Cache management (query, request, field data) in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of query cache in Elasticsearch?

Query cache stores the results of frequently run queries to speed up response times by avoiding repeated computation.

intermediate

How does request cache differ from query cache in Elasticsearch?

Request cache stores the results of entire search requests, including filters and aggregations, while query cache focuses on caching filter results only.

beginner

What type of data does the field data cache hold in Elasticsearch?

Field data cache holds in-memory data structures for sorting, aggregations, and scripting on text or keyword fields.

intermediate

Why is it important to monitor and manage field data cache in Elasticsearch?

Because field data cache can consume a lot of heap memory, unmanaged growth can cause out-of-memory errors and degrade cluster performance.

intermediate

How can you disable request cache for a specific search request in Elasticsearch?

By setting the parameter "request_cache": false in the search request body, you can disable request caching for that request.

Which cache in Elasticsearch stores results of filters to speed up queries?

A Shard cache

B Query cache

C Field data cache

D Request cache

Query cache stores filter results to speed up repeated queries.

What does the request cache in Elasticsearch store?

A Only filter results

B Index mappings

C Field data for sorting

D Entire search request results including aggregations

Request cache stores full search request results, including filters and aggregations.

Why should field data cache be carefully managed?

A It consumes heap memory and can cause out-of-memory errors

B It can cause slow network traffic

C It uses disk space

D It slows down indexing

Field data cache uses heap memory and unmanaged growth can cause memory errors.

How do you disable request cache for a search request?

A Set "request_cache": false

B Set "query_cache": false

C Set "cache": false

D Set "fielddata_cache": false

Setting "request_cache": false disables request caching for that search request.

Which cache is used to speed up sorting and aggregations on text fields?

A Query cache

B Request cache

C Field data cache

D Filter cache

Field data cache holds data structures needed for sorting and aggregations.

Explain the differences between query cache, request cache, and field data cache in Elasticsearch.

Think about what each cache stores and why.

Describe why managing field data cache is important and how it affects Elasticsearch performance.

Consider memory usage and cluster health.

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