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<?php
/**
* This file contains database access object related constants.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
* http://www.gnu.org/copyleft/gpl.html
*
* @file
* @ingroup Database
*/
/**
* Interface for database access objects.
*
* Classes using this support a set of constants in a bitfield argument to their data loading
* functions. In general, objects should assume READ_NORMAL if no flags are explicitly given,
* though certain objects may assume READ_LATEST for common use case or legacy reasons.
*
* There are four types of reads:
* - READ_NORMAL : Potentially cached read of data (e.g. from a replica DB or stale replica)
* - READ_LATEST : Up-to-date read as of transaction start (e.g. from master or a quorum read)
* - READ_LOCKING : Up-to-date read as of now, that locks (shared) the records
* - READ_EXCLUSIVE : Up-to-date read as of now, that locks (exclusive) the records
* All record locks persist for the duration of the transaction.
*
* A special constant READ_LATEST_IMMUTABLE can be used for fetching append-only data. Such
* data is either (a) on a replica DB and up-to-date or (b) not yet there, but on the master/quorum.
* Because the data is append-only, it can never be stale on a replica DB if present.
*
* Callers should use READ_NORMAL (or pass in no flags) unless the read determines a write.
* In theory, such cases may require READ_LOCKING, though to avoid contention, READ_LATEST is
* often good enough. If UPDATE race condition checks are required on a row and expensive code
* must run after the row is fetched to determine the UPDATE, it may help to do something like:
* - a) Start transaction
* - b) Read the current row with READ_LATEST
* - c) Determine the new row (expensive, so we don't want to hold locks now)
* - d) Re-read the current row with READ_LOCKING; if it changed then bail out
* - e) otherwise, do the updates
* - f) Commit transaction
*
* @since 1.20
*/
interface IDBAccessObject {
/** Constants for object loading bitfield flags (higher => higher QoS) */
/** @var int Read from a replica DB/non-quorum */
const READ_NORMAL = 0;
/** @var int Read from the master/quorum */
const READ_LATEST = 1;
/* @var int Read from the master/quorum and lock out other writers */
const READ_LOCKING = self::READ_LATEST | 2; // READ_LATEST (1) and "LOCK IN SHARE MODE" (2)
/** @var int Read from the master/quorum and lock out other writers and locking readers */
const READ_EXCLUSIVE = self::READ_LOCKING | 4; // READ_LOCKING (3) and "FOR UPDATE" (4)
/** @var int Read from a replica DB or without a quorum, using the master/quorum on miss */
const READ_LATEST_IMMUTABLE = 8;
// Convenience constant for tracking how data was loaded (higher => higher QoS)
const READ_NONE = -1; // not loaded yet (or the object was cleared)
}
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