadbalancer.backend.disabling.unreachable", backend.getName()));
          backend.disable();
          throw new UnreachableBackendException(Translate.get(
              "loadbalancer.backend.unreacheable", backend.getName()));
        }

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.backend.no.connection", backend.getName()));

        // Execute Query
        try
        {
          rs = executeSelectRequestOnBackend(request, backend, c, metadataCache);
          cm.releaseConnection(c);
        }
        catch (SQLException e)
        {
          cm.releaseConnection(c);
          throw new SQLException(Translate.get(
              "loadbalancer.request.failed.on.backend", new String[]{
                  request.getSQLShortForm(vdb.getSQLShortFormLength()),
                  backend.getName(), e.getMessage()}));
        }
        catch (BadConnectionException e)
        { // Get rid of the bad connection
          cm.deleteConnection(c);
          badConnection = true;
        }
      }
      while (badConnection);
      if (logger.isDebugEnabled())
        logger.debug(Translate.get("loadbalancer.execute.on", new String[]{
            String.valueOf(request.getId()), backend.getName()}));
      return rs;
    }
    else
    { // Inside a transaction
      Connection c;
      long tid = request.getTransactionId();
      Long lTid = new Long(tid);

      if (!backend.isStartedTransaction(lTid))
      { // transaction has not been started yet on this backend
        try
        {
          c = cm.getConnection(tid);
        }
        catch (UnreachableBackendException e1)
        {
          logger.error(Translate.get(
              "loadbalancer.backend.disabling.unreachable", backend.getName()));
          backend.disable();
          throw new UnreachableBackendException(Translate.get(
              "loadbalancer.backend.unreacheable", backend.getName()));
        }

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.unable.get.connection", new String[]{
                  String.valueOf(tid), backend.getName()}));

        // begin transaction
        backend.startTransaction(lTid);
        c.setAutoCommit(false);
      }
      else
      { // Re-use the connection used by this transaction
        c = cm.retrieveConnection(tid);

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.unable.retrieve.connection", new String[]{
                  String.valueOf(tid), backend.getName()}));
      }

      // Execute Query
      ControllerResultSet rs = null;
      try
      {
        rs = executeSelectRequestOnBackend(request, backend, c, metadataCache);
      }
      catch (SQLException e)
      {
        throw new SQLException(Translate.get(
            "loadbalancer.request.failed.on.backend", new String[]{
                request.getSQLShortForm(vdb.getSQLShortFormLength()),
                backend.getName(), e.getMessage()}));
      }
      catch (BadConnectionException e)
      { // Connection failed, so did the transaction
        // Disable the backend.
        cm.deleteConnection(tid);
        String msg = Translate.get(
            "loadbalancer.backend.disabling.connection.failure", backend
                .getName());
        logger.error(msg);
        backend.disable();
        throw new SQLException(msg);
      }
      if (logger.isDebugEnabled())
        logger.debug(Translate.get("loadbalancer.execute.transaction.on",
            new String[]{String.valueOf(tid), String.valueOf(request.getId()),
                backend.getName()}));
      return rs;
    }
  }

  private ControllerResultSet executeReadStoredProcedureOnBackend(
      StoredProcedure proc, DatabaseBackend backend, MetadataCache metadataCache)
      throws SQLException, UnreachableBackendException
  {
    // Ok, we have a backend, let's execute the request
    AbstractConnectionManager cm = backend
        .getConnectionManager(proc.getLogin());

    // Sanity check
    if (cm == null)
    {
      String msg = Translate.get("loadbalancer.connectionmanager.not.found",
          new String[]{proc.getLogin(), backend.getName()});
      logger.error(msg);
      throw new SQLException(msg);
    }

    // Execute the query
    if (proc.isAutoCommit())
    {
      // Use a connection just for this request
      Connection c = null;
      try
      {
        c = cm.getConnection();
      }
      catch (UnreachableBackendException e1)
      {
        logger.error(Translate.get(
            "loadbalancer.backend.disabling.unreachable", backend.getName()));
        backend.disable();
        throw new UnreachableBackendException(Translate.get(
            "loadbalancer.backend.unreacheable", backend.getName()));
      }

