Caches and Query Warming

Solr comes with a default SolrCache implementation that is used for different types of caches.

The CaffeineCache is an implementation backed by the Caffeine caching library. By default it uses a Window TinyLFU (W-TinyLFU) eviction policy, which allows the eviction based on both frequency and recency of use in O(1) time with a small footprint. Generally this cache usually offers lower memory footprint, higher hit ratio, and better multi-threaded performance over legacy caches.

CaffeineCache uses an auto-warm count that supports both integers and percentages which get evaluated relative to the current size of the cache when warming happens.

The Plugins & Stats Screen in the Solr Admin UI will display information about the performance of all the active caches. This information can help you fine-tune the sizes of the various caches appropriately for your particular application. When a Searcher terminates, a summary of its cache usage is also written to the log.

Each cache has settings to define its initial size (initialSize), maximum size (size), and number of items to use for during warming (autowarmCount). For autowarmCount this can be also expressed as a percentage instead of an absolute value.

A maxIdleTime attribute controls the automatic eviction of entries that haven’t been used for a while. This attribute is expressed in seconds, with the default value of 0 meaning no entries are automatically evicted due to exceeded idle time. Smaller values of this attribute will cause older entries to be evicted quickly, which will reduce cache memory usage but may instead cause thrashing due to a repeating eviction-lookup-miss-insertion cycle of the same entries. Larger values will cause entries to stay around longer, waiting to be reused, at the cost of increased memory usage. Reasonable values, depending on the query volume and patterns, may lie somewhere between 60-3600.

The maxRamMB attribute limits the maximum amount of memory a cache may consume. When both size and maxRamMB limits are specified the maxRamMB limit will take precedence and the size limit will be ignored.

The async attribute determines whether the cache stores direct results (async=false, disabled) or whether it will store indirect references to the computation (async=true, enabled by default). If your queries include child documents or join queries, then async cache must be enabled to function properly. Disabling the async option may use slightly less memory per cache entry at the expense of increased CPU. The async cache provides most significant improvement with many concurrent queries requesting the same result set that has not yet been cached, as an alternative to larger cache sizes or increased auto-warming counts. However, the async cache will not prevent data races for time-limited queries, since those are expected to provide partial results.

All caches can be disabled using the parameter enabled with a value of false. Caches can also be disabled on a query-by-query basis with the cache parameter, as described in the section cache Local Parameter.

Details of each cache are described below.

This cache holds parsed queries paired with an unordered set of all documents that match it. Unless such a set is trivially small, the set implementation is a bitset.

The most typical way Solr uses the filterCache is to cache results of each fq search parameter, though there are some other cases as well. Subsequent queries using the same parameter filter query result in cache hits and rapid returns of results. See fq (Filter Query) Parameter for a detailed discussion of fq. Use of this cache can be disabled for a fq using the cache local parameter.

Another Solr feature using this cache is the filter(…​) syntax in the default Lucene query parser.

Solr also uses this cache for faceting when the configuration parameter facet.method is set to fc. For a discussion of faceting parameters, see Field-Value Faceting Parameters.

The cache supports a maxRamMB parameter which restricts the maximum amount of heap used by this cache. The CaffeineCache only supports evictions by either heap usage or size, but not both. Therefore, the size parameter is ignored if maxRamMB is specified.

The filter cache is a good candidate for enabling async computation.

The queryResultCache holds the results of previous searches: ordered lists of document IDs (DocList) based on a query, a sort, and the range of documents requested.

The queryResultCache has an optional setting to limit the maximum amount of RAM used (maxRamMB). This lets you specify the maximum heap size, in megabytes, used by the contents of this cache. When the cache grows beyond this size, oldest accessed queries will be evicted until the heap usage of the cache decreases below the specified limit. If a size is specified in addition to maxRamMB then only the heap usage limit is respected.

Use of this cache can be disabled on a query-by-query basis in q using the cache local parameter.

