Index templates in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an index template in Elasticsearch?

An index template is a set of rules and settings that automatically apply to new indices matching a pattern. It helps keep index settings, mappings, and aliases consistent.

beginner

Which parts can you define in an Elasticsearch index template?

You can define index patterns, settings (like number of shards), mappings (field types), and aliases in an index template.

intermediate

How does Elasticsearch decide which template to apply when creating a new index?

Elasticsearch applies templates whose index patterns match the new index name. If multiple templates match, it merges them by priority.

intermediate

What is the difference between index_patterns and template in an index template?

index_patterns defines which index names the template applies to. template contains the actual settings, mappings, and aliases to apply.

beginner

Why use index templates instead of setting index settings manually each time?

Index templates save time and avoid mistakes by automatically applying consistent settings and mappings to new indices matching patterns.

What does an Elasticsearch index template primarily control?

A Cluster-wide configurations

B Data stored inside an index

C User access permissions

D Settings, mappings, and aliases for new indices

Index templates define settings, mappings, and aliases that apply automatically to new indices matching the template pattern.

Which field in an index template specifies which indices it applies to?

A index_patterns

B template

C aliases

D settings

index_patterns lists the index name patterns that the template matches.

If two templates match the same index, how does Elasticsearch decide which settings to use?

A It merges templates by priority, higher priority wins

B It uses the template with the lowest priority

C It randomly picks one template

D It ignores both templates

Elasticsearch merges matching templates by priority, applying higher priority templates last.

Can index templates be updated after creation?

A No, they are fixed once created

B Yes, you can update them and new indices will use updated templates

C Only mappings can be updated

D Only settings can be updated

You can update index templates anytime; new indices created after update use the new template.

What happens if no index template matches a new index name?

A Index creation fails

B Elasticsearch applies a random template

C Index is created with default settings

D Index is created but with no mappings

If no template matches, Elasticsearch creates the index with default settings and mappings.

Explain what an Elasticsearch index template is and why it is useful.

Think about how you can save time and avoid errors when creating many similar indices.

Describe how Elasticsearch applies multiple index templates when creating a new index.

Consider what happens if two templates want to set different shard counts.

Practice

1.

What is the main purpose of an index template in Elasticsearch?

easy

A. To delete old indexes automatically

B. To run queries faster on existing indexes

C. To backup data from indexes

D. To automatically apply settings and mappings to new indexes matching a pattern

Solution

1. Step 1: Understand what index templates do

   Index templates define settings and mappings that apply automatically to new indexes matching a pattern.

2. Step 2: Compare options with this purpose

   Only To automatically apply settings and mappings to new indexes matching a pattern describes this automatic application of settings and mappings.

3. Final Answer:

   To automatically apply settings and mappings to new indexes matching a pattern -> Option D

4. Quick Check:

   Index templates = auto apply settings [OK]

Hint: Index templates set rules for new indexes automatically [OK]

Common Mistakes:

• Thinking templates delete or backup data
• Confusing templates with query optimization
• Believing templates apply to existing indexes only

2.

Which of the following is the correct way to specify an index pattern in an Elasticsearch index template?

{
  "index_patterns": ["logs-*"]
}

easy

A. "index_patterns": "logs-*"

B. "index_patterns": ["logs-*"]

C. "index_patterns": {"logs-*"}

D. "index_patterns": logs-*

Solution

1. Step 1: Recall correct JSON syntax for index_patterns

   The index_patterns field expects an array of strings, so it must be inside square brackets.

2. Step 2: Check each option's syntax

   Only "index_patterns": ["logs-*"] uses an array with a string inside: ["logs-*"] which is correct JSON syntax.

3. Final Answer:

   "index_patterns": ["logs-*"] -> Option B

4. Quick Check:

   Patterns must be in array format [OK]

Hint: Patterns always go inside square brackets as a list [OK]

Common Mistakes:

• Using a string instead of an array
• Using curly braces instead of square brackets
• Omitting quotes around the pattern

3.

Given this index template:

{
  "index_patterns": ["app-logs-*"],
  "template": {
    "settings": {"number_of_shards": 2},
    "mappings": {"properties": {"user": {"type": "keyword"}}}
  }
}

What will be the number of shards and mapping type for the index app-logs-2024?

medium

A. 2 shards, user field as text

B. 1 shard, user field as text

C. 2 shards, user field as keyword

D. 1 shard, user field as keyword

Solution

1. Step 1: Match index name with pattern

   The index name app-logs-2024 matches the pattern app-logs-* in the template.

2. Step 2: Apply template settings and mappings

   The template sets number_of_shards to 2 and maps the user field as keyword type.

3. Final Answer:

   2 shards, user field as keyword -> Option C

4. Quick Check:

   Matching pattern applies template settings [OK]

Hint: Check if index name fits pattern, then apply template settings [OK]

Common Mistakes:

• Assuming default shard count instead of template value
• Confusing keyword with text mapping type
• Ignoring pattern matching rules

4.

Identify the error in this index template snippet:

{
  "index_patterns": "logs-*",
  "template": {
    "settings": {"number_of_replicas": 1}
  }
}

medium

A. "index_patterns" should be an array, not a string

B. "number_of_replicas" is not a valid setting

C. "template" key is missing required "mappings"

D. The pattern "logs-*" is invalid

Solution

1. Step 1: Check the type of index_patterns

   The index_patterns field must be an array of strings, but here it is a single string.

2. Step 2: Validate other fields

   number_of_replicas is a valid setting, mappings is optional, and the pattern syntax is correct.

3. Final Answer:

   "index_patterns" should be an array, not a string -> Option A

4. Quick Check:

   index_patterns must be an array [OK]

Hint: index_patterns always needs square brackets [OK]

Common Mistakes:

• Using string instead of array for index_patterns
• Thinking mappings are always required
• Assuming pattern syntax is wrong when it's correct

5.

You want to create an index template that applies to all indexes starting with sales- but excludes those ending with -archive. Which approach correctly achieves this?

hard

A. Use index_patterns": ["sales-*"] and create a separate template with higher priority for sales-*-archive that overrides settings

B. Use index_patterns": ["sales-*"] and rely on manual exclusion when creating indexes

C. Use index_patterns": ["sales-*"] and add a composed_of template excluding sales-*-archive

D. Use index_patterns": ["sales-*"] and add a priority template for sales-*-archive with no settings

Solution

1. Step 1: Understand template priority and pattern matching

   Elasticsearch applies templates by priority; higher priority templates override lower ones.

2. Step 2: Exclude indexes by overriding with higher priority template

   To exclude sales-*-archive, create a separate template with higher priority that overrides or clears settings for those indexes.

3. Final Answer:

   Create a higher priority template for sales-*-archive to override settings -> Option A

4. Quick Check:

   Use priority to override unwanted index templates [OK]

Hint: Use higher priority template to override exclusions [OK]

Common Mistakes:

• Assuming manual exclusion works automatically
• Using composed_of incorrectly for exclusion
• Not setting template priority to control overrides