      // Sanity check
      if (c == null)
        throw new UnreachableBackendException(Translate.get(
            "loadbalancer.backend.no.connection", backend.getName()));

      // Execute Query
      ControllerResultSet rs = null;
      try
      {
        rs = AbstractLoadBalancer.executeReadStoredProcedureOnBackend(proc,
            backend, c, metadataCache);
      }
      catch (Exception e)
      {
        throw new SQLException(Translate.get(
            "loadbalancer.storedprocedure.failed.on.backend", new String[]{
                proc.getSQLShortForm(vdb.getSQLShortFormLength()),
                backend.getName(), e.getMessage()}));
      }
      finally
      {
        cm.releaseConnection(c);
      }
      if (logger.isDebugEnabled())
        logger.debug(Translate.get("loadbalancer.storedprocedure.on",
            new String[]{String.valueOf(proc.getId()), backend.getName()}));
      return rs;
    }
    else
    { // Inside a transaction
      Connection c;
      long tid = proc.getTransactionId();
      Long lTid = new Long(tid);

      if (!backend.isStartedTransaction(lTid))
      { // transaction has not been started yet on this backend
        try
        {
          c = cm.getConnection(tid);
        }
        catch (UnreachableBackendException e1)
        {
          logger.error(Translate.get(
              "loadbalancer.backend.disabling.unreachable", backend.getName()));
          backend.disable();
          throw new UnreachableBackendException(Translate.get(
              "loadbalancer.backend.unreacheable", backend.getName()));
        }

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.unable.get.connection", new String[]{
                  String.valueOf(tid), backend.getName()}));

        // begin transaction
        backend.startTransaction(lTid);
        c.setAutoCommit(false);
      }
      else
      { // Re-use the connection used by this transaction
        c = cm.retrieveConnection(tid);

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.unable.retrieve.connection", new String[]{
                  String.valueOf(tid), backend.getName()}));
      }

      // Execute Query
      ControllerResultSet rs;
      try
      {
        rs = AbstractLoadBalancer.executeReadStoredProcedureOnBackend(proc,
            backend, c, metadataCache);
      }
      catch (Exception e)
      {
        throw new SQLException(Translate.get(
            "loadbalancer.storedprocedure.failed.on.backend", new String[]{
                proc.getSQLShortForm(vdb.getSQLShortFormLength()),
                backend.getName(), e.getMessage()}));
      }
      if (logger.isDebugEnabled())
        logger.debug(Translate.get("loadbalancer.execute.transaction.on",
            new String[]{String.valueOf(tid), String.valueOf(proc.getId()),
                backend.getName()}));
      return rs;
    }
  }

  private int executeWriteRequestOnBackend(AbstractWriteRequest request,
      DatabaseBackend backend) throws SQLException, UnreachableBackendException
  {
    if (backend == null)
      throw new SQLException(Translate.get(
          "loadbalancer.execute.no.backend.available", request.getId()));

    try
    {
      AbstractConnectionManager cm = backend.getConnectionManager(request
          .getLogin());
      if (request.isAutoCommit())
      { // Use a connection just for this request
        Connection c = null;
        try
        {
          c = cm.getConnection();
        }
        catch (UnreachableBackendException e1)
        {
          String backendName = backend.getName();
          logger.error(Translate.get(
              "loadbalancer.backend.disabling.unreachable", backendName));
          backend.disable();
          throw new UnreachableBackendException(Translate.get(
              "loadbalancer.backend.unreacheable", backendName));
        }

        // Sanity check
        if (c == null)
          throw new UnreachableBackendException(Translate.get(
              "loadbalancer.backend.no.connection", backend.getName()));

