Partial updates in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this partial update script?

Given the following Elasticsearch update request, what will be the value of the counter field after the update?

Elasticsearch

{
  "script": {
    "source": "ctx._source.counter += params.increment",
    "params": {"increment": 3}
  }
}

A The <code>counter</code> field will be set to 3 regardless of its previous value.

B The <code>counter</code> field will increase by 3 from its current value.

C The update will fail because <code>ctx._source.counter</code> is not defined.

D The <code>counter</code> field will be removed from the document.

💡 Hint

Think about how the script modifies the existing field value.

The script uses ctx._source.counter += params.increment which adds 3 to the existing counter value. It does not overwrite or remove it.

❓ Predict Output

intermediate

What happens if you use a partial update with a nested field?

Consider this partial update request to change a nested field address.city. What will be the result?

Elasticsearch

{
  "doc": {
    "address": {
      "city": "Newville"
    }
  }
}

A Only the <code>city</code> inside <code>address</code> is updated; other <code>address</code> fields remain unchanged.

B The entire <code>address</code> object is replaced with {"city": "Newville"}.

C The update will fail because nested fields cannot be updated partially.

D The <code>city</code> field is added at the root level, not inside <code>address</code>.

💡 Hint

Partial updates merge the provided fields into the existing document.

Partial updates merge the provided nested fields into the existing object, so only city changes while other address fields stay the same.

🔧 Debug

advanced

Why does this partial update script cause an error?

This update script is intended to add a tag to a list field tags. Why does it fail?

Elasticsearch

{
  "script": {
    "source": "ctx._source.tags.add(params.new_tag)",
    "params": {"new_tag": "urgent"}
  }
}

A The <code>add</code> method is not supported in Elasticsearch scripts.

B The script syntax is invalid because it lacks a return statement.

C The parameter <code>new_tag</code> is not passed correctly.

D The <code>tags</code> field is null or missing, so calling <code>add</code> causes a NullPointerException.

💡 Hint

Check if the tags field exists and is initialized as a list.

If tags is missing or null, calling add on it causes an error. You must ensure tags is a list before adding.

🧠 Conceptual

advanced

What is the effect of using doc_as_upsert in a partial update?

When you set doc_as_upsert to true in an update request, what happens if the document does not exist?

A The partial document is inserted as a new document.

B The update fails with a document not found error.

C The update request is ignored silently.

D A new empty document is created without the partial fields.

💡 Hint

Think about how upsert works in Elasticsearch.

Setting doc_as_upsert to true means if the document does not exist, Elasticsearch inserts the partial document as a new one.

❓ Predict Output

expert

What is the final document after this partial update with a script?

Given the document {"count": 5, "tags": ["old"]} and this update request, what is the final document?

Elasticsearch

{
  "script": {
    "source": "ctx._source.count = params.count; ctx._source.tags.add(params.tag)",
    "params": {"count": 10, "tag": "new"}
  }
}

A {"count": 5, "tags": ["old", "new"]}

B {"count": 10, "tags": ["new"]}

C {"count": 10, "tags": ["old", "new"]}

D The update fails with a NullPointerException.

💡 Hint

The script sets count and adds a tag to the existing list.

The script sets count to 10 and adds "new" to the existing tags list, resulting in {"count": 10, "tags": ["old", "new"]}.

Practice

1. What is the main purpose of using partial updates in Elasticsearch?

easy

A. To delete a document completely from the index

B. To create a new document with all fields specified

C. To update only specific fields of a document without replacing the whole document

D. To retrieve documents matching a query

Solution

1. Step 1: Understand partial update concept

   Partial updates allow changing only some fields of a document instead of replacing the entire document.

2. Step 2: Identify the main purpose

   This saves bandwidth and time by sending only the changed fields, not the whole document.