The documentCache holds Lucene Document objects (the stored fields for each document). Since Lucene internal document IDs are transient, this cache is not auto-warmed.

The size for the documentCache should always be greater than max_results times the max_concurrent_queries, to ensure that Solr does not need to refetch a document during a request. The more fields you store in your documents, the higher the memory usage of this cache will be.

You can also define named caches for your own application code to use. You can locate and use your cache object by name by calling the SolrIndexSearcher methods getCache(), cacheLookup() and cacheInsert().

If you want auto-warming of your cache, include a regenerator attribute with the fully qualified name of a class that implements solr.search.CacheRegenerator. You can also use the NoOpRegenerator, which simply repopulates the cache with old items. Define it with the regenerator parameter as regenerator="solr.NoOpRegenerator".

The section Cache Statistics describes the metrics available for each cache. The metrics can be accessed in the Plugins & Stats Screen or with the Metrics API.

The most important metrics to review when assessing caches are the size and the hit ratio.

The size indicates how many items are in the cache. Some caches support setting the maximum cache size in MB of RAM.

The hit ratio is a percentage of queries served by the cache, shown as a number between 0 and 1. Higher values indicate that the cache is being used often, while lower values would show that the cache isn’t helping queries very much. Ideally, this number should be as close to 1 as possible.

If you find that you have a low hit ratio but you’ve set your cache size high, you can optimize by reducing the cache size - there’s no need to keep those objects in memory when they are not being used.

Another useful metric is the cache evictions, which measures the objects removed from the cache. A high rate of evictions can indicate that your cache is too small and increasing it may show a higher hit ratio. Alternatively, if your hit ratio is high but your evictions are low, your cache might be too large and you may benefit from reducing the size.

A low hit ratio is not always a sign of a specific cache problem. If your queries are not repeated often, a low hit ratio would be expected because it’s less likely that cached objects will need to be reused. In these cases, a smaller cache size may be ideal for your system.

Sets the maximum number of clauses allowed when parsing a boolean query string.

This limit only impacts boolean queries specified by a user as part of a query string, and provides per-collection controls on how complex user specified boolean queries can be. Query strings that specify more clauses than this will result in an error.

If this per-collection limit is greater than the global maxBooleanClauses limit specified in solr.xml, it will have no effect, as that setting also limits the size of user specified boolean queries.

In default configurations this property uses the value of the solr.max.booleanClauses system property if specified. This is the same system property used in the global maxBooleanClauses setting in the default solr.xml making it easy for Solr administrators to increase both values (in all collections) without needing to search through and update the solrconfig.xml files in each collection.

When this parameter is set to true, fields that are not directly requested will be loaded only as needed.

This can boost performance if the most common queries only need a small subset of fields, especially if infrequently accessed fields are large in size.

This setting only affects queries where the requested sort does not include "score" (or for which score is irrelevant — e.g., no docs requested, query outputs a constant score). In such cases, configuring this element to true causes the filterCache to be consulted for a filter matching the main query. This can be useful if the same search is issued repeatedly with different sort options or different pagination via offset or cursorMark (again, provided that "score" is not included or not relevant to the requested sort). If enabling this option, ensure that the filterCache has sufficient capacity to support expected use patterns.

Used with the queryResultCache, this will cache a superset of the requested number of document IDs.

For example, if a query requests documents 10 through 19, and queryWindowSize is 50, documents 0 through 49 will be cached.

This parameter sets the maximum number of documents to cache for any entry in the queryResultCache.

This setting controls whether search requests for which there is not a currently registered searcher should wait for a new searcher to warm up (false) or proceed immediately (true). When set to "false`, requests will block until the searcher has warmed its caches.

This parameter sets the maximum number of searchers that may be warming up in the background at any given time. Exceeding this limit will raise an error.

For read-only followers, a value of 2 is reasonable. Leaders should probably be set a little higher.

Warming queries may be managed using the Config API using commands such as the below.

Multiple sets with different name attributes may be provided and any changes will take place immediately without a restart being necessary.