        // Execute Query
        int result;
        try
        {
          result = executeUpdateRequestOnBackend(request, backend, c);
        }
        catch (Exception e)
        {
          throw new SQLException(Translate.get(
              "loadbalancer.request.failed.on.backend", new String[]{
                  request.getSQLShortForm(vdb.getSQLShortFormLength()),
                  backend.getName(), e.getMessage()}));
        }
        finally
        {
          cm.releaseConnection(c);
        }
        return result;
      }
      else
      { // Re-use the connection used by this transaction
        Connection c = cm.retrieveConnection(request.getTransactionId());

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.unable.retrieve.connection",
              new String[]{String.valueOf(request.getTransactionId()),
                  backend.getName()}));

        // Execute Query
        int result;
        try
        {
          result = executeUpdateRequestOnBackend(request, backend, c);
          return result;
        }
        catch (Exception e)
        {
          throw new SQLException(Translate.get(
              "loadbalancer.request.failed.on.backend", new String[]{
                  request.getSQLShortForm(vdb.getSQLShortFormLength()),
                  backend.getName(), e.getMessage()}));
        }
      }
    }
    catch (RuntimeException e)
    {
      String msg = Translate.get("loadbalancer.request.failed.on.backend",
          new String[]{request.getSQLShortForm(vdb.getSQLShortFormLength()),
              backend.getName(), e.getMessage()});
      logger.fatal(msg, e);
      throw new SQLException(msg);
    }
  }

  private ControllerResultSet executeWriteRequestWithKeysOnBackend(
      AbstractWriteRequest request, DatabaseBackend backend,
      MetadataCache metadataCache) throws SQLException,
      UnreachableBackendException
  {
    if (backend == null)
      throw new SQLException(Translate.get(
          "loadbalancer.execute.no.backend.available", request.getId()));

    if (!backend.getDriverCompliance().supportGetGeneratedKeys())
      throw new SQLException(Translate.get(
          "loadbalancer.backend.autogeneratedkeys.unsupported", backend
              .getName()));

    try
    {
      AbstractConnectionManager cm = backend.getConnectionManager(request
          .getLogin());
      if (request.isAutoCommit())
      { // Use a connection just for this request
        Connection c = null;
        try
        {
          c = cm.getConnection();
        }
        catch (UnreachableBackendException e1)
        {
          String backendName = backend.getName();
          logger.error(Translate.get(
              "loadbalancer.backend.disabling.unreachable", backendName));
          backend.disable();
          throw new UnreachableBackendException(Translate.get(
              "loadbalancer.backend.unreacheable", backendName));
        }

        // Sanity check
        if (c == null)
          throw new UnreachableBackendException(Translate.get(
              "loadbalancer.backend.no.connection", backend.getName()));

        // Execute Query
        ControllerResultSet result;
        try
        {
          result = new ControllerResultSet(request,
              executeUpdateRequestOnBackendWithKeys(request, backend, c),
              metadataCache);
        }
        catch (Exception e)
        {
          throw new SQLException(Translate.get(
              "loadbalancer.request.failed.on.backend", new String[]{
                  request.getSQLShortForm(vdb.getSQLShortFormLength()),
                  backend.getName(), e.getMessage()}));
        }
        finally
        {
          cm.releaseConnection(c);
        }
        return result;
      }
      else
      { // Re-use the connection used by this transaction
        Connection c = cm.retrieveConnection(request.getTransactionId());

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.unable.retrieve.connection",
              new String[]{String.valueOf(request.getTransactionId()),
                  backend.getName()}));

        // Execute Query
        try
        {
          return new ControllerResultSet(request,
              executeUpdateRequestOnBackendWithKeys(request, backend, c),
              metadataCache);
        }
        catch (Exception e)
        {
          throw new SQLException(Translate.get(
              "loadbalancer.request.failed.on.backend", new String[]{
                  request.getSQLShortForm(vdb.getSQLShortFormLength()),
                  backend.getName(), e.getMessage()}));
        }
      }
    }
    catch (RuntimeException e)
    {
      String msg = Translate
          .get("loadbalancer.request.failed", new String[]{
              request.getSQLShortForm(vdb.getSQLShortFormLength()),
              e.getMessage()});
      logger.fatal(msg, e);
      throw new SQLException(msg);
    }
  }

  private int executeWriteStoredProcedureOnBackend(StoredProcedure proc,
      DatabaseBackend backend) throws SQLException, UnreachableBackendException
  {
    // Ok, we have a backend, let's execute the request
    AbstractConnectionManager cm = backend
        .getConnectionManager(proc.getLogin());

