Help:Query cache

Semantic MediaWiki 2.5.0Released on an unknown date unknown versions of MediaWiki adds an experimental feature to support a simple query cache1 with the objective to minimize the computational effort for queries that share the same query signature. A query signature is a fingerprint that identifies each query on characteristics that influences (e.g. condition, limit, offset, and sort/order) the result set of a query. This experimental feature is disabled by default.

Usage

The simple query cache is used for:

• Queries that have the same signature will be redirected to the cache instead of seeking an active "QueryEngine" connection and hereby reduce DB connections and computation effort
• Only the subject list is being cached as result of the query answering, leaving individual "PrintRequest's" to the distinctive printer and avoids conflict with existing or future "ResultPrinter" when generating the output representation

When the query result cache is enabled, current queries will be reassigned a new query ID 23 to increase the pool of queries that share potentially the same signature and hereby broaden the range of a cache reuse and minimize cache fragmentation.

The eviction4 of a cache is normally triggered by:

• A time constraint (see configuration parameter $smwgQueryResultCacheLifetimeSets the cache lifetime of embedded queries and their results fetched from the query engine})
• A manually intervention (see configuration parameter $smwgQueryResultCacheRefreshOnPurgeSets whether it is possible to manually refresh the query cache by doing a "purge" action), or
• An embedded query update (see configuration parameter $smwgEnabledQueryDependencyLinksStoreSets whether tracking and storing of dependencies of embedded queries may be used) if enabled with any monitored change to a dependency chain to trigger an automatic purge of related queries

A recommended cache provider is redis5 to account for an optimal response time, storage capacity, and independence from the DB master. The cache statistics with its relative small size will be stored using CACHE_DB to improve durability of the collected data and make it independent of the query cache operations.

Operational relevance and statistics

To make a decision about whether the query result cache bears some operational relevance (meaning the benefits outweigh the cost of keeping an additional cache instance, or the median response time proves some generalisable advantage) with cache statistics being kept for the duration of the query cache lifetime, allowing it to gather some data about performance and use.

Example

{
    "misses": 545,
    "deletes": 56,
    "hits": {
        "nonEmbedded": 636,
        "embedded": 39
    },
    "medianRetrievalResponseTime": {
        "uncached": 0.030940875770042,
        "cached": 0.0069486399265745
    },
    "noCache": {
        "byLimit": 30
    },
    "ratio": {
        "hit": 0.5533,
        "miss": 0.4467
    },
    "meta": {
        "version": "0.2",
        "cacheLifetime": {
            "embedded": 86400,
            "nonEmbedded": 600
        },
        "collectionDate": {
            "start": "2016-11-30 14:17:21",
            "update": "2016-12-07 10:13:54"
        }
    }
}

Configuration parameters

• Configuration parameter $smwgQueryResultCacheTypeSets whether the query result cache may be used − Allows to enable the feature
• Configuration parameter $smwgQueryResultCacheLifetimeSets the cache lifetime of embedded queries and their results fetched from the query engine − Allows to set the lifetime of embedded queries and their results
• Configuration parameter $smwgQueryResultNonEmbeddedCacheLifetimeSets the cache lifetime of non-embedded queries and their results fetched from the query engine − Allows to set the lifetime of non-embedded queries and their results
• Configuration parameter $smwgQueryResultCacheRefreshOnPurgeSets whether it is possible to manually refresh the query cache by doing a "purge" action − Allows to control the option to manually refresh embedded queries and their results

The default values to these configuration parameters may not be applicable for each situation therefore it is suggested to monitor the statistics and make a decision about appropriate values for embedded and non-embedded cached queries.

See also

• Help page on cache statistics
• Help pages on query management and query dependency
• Help page on concept caching
• Help page on speeding up Semantic MediaWiki
• Help page on caching
• Help page on embedded query updates
• Help page on query performance
• Help page on page loading time
• Help page on the parser cache

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