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
- Enable request caching for queries that are frequently repeated and have consistent results.
- Use the
?request_cache=true
parameter in your API calls to explicitly enable caching for specific requests. - Monitor cache usage and hit rates to optimize cache size and effectiveness.
- Regularly clear the cache for indices with frequently changing data to ensure fresh results.
- Use the
size
parameter judiciously, as requests with large result sets are not cached.
Common Issues or Misuses
- Caching queries on rapidly changing data, leading to stale results.
- Overreliance on caching without considering its impact on memory usage.
- Failing to clear the cache when necessary, especially after bulk updates.
- Caching large result sets, which can quickly fill up the cache and reduce its effectiveness.
- 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.