    // Sanity check
    if (cm == null)
    {
      String msg = Translate.get("loadbalancer.connectionmanager.not.found",
          new String[]{proc.getLogin(), backend.getName()});
      logger.error(msg);
      throw new SQLException(msg);
    }

    // Execute the query
    if (proc.isAutoCommit())
    {
      // Use a connection just for this request
      Connection c = null;
      try
      {
        c = cm.getConnection();
      }
      catch (UnreachableBackendException e1)
      {
        logger.error(Translate.get(
            "loadbalancer.backend.disabling.unreachable", backend.getName()));
        backend.disable();
        throw new UnreachableBackendException(Translate.get(
            "loadbalancer.backend.unreacheable", backend.getName()));
      }

      // Sanity check
      if (c == null)
        throw new UnreachableBackendException(Translate.get(
            "loadbalancer.backend.no.connection", backend.getName()));

      // Execute Query
      int result;
      try
      {
        result = AbstractLoadBalancer.executeWriteStoredProcedureOnBackend(
            proc, backend, c);

        // Warning! No way to detect if schema has been modified unless
        // we ask the backend again using DatabaseMetaData.getTables().
      }
      catch (Exception e)
      {
        throw new SQLException(Translate.get(
            "loadbalancer.storedprocedure.failed.on.backend", new String[]{
                proc.getSQLShortForm(vdb.getSQLShortFormLength()),
                backend.getName(), e.getMessage()}));
      }
      finally
      {
        cm.releaseConnection(c);
      }
      if (logger.isDebugEnabled())
        logger.debug(Translate.get("loadbalancer.storedprocedure.on",
            new String[]{String.valueOf(proc.getId()), backend.getName()}));
      return result;
    }
    else
    { // Inside a transaction
      Connection c;
      long tid = proc.getTransactionId();
      Long lTid = new Long(tid);

      if (!backend.isStartedTransaction(lTid))
      { // transaction has not been started yet on this backend
        try
        {
          c = cm.getConnection(tid);
        }
        catch (UnreachableBackendException e1)
        {
          logger.error(Translate.get(
              "loadbalancer.backend.disabling.unreachable", backend.getName()));
          backend.disable();
          throw new SQLException(Translate.get(
              "loadbalancer.backend.unreacheable", backend.getName()));
        }

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.unable.get.connection", new String[]{
                  String.valueOf(tid), backend.getName()}));

        // begin transaction
        backend.startTransaction(lTid);
        c.setAutoCommit(false);
      }
      else
      { // Re-use the connection used by this transaction
        c = cm.retrieveConnection(tid);

        // Sanity check
        if (c == null)
          throw new SQLException(Translate.get(
              "loadbalancer.unable.retrieve.connection", new String[]{
                  String.valueOf(tid), backend.getName()}));
      }

      // Execute Query
      int result;
      try
      {
        result = AbstractLoadBalancer.executeWriteStoredProcedureOnBackend(
            proc, backend, c);

        // Warning! No way to detect if schema has been modified unless
        // we ask the backend again using DatabaseMetaData.getTables().
      }
      catch (Exception e)
      {
        throw new SQLException(Translate.get(
            "loadbalancer.storedprocedure.failed.on.backend", new String[]{
                proc.getSQLShortForm(vdb.getSQLShortFormLength()),
                backend.getName(), e.getMessage()}));
      }
      if (logger.isDebugEnabled())
        logger.debug(Translate.get("loadbalancer.execute.transaction.on",
            new String[]{String.valueOf(tid), String.valueOf(proc.getId()),
                backend.getName()}));
      return result;
    }
  }

