Cache management (query, request, field data) in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Elasticsearch query cache stats request?

Given the following request to check query cache statistics, what will be the value of query_cache.memory_size_in_bytes if no queries have been cached yet?

Elasticsearch

GET /_nodes/stats/query_cache

A {"nodes":{"<node_id>":{"indices":{"query_cache":{"memory_size_in_bytes":null}}}}}

B {"nodes":{"<node_id>":{"indices":{"query_cache":{"memory_size_in_bytes":0}}}}}

C {"nodes":{"<node_id>":{"indices":{"query_cache":{"memory_size_in_bytes":1024}}}}}

D {"nodes":{"<node_id>":{"indices":{"query_cache":{"memory_size_in_bytes":-1}}}}}

💡 Hint

Think about what the cache size would be before any queries are cached.

When no queries are cached, the memory size used by the query cache is zero bytes.

🧠 Conceptual

intermediate

Which cache type is cleared when a document is updated in Elasticsearch?

In Elasticsearch, when you update a document, which cache is automatically cleared to ensure fresh results?

A Field data cache

B Request cache

C Query cache

D Shard request cache

💡 Hint

Think about which cache holds field values used for sorting and aggregations.

Field data cache is cleared on document updates because it holds in-memory field values that may change.

❓ Predict Output

advanced

What is the output of this request cache usage query?

Consider this request to check the request cache stats for an index. What is the value of hit_count if the cache was never used?

Elasticsearch

GET /my_index/_stats/request_cache

A {"_all":{"total":{"request_cache":{"hit_count":1}}}}

B {"_all":{"total":{"request_cache":{"hit_count":-1}}}}

C {"_all":{"total":{"request_cache":{"hit_count":null}}}}

D {"_all":{"total":{"request_cache":{"hit_count":0}}}}

💡 Hint

If the cache was never hit, the hit count should be zero.

Hit count starts at zero and increments when cache hits occur.

🔧 Debug

advanced

Why does this query cache not improve performance?

You enabled query cache on an index, but queries are not being cached. Which of the following reasons explains this behavior?

Elasticsearch

GET /my_index/_search
{
  "query": {
    "match_all": {}
  },
  "request_cache": true
}

A The query cache is disabled by default and must be enabled explicitly per index.

B The query is not cacheable because it uses a match_all query.

C The request cache only caches filter context queries, not match queries.

D The request cache caches only aggregations, not queries.

💡 Hint

Request cache works only for certain query types.

Request cache caches filter context queries, not full queries like match_all.

🚀 Application

expert

How to reduce field data cache memory usage for a large text field?

You have a large text field used for sorting and aggregations causing high field data cache memory usage. Which approach will reduce memory usage effectively?

A Enable doc_values on the field and disable fielddata.

B Increase the fielddata cache size limit in Elasticsearch settings.

C Disable the query cache for the index.

D Use the request cache instead of field data cache.

💡 Hint

Doc_values store field data on disk in a columnar format, reducing heap usage.

Enabling doc_values stores field data on disk, reducing heap memory usage compared to fielddata cache.