Reindexing data in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this reindexing task response?

Given the following Elasticsearch reindex API request, what will be the value of response['created'] after the task completes successfully?

Elasticsearch

{
  "source": {"index": "old_index"},
  "dest": {"index": "new_index"}
}

// Assume old_index has 100 documents and no errors occur.

A 0

B An error is thrown

C 100

D null

💡 Hint

Reindexing copies documents from source to destination. The created field shows how many documents were added.

The created field in the response indicates how many documents were successfully reindexed. Since the source index has 100 documents and no errors occur, created will be 100.

🧠 Conceptual

intermediate

Which option correctly describes the purpose of the 'wait_for_completion' parameter in Elasticsearch reindex API?

What does setting wait_for_completion to false do when running a reindex task?

A Waits for the reindex task to finish before returning the response.

B Runs the reindex task asynchronously and returns a task ID immediately.

C Cancels the reindex task immediately.

D Runs the reindex task only if the destination index exists.

💡 Hint

Think about how asynchronous tasks work in Elasticsearch.

Setting wait_for_completion to false makes the reindex API return immediately with a task ID, allowing the task to run in the background.

🔧 Debug

advanced

Why does this reindex request fail with a 'version_conflict_engine_exception' error?

Consider this reindex request:

{
  "source": {"index": "source_index"},
  "dest": {"index": "dest_index", "op_type": "create"}
}

Both indices have documents with overlapping IDs. What causes the error?

A Because <code>op_type: create</code> forbids overwriting existing documents, causing conflicts on duplicate IDs.

B Because the source index is empty.

C Because the destination index does not exist.

D Because the reindex API does not support <code>op_type</code> parameter.

💡 Hint

Check what op_type: create means in Elasticsearch indexing.

The op_type: create option tells Elasticsearch to only create documents if they don't exist. If a document with the same ID exists in the destination, a version conflict error occurs.

📝 Syntax

advanced

Which option is the correct syntax to reindex documents with a script that adds a new field 'status' with value 'archived'?

Choose the valid reindex request that adds a new field status with value archived to each document.

A

{
  "source": {"index": "old_index"},
  "dest": {"index": "new_index"},
  "script": {
    "source": "ctx._source.status = 'archived'"
  }
}

B

{
  "source": {"index": "old_index"},
  "dest": {"index": "new_index"},
  "script": "ctx._source.status = 'archived'"
}

C

{
  "source": {"index": "old_index"},
  "dest": {"index": "new_index"},
  "script": {
    "inline": "ctx._source.status = 'archived'"
  }
}

D

{
  "source": {"index": "old_index"},
  "dest": {"index": "new_index"},
  "script": {
    "code": "ctx._source.status = 'archived'"
  }
}

💡 Hint

Check the correct key for the script source in Elasticsearch reindex API.

The correct key for the script source in recent Elasticsearch versions is source. Using inline or code is deprecated or invalid in this context.

🚀 Application

expert

How to reindex only documents where 'status' is 'active' and update 'last_updated' field to current date?

You want to reindex documents from users index to users_archive index. Only documents with status equal to active should be copied. Also, add or update a field last_updated with the current date during reindexing.

Which request accomplishes this?

A

{
  "source": {
    "index": "users",
    "query": { "term": { "status": "active" } }
  },
  "dest": {"index": "users_archive"},
  "script": {
    "source": "ctx._source.last_updated = params.now",
    "params": {"now": "{{ctx.execution_time}}"}
  }
}

B

{
  "source": {
    "index": "users",
    "query": { "match": { "status": "active" } }
  },
  "dest": {"index": "users_archive"},
  "script": {
    "source": "ctx._source.last_updated = new Date()"
  }
}

C

{
  "source": {
    "index": "users",
    "query": { "term": { "status": "active" } }
  },
  "dest": {"index": "users_archive"},
  "script": {
    "source": "ctx._source.last_updated = params.now",
    "params": {"now": "2024-01-01T00:00:00Z"}
  }
}

D

{
  "source": {
    "index": "users",
    "query": { "term": { "status": "active" } }
  },
  "dest": {"index": "users_archive"},
  "script": {
    "source": "ctx._source.last_updated = Instant.now()"
  }
}

💡 Hint

Painless scripts support Instant.now() for current time. Avoid JavaScript-like syntax or invalid placeholders.

Option D correctly filters documents with a term query and sets last_updated to the current time using Instant.now(), which is supported in Painless scripts. Option D uses an invalid placeholder {{ctx.execution_time}}. Option D uses unsupported new Date() and a less precise match query. Option D uses a fixed date instead of the current date.

Practice

1.

What is the main purpose of _reindex API in Elasticsearch?

easy

A. To delete an index permanently

B. To copy data from one index to another

C. To update the Elasticsearch cluster settings

D. To create a new index with default settings

Solution

1. Step 1: Understand the function of _reindex

   The _reindex API is designed to copy documents from a source index to a destination index.

2. Step 2: Differentiate from other operations

   Deleting an index, updating cluster settings, or creating new indexes are different APIs and not the purpose of _reindex.

3. Final Answer:

   To copy data from one index to another -> Option B

4. Quick Check:

   _reindex copies data [OK]

Hint: Remember: reindex means copying data between indexes [OK]

Common Mistakes:

• Confusing reindex with deleting indexes
• Thinking reindex updates cluster settings
• Assuming reindex creates new indexes without data

2.