  //
  // Transaction Management
  //

  public void begin(TransactionMarkerMetaData tm) throws SQLException
  {
    Long lTid = new Long(tm.getTransactionId());
    if (backendPerTransactionId.containsKey(lTid))
      throw new SQLException(Translate.get(
          "loadbalancer.transaction.already.started", lTid.toString()));

    DatabaseBackend backend = chooseBackendForReadRequest(new UnknownRequest(
        "begin", false, 0, "\n"));
    backendPerTransactionId.put(lTid, backend);
    backend.startTransaction(lTid);
  }

  public void commit(TransactionMarkerMetaData tm) throws SQLException
  {
    long tid = tm.getTransactionId();
    Long lTid = new Long(tid);
    DatabaseBackend db = (DatabaseBackend) backendPerTransactionId.remove(lTid);

    AbstractConnectionManager cm = db.getConnectionManager(tm.getLogin());
    Connection c = cm.retrieveConnection(tid);

    // Sanity check
    if (c == null)
    { // Bad connection
      db.stopTransaction(lTid);

      throw new SQLException(Translate.get(
          "loadbalancer.unable.retrieve.connection", new String[]{
              String.valueOf(tid), db.getName()}));
    }

    // Execute Query
    try
    {
      c.commit();
      c.setAutoCommit(true);
    }
    catch (Exception e)
    {
      String msg = Translate.get("loadbalancer.commit.failed", new String[]{
          String.valueOf(tid), db.getName(), e.getMessage()});
      logger.error(msg);
      throw new SQLException(msg);
    }
    finally
    {
      cm.releaseConnection(tid);
      db.stopTransaction(lTid);
    }
  }

  public void rollback(TransactionMarkerMetaData tm) throws SQLException
  {
    long tid = tm.getTransactionId();
    Long lTid = new Long(tid);
    DatabaseBackend db = (DatabaseBackend) backendPerTransactionId.remove(lTid);

    AbstractConnectionManager cm = db.getConnectionManager(tm.getLogin());
    Connection c = cm.retrieveConnection(tid);

    // Sanity check
    if (c == null)
    { // Bad connection
      db.stopTransaction(lTid);

      throw new SQLException(Translate.get(
          "loadbalancer.unable.retrieve.connection", new String[]{
              String.valueOf(tid), db.getName()}));
    }

    // Execute Query
    try
    {
      c.rollback();
      c.setAutoCommit(true);
    }
    catch (Exception e)
    {
      String msg = Translate.get("loadbalancer.rollback.failed", new String[]{
          String.valueOf(tid), db.getName(), e.getMessage()});
      logger.error(msg);
      throw new SQLException(msg);
    }
    finally
    {
      cm.releaseConnection(tid);
      db.stopTransaction(lTid);
    }
  }

  public void enableBackend(DatabaseBackend db, boolean writeEnabled)
      throws SQLException
  {
    logger.info(Translate.get("loadbalancer.backend.enabling", db.getName()));
    if (!db.isInitialized())
      db.initializeConnections();
    db.enableRead();
    if (writeEnabled)
      db.enableWrite();
  }

  public void disableBackend(DatabaseBackend db) throws SQLException
  {
    logger.info(Translate.get("loadbalancer.backend.disabling", db.getName()));
    db.disable();
    if (db.isInitialized())
      db.finalizeConnections();
  }

  public String getXmlImpl()
  {
    StringBuffer info = new StringBuffer();
    info.append("<" + DatabasesXmlTags.ELT_ParallelDB + ">");
    info.append(getParallelDBXml());
    info.append("</" + DatabasesXmlTags.ELT_ParallelDB + ">");
    return info.toString();
  }

  public abstract String getParallelDBXml();

  public abstract DatabaseBackend chooseBackendForReadRequest(
      AbstractRequest request) throws SQLException;

  public abstract DatabaseBackend chooseBackendForWriteRequest(
      AbstractWriteRequest request) throws SQLException;

}

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