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   1  <?php
   2  /**
   3   * This file contains database access object related constants.
   4   *
   5   * This program is free software; you can redistribute it and/or modify
   6   * it under the terms of the GNU General Public License as published by
   7   * the Free Software Foundation; either version 2 of the License, or
   8   * (at your option) any later version.
   9   *
  10   * This program is distributed in the hope that it will be useful,
  11   * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13   * GNU General Public License for more details.
  14   *
  15   * You should have received a copy of the GNU General Public License along
  16   * with this program; if not, write to the Free Software Foundation, Inc.,
  17   * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18   * http://www.gnu.org/copyleft/gpl.html
  19   *
  20   * @file
  21   * @ingroup Database
  22   */
  23  
  24  /**
  25   * Interface for database access objects.
  26   *
  27   * Classes using this support a set of constants in a bitfield argument to their data loading
  28   * functions. In general, objects should assume READ_NORMAL if no flags are explicitly given,
  29   * though certain objects may assume READ_LATEST for common use case or legacy reasons.
  30   *
  31   * There are four types of reads:
  32   *   - READ_NORMAL    : Potentially cached read of data (e.g. from a slave or stale replica)
  33   *   - READ_LATEST    : Up-to-date read as of transaction start (e.g. from master or a quorum read)
  34   *   - READ_LOCKING   : Up-to-date read as of now, that locks (shared) the records
  35   *   - READ_EXCLUSIVE : Up-to-date read as of now, that locks (exclusive) the records
  36   * All record locks persist for the duration of the transaction.
  37   *
  38   * Callers should use READ_NORMAL (or pass in no flags) unless the read determines a write.
  39   * In theory, such cases may require READ_LOCKING, though to avoid contention, READ_LATEST is
  40   * often good enough. If UPDATE race condition checks are required on a row and expensive code
  41   * must run after the row is fetched to determine the UPDATE, it may help to do something like:
  42   *   - a) Read the current row
  43   *   - b) Determine the new row (expensive, so we don't want to hold locks now)
  44   *   - c) Re-read the current row with READ_LOCKING; if it changed then bail out
  45   *   - d) otherwise, do the updates
  46   *
  47   * @since 1.20
  48   */
  49  interface IDBAccessObject {
  50      // Constants for object loading bitfield flags (higher => higher QoS)
  51      const READ_LATEST = 1; // read from the master
  52      const READ_LOCKING = 3; // READ_LATEST (1) and "LOCK IN SHARE MODE" (2)
  53      const READ_EXCLUSIVE = 7; // READ_LOCKING (3) and "FOR UPDATE" (4)
  54  
  55      // Convenience constant for callers to explicitly request slave data
  56      const READ_NORMAL = 0; // read from the slave
  57  
  58      // Convenience constant for tracking how data was loaded (higher => higher QoS)
  59      const READ_NONE = -1; // not loaded yet (or the object was cleared)
  60  }