Which of the following is the correct minimal syntax to reindex data from old_index to new_index using Elasticsearch's REST API?

{
  "source": {"index": "old_index"},
  "dest": {"index": "new_index"}
}

easy

A. { "source": { "index": "old_index" }, "dest": { "index": "new_index" } }

B. { "source": { "index": "new_index" }, "dest": { "index": "old_index" } }

C. { "source": { "index": "old_index" }, "destination": { "index": "new_index" } }

D. { "src": { "index": "old_index" }, "dest": { "index": "new_index" } }

Solution

1. Step 1: Check the required keys for reindex

   The correct keys are source and dest with nested index names.

2. Step 2: Validate the index names and key spelling

   { "source": { "index": "old_index" }, "dest": { "index": "new_index" } } uses correct keys and index names. { "source": { "index": "new_index" }, "dest": { "index": "old_index" } } swaps source and destination incorrectly. { "source": { "index": "old_index" }, "destination": { "index": "new_index" } } uses wrong key destination. { "src": { "index": "old_index" }, "dest": { "index": "new_index" } } uses wrong key src.

3. Final Answer:

   { "source": { "index": "old_index" }, "dest": { "index": "new_index" } } -> Option A

4. Quick Check:

   Use source and dest keys exactly [OK]

Hint: Use exact keys: source and dest for reindex JSON [OK]

Common Mistakes:

• Swapping source and destination indexes
• Using wrong keys like 'src' or 'destination'
• Misspelling 'dest' or 'source'

3.

Given the following reindex request, what will be the result?

{
  "source": { "index": "products" },
  "dest": { "index": "products_new" },
  "script": {
    "source": "ctx._source.price = ctx._source.price * 1.1"
  }
}

Assuming products has documents with a price field.

medium

A. Only documents with price > 1.1 are copied

B. Documents copied without any price change

C. Reindex fails due to script syntax error

D. All documents copied with price increased by 10%

Solution

1. Step 1: Understand the script in reindex

   The script modifies each document's price by multiplying it by 1.1 during reindexing.

2. Step 2: Effect on documents

   All documents from products are copied to products_new with updated price values increased by 10%.

3. Final Answer:

   All documents copied with price increased by 10% -> Option D

4. Quick Check:

   Script modifies price during reindex [OK]

Hint: Scripts can modify fields during reindexing [OK]

Common Mistakes:

• Assuming script filters documents instead of modifying
• Thinking script causes syntax error
• Believing price remains unchanged

4.

Identify the error in this reindex request:

{
  "source": { "index": "old_index" },
  "dest": { "index": "new_index" },
  "query": {
    "match": { "status": "active" }
  }
}

medium

A. The query should be inside the "source" object

B. "query" is not allowed inside reindex requests

C. The "match" syntax is incorrect

D. The "dest" index name cannot be "new_index"

Solution

1. Step 1: Check placement of query in reindex

   The query must be inside the source object to filter documents.

2. Step 2: Validate other parts

   The match syntax is correct, and dest index name is valid.

3. Final Answer:

   The query should be inside the "source" object -> Option A

4. Quick Check:

   Put query inside source to filter during reindex [OK]

Hint: Put filters inside source, not at top level [OK]

Common Mistakes:

• Placing query outside source object
• Misunderstanding match query syntax
• Thinking destination index name is invalid

5.

You want to reindex data from logs_2023 to logs_archive but only include documents where level is "error" and add a new field archived set to true. Which reindex request achieves this?

hard

A. { "source": { "index": "logs_2023", "query": { "match": { "level": "error" } } }, "dest": { "index": "logs_archive" }, "script": { "source": "ctx._source.archived = false" } }

B. { "source": { "index": "logs_2023" }, "dest": { "index": "logs_archive" }, "query": { "term": { "level": "error" } }, "script": { "source": "ctx._source.archived = true" } }

C. { "source": { "index": "logs_2023", "query": { "term": { "level": "error" } } }, "dest": { "index": "logs_archive" }, "script": { "source": "ctx._source.archived = true" } }

D. { "source": { "index": "logs_2023", "query": { "term": { "level": "error" } } }, "dest": { "index": "logs_archive" }, "script": { "source": "ctx._source.archived = false" } }

Solution

1. Step 1: Filter documents by level "error" inside source

   The query with term filter must be inside the source object to select only error level logs.

2. Step 2: Add new field "archived" with value true via script

   The script sets ctx._source.archived = true to add the new field during reindex.

3. Step 3: Validate requests

   The request that places query inside source with term filter for level: "error" and script setting ctx._source.archived = true is correct. Placing query at top level is invalid. Using match instead of term or setting archived to false is incorrect.

4. Final Answer:

   { "source": { "index": "logs_2023", "query": { "term": { "level": "error" } } }, "dest": { "index": "logs_archive" }, "script": { "source": "ctx._source.archived = true" } } -> Option C

5. Quick Check:

   Filter in source + script sets field true [OK]

Hint: Filter inside source; script adds fields during reindex [OK]

Common Mistakes:

• Placing query outside source object
• Setting wrong value for new field
• Using match instead of term for exact filter