Index aliases in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an index alias in Elasticsearch?

An index alias is a name that points to one or more Elasticsearch indices. It acts like a shortcut or nickname to access those indices without using their real names.

beginner

How can index aliases help in managing Elasticsearch indexes?

Index aliases let you switch which index your application uses without changing the application code. This helps when updating or reindexing data, making the process smooth and safe.

intermediate

Can an index alias point to multiple indexes at the same time?

Yes, an alias can point to multiple indices. This allows you to search across several indices using one alias name, simplifying queries.

intermediate

What is a common use case for using index aliases with filters?

You can create an alias with a filter to show only certain documents from an index. This lets different users or applications see only the data they need through the same alias.

beginner

How do you add an alias to an index in Elasticsearch?

You use the Elasticsearch API with a command like POST /_aliases and specify the index and alias names. This creates or updates the alias to point to the index.

What does an Elasticsearch index alias do?

A Deletes an index permanently

B Points to one or more indexes as a shortcut

C Creates a new index with a different name

D Backs up an index automatically

An index alias acts as a shortcut name that points to one or more indexes.

Can an alias point to multiple indexes at once?

A Yes, it can point to many indexes

B No, only one index per alias

C Only if indexes have the same name

D Only if indexes are empty

Aliases can point to multiple indexes, allowing combined searches.

Why use an alias with a filter in Elasticsearch?

A To delete filtered documents

B To rename the index permanently

C To show only certain documents through the alias

D To speed up index creation

Filtered aliases let users see only specific documents via the alias.

How do you create or update an alias in Elasticsearch?

A By restarting Elasticsearch

B By renaming the index directly

C By deleting the old index

D Using the <code>POST /_aliases</code> API call

You use the POST /_aliases API to add or update aliases.

What is a key benefit of using index aliases?

A They allow changing indexes without changing application code

B They increase index size automatically

C They encrypt the index data

D They delete old indexes automatically

Aliases let you switch indexes behind the scenes without changing your app.

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

Think about how a nickname can replace a full name to make things easier.

Describe how you would create an alias with a filter and why you might do that.

Imagine giving different people access to only parts of a big photo album.

Practice

1.

What is the main purpose of using index aliases in Elasticsearch?

easy

A. To use simple names instead of real index names for easier access

B. To permanently rename an index

C. To delete multiple indexes at once

D. To backup indexes automatically

Solution

1. Step 1: Understand what index aliases do

   Index aliases provide a way to refer to one or more indexes using a simple name.

2. Step 2: Compare options with this purpose

   Only To use simple names instead of real index names for easier access describes using simple names instead of real index names, which matches the purpose of aliases.

3. Final Answer:

   To use simple names instead of real index names for easier access -> Option A

4. Quick Check:

   Index aliases simplify index names = D [OK]

Hint: Aliases are easy names for indexes, not permanent renames [OK]

Common Mistakes:

• Thinking aliases rename indexes permanently
• Confusing aliases with backup or delete functions
• Assuming aliases are only for single indexes

2.

Which of the following is the correct syntax to add an alias named logs_current to an index called logs_2024 using Elasticsearch API?

{
  "actions": [
    {
      "add": {
        "index": "logs_2024",
        "alias": "logs_current"
      }
    }
  ]
}

easy

A. PUT /logs_current { "index": "logs_2024" }

B. POST /_aliases { "actions": [{ "add": { "index": "logs_2024", "alias": "logs_current" } }] }

C. GET /logs_2024/_alias/logs_current

D. DELETE /_aliases { "actions": [{ "remove": { "index": "logs_2024", "alias": "logs_current" } }] }

Solution

1. Step 1: Identify the correct API endpoint and method

   Adding an alias uses POST on /_aliases with an actions array.

2. Step 2: Match the syntax with the options

   POST /_aliases { "actions": [{ "add": { "index": "logs_2024", "alias": "logs_current" } }] } correctly uses POST /_aliases with the add action and proper JSON structure.

3. Final Answer:

   POST /_aliases with add action JSON -> Option B

4. Quick Check:

   Adding alias uses POST /_aliases = A [OK]

Hint: Add aliases with POST /_aliases and actions array [OK]

Common Mistakes:

• Using PUT on alias name instead of POST on /_aliases
• Using GET to add an alias (GET is for reading)
• Confusing remove action with add

3.

Given these aliases and indexes:
logs_2023, logs_2024
Alias logs_all points to both indexes.
Which index will be searched when querying logs_all?

medium

A. Only logs_2023

B. Only logs_2024

C. No index, alias is invalid

D. Both logs_2023 and logs_2024

Solution

1. Step 1: Understand alias pointing to multiple indexes

   An alias can point to many indexes, so searching the alias searches all linked indexes.

2. Step 2: Apply to given alias logs_all

   Since logs_all points to both logs_2023 and logs_2024, queries to it search both indexes.

3. Final Answer:

   Both logs_2023 and logs_2024 -> Option D

4. Quick Check:

   Alias to multiple indexes searches all = C [OK]

Hint: Aliases can point to many indexes, searching all at once [OK]

Common Mistakes:

• Assuming alias searches only one index
• Thinking alias must be single index
• Believing alias is invalid if multiple indexes linked

4.

What is wrong with this alias update request?

{
  "actions": [
    {
      "add": {
        "alias": "current_logs",
        "index": "logs_2024"
      }
    },
    {
      "remove": {
        "alias": "old_logs",
        "index": "logs_2023"
      }
    }
  ]
}

Sent as POST to /_aliases.

medium

A. The request is correct and will work as expected

B. The JSON keys alias and index are swapped

C. The order of add and remove actions must be reversed

D. The HTTP method should be PUT, not POST

Solution

1. Step 1: Check JSON structure for alias actions

   The actions array correctly uses add and remove with keys alias and index in correct places.

2. Step 2: Verify HTTP method and endpoint

   POST to /_aliases is the correct way to update aliases.

3. Final Answer:

   The request is correct and will work as expected -> Option A

4. Quick Check:

   Correct alias update uses POST /_aliases with add/remove = A [OK]

Hint: POST /_aliases with add/remove actions is correct syntax [OK]

Common Mistakes:

• Thinking order of actions matters
• Swapping alias and index keys
• Using PUT instead of POST for alias updates

5.

You want to switch an alias active_logs from pointing to logs_2023 to logs_2024 without downtime. Which is the best approach?

hard

A. Remove alias from logs_2023 then add alias to logs_2024 in two separate requests

B. Delete logs_2023 index and then add alias to logs_2024

C. Use a single atomic alias update request with both remove and add actions

D. Create a new alias with a different name for logs_2024

Solution

1. Step 1: Understand alias switching without downtime

   Switching alias atomically avoids downtime by updating both remove and add in one request.

2. Step 2: Evaluate options for atomic update

   Use a single atomic alias update request with both remove and add actions uses a single request with remove and add actions, ensuring no downtime.

3. Final Answer:

   Use a single atomic alias update request with both remove and add actions -> Option C

4. Quick Check:

   Atomic alias update = remove + add in one request = B [OK]

Hint: Switch aliases atomically with one request to avoid downtime [OK]

Common Mistakes:

• Doing remove and add in separate requests causing downtime
• Deleting index instead of switching alias
• Creating new alias instead of switching existing one