includes/dao/IDBAccessObject.php

   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 replica DB 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  * A special constant READ_LATEST_IMMUTABLE can be used for fetching append-only data. Such
  39  * data is either (a) on a replica DB and up-to-date or (b) not yet there, but on the master/quorum.
  40  * Because the data is append-only, it can never be stale on a replica DB if present.
  41  *
  42  * Callers should use READ_NORMAL (or pass in no flags) unless the read determines a write.
  43  * In theory, such cases may require READ_LOCKING, though to avoid contention, READ_LATEST is
  44  * often good enough. If UPDATE race condition checks are required on a row and expensive code
  45  * must run after the row is fetched to determine the UPDATE, it may help to do something like:
  46  *   - a) Start transaction
  47  *   - b) Read the current row with READ_LATEST
  48  *   - c) Determine the new row (expensive, so we don't want to hold locks now)
  49  *   - d) Re-read the current row with READ_LOCKING; if it changed then bail out
  50  *   - e) otherwise, do the updates
  51  *   - f) Commit transaction
  52  *
  53  * @since 1.20
  54  */
  55 interface IDBAccessObject {
  56         /** Constants for object loading bitfield flags (higher => higher QoS) */
  57         /** @var int Read from a replica DB/non-quorum */
  58         const READ_NORMAL = 0;
  59         /** @var int Read from the master/quorum */
  60         const READ_LATEST = 1;
  61         /* @var int Read from the master/quorum and lock out other writers */
  62         const READ_LOCKING = 3; // READ_LATEST (1) and "LOCK IN SHARE MODE" (2)
  63         /** @var int Read from the master/quorum and lock out other writers and locking readers */
  64         const READ_EXCLUSIVE = 7; // READ_LOCKING (3) and "FOR UPDATE" (4)
  65
  66         /** @var int Read from a replica DB or without a quorum, using the master/quorum on miss */
  67         const READ_LATEST_IMMUTABLE = 8;
  68
  69         // Convenience constant for tracking how data was loaded (higher => higher QoS)
  70         const READ_NONE = -1; // not loaded yet (or the object was cleared)
  71 }