d83a7bcd09
The `SettingsBuilder` now accepts a PSR-16 cache interface with which to store and query settings before attempting to load from each source. By default, no cache is used, but any object that implements the `Psr\SimpleCache\CacheInterface` may be provided to the constructor. An explicit dependency on "psr/simple-cache" has been added to `composer.json`. Note that this dependency already existed in vendor albeit it as a transitive one. An APCu based `SharedMemoryCache` adapter is provided as a canonical PSR-16 compliant interface for production use. Sources are now queued by the `SettingsBuilder` when calling `load()`. If a cache interface has been provided, and the source is considered cacheable (implements `CacheableSource`), then it is wrapped as a `CachedSource` which will query the cache first before loading from the wrapped source. Cache stampedes are mitigated using probabilistic early expiry. The implementation for this was partially based on symfony/cache-contract source code but also from the Wikipedia article and paper referenced therein. See https://en.wikipedia.org/wiki/Cache_stampede#Probabilistic_early_expiration Bug: T294748 Change-Id: I52ab3899731546876ee58265bd4a1927886746dc
52 lines
1.9 KiB
PHP
52 lines
1.9 KiB
PHP
<?php
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namespace MediaWiki\Settings\Cache;
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use MediaWiki\Settings\Source\SettingsSource;
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/**
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* A {@link SettingsSource} that can be cached. It must return a unique
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* (enough) and deterministic hash key for cache indexing.
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*
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* @since 1.38
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* @todo mark as stable before the 1.38 release
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*/
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interface CacheableSource extends SettingsSource {
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/**
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* Coefficient used in determining early expiration of cached settings to
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* avoid stampedes.
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*
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* Increasing this value will cause the random early election to happen by
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* a larger margin of lead time before normal expiry, relative to the
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* cache value's generation duration. Conversely, returning a lesser value
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* will narrow the margin of lead time, making the cache hold items for
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* slightly longer but with more likelihood that concurrent regenerations
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* and set overwrites will occur. Returning <code>0</code> will
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* effectively disable early expiration, and by extension disable stampede
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* mitigation altogether.
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*
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* A greater value may be suitable if a source has a highly variable
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* generation duration, but most implementations should simply return
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* <code>1.0</code>.
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*
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* @link https://en.wikipedia.org/wiki/Cache_stampede#Probabilistic_early_expiration
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* @link https://cseweb.ucsd.edu/~avattani/papers/cache_stampede.pdf
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*
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* Optimal Probabilistic Cache Stampede Prevention
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* Vattani, A.; Chierichetti, F.; Lowenstein, K. (2015), "Optimal
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* Probabilistic Cache Stampede Prevention" (PDF), Proceedings of the VLDB
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* Endowment, VLDB, 8 (8): 886–897, doi:10.14778/2757807.2757813, ISSN
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* 2150-8097 https://cseweb.ucsd.edu/~avattani/papers/cache_stampede.pdf
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*
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* @return float
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*/
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public function getExpiryWeight(): float;
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/**
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* Returns a deterministically computed key for use in caching settings
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* from this source.
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*
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* @return string
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*/
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public function getHashKey(): string;
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}
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