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69 lines
3.4 KiB
69 lines
3.4 KiB
/*
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XOWA: the XOWA Offline Wiki Application
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Copyright (C) 2012-2020 gnosygnu@gmail.com
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XOWA is licensed under the terms of the General Public License (GPL) Version 3,
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or alternatively under the terms of the Apache License Version 2.0.
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You may use XOWA according to either of these licenses as is most appropriate
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for your project on a case-by-case basis.
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The terms of each license can be found in the source code repository:
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GPLv3 License: https://github.com/gnosygnu/xowa/blob/master/LICENSE-GPLv3.txt
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Apache License: https://github.com/gnosygnu/xowa/blob/master/LICENSE-APACHE2.txt
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*/
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package gplx.xowa.mediawiki.includes.dao;
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/**
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* Interface for database access objects.
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*
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* Classes using this support a set of constants in a bitfield argument to their data loading
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* functions. In general, objects should assume READ_NORMAL if no flags are explicitly given,
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* though certain objects may assume READ_LATEST for common use case or legacy reasons.
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*
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* There are four types of reads:
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* - READ_NORMAL : Potentially cached read of data (e.g. from a replica DB or stale replica)
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* - READ_LATEST : Up-to-date read as of transaction start (e.g. from master or a quorum read)
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* - READ_LOCKING : Up-to-date read as of now, that locks (shared) the records
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* - READ_EXCLUSIVE : Up-to-date read as of now, that locks (exclusive) the records
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* All record locks persist for the duration of the transaction.
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*
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* A special constant READ_LATEST_IMMUTABLE can be used for fetching append-only data. Such
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* data is either (a) on a replica DB and up-to-date or (b) not yet there, but on the master/quorum.
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* Because the data is append-only, it can never be stale on a replica DB if present.
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*
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* Callers should use READ_NORMAL (or pass in no flags) unless the read determines a write.
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* In theory, such cases may require READ_LOCKING, though to avoid contention, READ_LATEST is
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* often good enough. If UPDATE race condition checks are required on a row and expensive code
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* must run after the row is fetched to determine the UPDATE, it may help to do something like:
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* - a) Start transaction
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* - b) Read the current row with READ_LATEST
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* - c) Determine the new row (expensive, so we don't want to hold locks now)
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* - d) Re-read the current row with READ_LOCKING; if it changed then bail out
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* - e) otherwise, do the updates
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* - f) Commit transaction
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*
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* @since 1.20
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*/
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public interface XomwIDBAccessObject {
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/** Constants for object loading bitfield flags (higher => higher QoS) */
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/** @var int Read from a replica DB/non-quorum */
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public static int READ_NORMAL = 0;
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/** @var int Read from the master/quorum */
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public static int READ_LATEST = 1;
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/* @var int Read from the master/quorum and lock out other writers */
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public static int READ_LOCKING = READ_LATEST | 2; // READ_LATEST (1) and "LOCK IN SHARE MODE" (2)
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/** @var int Read from the master/quorum and lock out other writers and locking readers */
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public static int READ_EXCLUSIVE = READ_LOCKING | 4; // READ_LOCKING (3) and "FOR UPDATE" (4)
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/** @var int Read from a replica DB or without a quorum, using the master/quorum on miss */
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public static int READ_LATEST_IMMUTABLE = 8;
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// Convenience constant for tracking how data was loaded (higher => higher QoS)
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public static int READ_NONE = -1; // not loaded yet (or the object was cleared)
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}
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