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.

Practice

1. What is the primary purpose of cache management in Elasticsearch?

easy

A. To store recent search data and speed up query responses

B. To permanently save all search results for future use

C. To delete all data from the Elasticsearch index

D. To increase the size of the Elasticsearch cluster automatically

Solution

1. Step 1: Understand what cache does in Elasticsearch

   Cache temporarily stores recent search data to avoid repeating expensive operations.

2. Step 2: Identify the main benefit of caching

   By storing recent data, Elasticsearch can respond faster to repeated queries.

3. Final Answer:

   To store recent search data and speed up query responses -> Option A

4. Quick Check:

   Cache speeds up queries = A [OK]

Hint: Cache is for speed, not permanent storage [OK]

Common Mistakes:

• Thinking cache saves data permanently
• Confusing cache with index storage
• Assuming cache increases cluster size

2. Which of the following is the correct syntax to clear the query cache for an index named products using Elasticsearch REST API?

easy

A. POST /products/_cache/clear?query=true

B. POST /products/_cache/clear/query

C. POST /products/_cache/clear/fielddata

D. POST /products/_clear_cache/query

Solution

1. Step 1: Recall the correct REST API endpoint for clearing cache

   Elasticsearch uses _cache/clear with query parameters to specify cache types.

2. Step 2: Identify the correct parameter for query cache

   The query cache is cleared by adding ?query=true to the endpoint.

3. Final Answer:

   POST /products/_cache/clear?query=true -> Option A

4. Quick Check:

   Use query=true parameter to clear query cache [OK]

Hint: Use query=true parameter to clear query cache [OK]

Common Mistakes:

• Using wrong endpoint like _clear_cache
• Confusing fielddata cache with query cache
• Missing query parameter in URL

3. Given this Elasticsearch request to clear caches:

POST /myindex/_cache/clear
{
  "fielddata": true,
  "query": true
}

What caches will be cleared?

medium

A. Only the query cache

B. Only the fielddata cache

C. Request cache only

D. Both query and fielddata caches

Solution

1. Step 1: Analyze the JSON body parameters

   The request sets both fielddata and query to true, indicating both caches should be cleared.

2. Step 2: Understand cache clearing behavior

   When multiple cache types are true, Elasticsearch clears all specified caches.

3. Final Answer:

   Both query and fielddata caches -> Option D

4. Quick Check:

   fielddata=true + query=true clears both caches [OK]

Hint: True flags clear all specified caches together [OK]

Common Mistakes:

• Assuming only one cache clears at a time
• Confusing request cache with fielddata cache
• Ignoring JSON body parameters

4. You run this command to clear the request cache:

POST /logs/_cache/clear
{
  "request": true
}

But the cache is not cleared. What is the likely problem?

medium

A. The index name logs is invalid for cache clearing

B. The syntax is incorrect; request cache cannot be cleared this way

C. Request cache is disabled by default and must be enabled first

D. You must specify fielddata as true to clear request cache

Solution

1. Step 1: Understand request cache behavior

   Request cache is off by default and must be enabled in index settings to be used and cleared.

2. Step 2: Identify why clearing fails

   If request cache is disabled, clearing it has no effect, so the command appears to do nothing.

3. Final Answer:

   Request cache is disabled by default and must be enabled first -> Option C

4. Quick Check:

   Request cache off by default = no clearing effect [OK]

Hint: Enable request cache before clearing it [OK]

Common Mistakes:

• Assuming request cache is always enabled
• Using wrong syntax to clear caches
• Thinking fielddata cache affects request cache

5. You want to optimize Elasticsearch performance by managing caches. Which strategy correctly balances cache clearing and system stability?

hard

A. Clear all caches frequently to free memory, even during heavy query load

B. Clear query and request caches during low traffic, and monitor fielddata cache size

C. Never clear caches to keep all data in memory permanently

D. Disable all caches to avoid stale data and improve stability

Solution

1. Step 1: Understand cache clearing impact

   Clearing caches frees memory but can slow queries if done too often or during heavy load.

2. Step 2: Identify best practice for cache management

   Clearing query and request caches during low traffic avoids performance hits; monitoring fielddata cache prevents memory issues.

3. Final Answer:

   Clear query and request caches during low traffic, and monitor fielddata cache size -> Option B

4. Quick Check:

   Clear caches smartly during low load + monitor fielddata [OK]

Hint: Clear caches during low load, monitor fielddata size [OK]

Common Mistakes:

• Clearing caches too often during heavy load
• Disabling caches completely
• Ignoring fielddata cache memory use