Elasticsearch Request Cache: Optimizing Query Performance

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What is Request Cache? Best Practices Common Issues or Misuses Additional Information Frequently Asked Questions

What is Request Cache?

The request cache in Elasticsearch stores the results of search requests, allowing subsequent identical requests to be served from the cache instead of re-executing the query. This caching mechanism significantly improves query performance and reduces the load on the cluster, especially for frequently executed queries on mostly static data.

Best Practices

  1. Enable request caching for queries that are frequently repeated and have consistent results.
  2. Use the ?request_cache=true parameter in your API calls to explicitly enable caching for specific requests.
  3. Monitor cache usage and hit rates to optimize cache size and effectiveness.
  4. Regularly clear the cache for indices with frequently changing data to ensure fresh results.
  5. Use the size parameter judiciously, as requests with large result sets are not cached.

Common Issues or Misuses

  1. Caching queries on rapidly changing data, leading to stale results.
  2. Overreliance on caching without considering its impact on memory usage.
  3. Failing to clear the cache when necessary, especially after bulk updates.
  4. Caching large result sets, which can quickly fill up the cache and reduce its effectiveness.
  5. Not considering the trade-off between cache freshness and query performance.

Additional Information

  • The request cache is enabled by default for aggregation and suggestion requests.
  • It's disabled by default for search requests but can be enabled at the index level or per-request.
  • The cache is managed on a per-node basis and is shared across all shards on that node.
  • Cached results are invalidated when the underlying index is modified.
  • The size of the request cache can be configured using the indices.requests.cache.size setting.

Frequently Asked Questions

Q: How does the request cache differ from the query cache?
A: The request cache stores entire search request results, while the query cache (now deprecated) stored only filter results. The request cache is more flexible and can cache a wider range of query types.

Q: Can I cache partial results of a query?
A: No, the request cache stores complete results of a query. Partial results or intermediate stages of query execution are not cached.

Q: How can I monitor the effectiveness of my request cache?
A: You can use Elasticsearch's stats API to check cache hit rates, memory usage, and evictions. This information helps in tuning cache size and identifying which queries benefit most from caching.

Q: Does the request cache work with near real-time (NRT) search?
A: Yes, but with cautions. The cache is invalidated when the index is modified, so for indices with frequent updates, the benefits of caching may be limited.

Q: Can I selectively enable or disable request caching for specific fields or query types?
A: Request caching is applied at the query level, not field level. However, you can control caching on a per-request basis using the request_cache parameter in your API calls.

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