Why Partial updates in Elasticsearch? - Purpose & Use Cases

The Big Idea

What if you could fix just one tiny detail without rewriting the whole story?

The Scenario

Imagine you have a huge document stored in Elasticsearch, like a detailed user profile with many fields. Now, you want to change just the user's phone number. Without partial updates, you'd have to fetch the entire document, change the phone number manually, and then send the whole document back to Elasticsearch.

The Problem

This manual way is slow because you transfer and rewrite the entire document even if only one small part changes. It's also risky because you might accidentally overwrite other fields or lose data if you miss something. Plus, it wastes bandwidth and processing power.

The Solution

Partial updates let you send only the changes you want to make. Elasticsearch applies these changes directly to the stored document without needing the full data. This makes updates faster, safer, and more efficient.

Before vs After

✗ Before

GET /index/_doc/1
// fetch full document
PUT /index/_doc/1
{ "name": "Alice", "phone": "12345", "email": "[e-mail]" }

✓ After

POST /index/_update/1
{ "doc": { "phone": "12345" } }

What It Enables

Partial updates enable quick, precise changes to documents without the overhead of resending or risking full document replacement.

Real Life Example

In a social media app, when a user changes their profile picture, partial updates let you update just the picture URL without touching the rest of their profile data.

Key Takeaways

Manual full-document updates are slow and risky.

Partial updates send only the changed fields.

This makes updates faster, safer, and more efficient.

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