3. Final Answer:

   To update only specific fields of a document without replacing the whole document -> Option C

4. Quick Check:

   Partial update = update some fields only [OK]

Hint: Partial updates change only some fields, not whole document [OK]

Common Mistakes:

• Confusing partial update with document deletion
• Thinking partial update creates new documents
• Believing partial update retrieves documents

2. Which of the following is the correct syntax to perform a partial update on a document with ID '1' in index 'products'?

easy

A. POST /products/_doc/1 { "price": 20 }

B. POST /products/_update/1 { "doc": { "price": 20 } }

C. PUT /products/_update/1 { "price": 20 }

D. GET /products/_update/1 { "doc": { "price": 20 } }

Solution

1. Step 1: Identify correct HTTP method and endpoint

   Partial updates use POST method on the _update API with the document ID.

2. Step 2: Check request body format

   The update body must include a "doc" key with the fields to update.

3. Final Answer:

   POST /products/_update/1 { "doc": { "price": 20 } } -> Option B

4. Quick Check:

   Use POST + _update + doc key [OK]

Hint: Use POST with _update and doc key for partial updates [OK]

Common Mistakes:

• Using GET instead of POST for update
• Omitting the 'doc' key in request body
• Using PUT on _update endpoint incorrectly
• Updating with _doc endpoint instead of _update

3. Given this partial update request on document ID '2' in index 'users':

POST /users/_update/2
{
  "doc": {
    "age": 30
  }
}

If the original document was {"name": "Alice", "age": 25}, what will be the document after the update?

medium

A. {"name": "Alice", "age": 30}

B. {"age": 30}

C. {"name": "Alice"}

D. {"name": "Alice", "age": 25, "updated": true}

Solution

1. Step 1: Understand partial update effect

   The partial update changes only the "age" field to 30, keeping other fields unchanged.

2. Step 2: Apply update to original document

   Original document {"name": "Alice", "age": 25} becomes {"name": "Alice", "age": 30} after update.

3. Final Answer:

   {"name": "Alice", "age": 30} -> Option A

4. Quick Check:

   Partial update changes only specified fields [OK]

Hint: Partial update changes only given fields, others stay same [OK]

Common Mistakes:

• Thinking update replaces whole document
• Assuming fields not in doc are removed
• Adding extra fields not in update request

4. You try this partial update:

POST /orders/_update/5
{
  "doc": {
    "status": "shipped"
  }
}

But you get an error. Which is the most likely cause?

medium

A. The document with ID '5' does not exist in 'orders' index

B. You used GET instead of POST

C. You forgot to include the 'doc' key in the body

D. The 'status' field cannot be updated

Solution

1. Step 1: Check common update errors

   Partial update errors often happen if the document ID does not exist.

2. Step 2: Analyze given request and error cause

   The syntax is correct, so the likely cause is missing document with ID '5'.

3. Final Answer:

   The document with ID '5' does not exist in 'orders' index -> Option A

4. Quick Check:

   Missing document ID causes update error [OK]

Hint: Check if document ID exists before updating [OK]

Common Mistakes:

• Confusing HTTP method errors with missing document
• Assuming all fields are immutable
• Omitting 'doc' key causing syntax errors

5. You want to increase the 'stock' field by 10 in a product document with ID '10' using a partial update. Which request body correctly achieves this?

hard

A. { "update": { "stock": 10 } }

B. { "doc": { "stock": 10 } }

C. { "doc": { "stock": "stock + 10" } }

D. { "script": { "source": "ctx._source.stock += params.count", "params": { "count": 10 } } }

Solution

1. Step 1: Understand increment operation in partial update

   To increase a numeric field, use a script that adds the value to the existing field.

2. Step 2: Identify correct syntax for scripted update

   { "script": { "source": "ctx._source.stock += params.count", "params": { "count": 10 } } } uses the 'script' key with source code to add 10 to 'stock'.

3. Final Answer:

   { "script": { "source": "ctx._source.stock += params.count", "params": { "count": 10 } } } -> Option D

4. Quick Check:

   Use script to increment numeric fields [OK]

Hint: Use script with ctx._source to increment fields [OK]

Common Mistakes:

• Trying to update with doc key using expression string
• Using 'update' key instead of 'doc' or 'script'
• Assigning value directly instead of incrementing