Retrieving a document by ID in Elasticsearch - Step-by-Step Execution

Concept Flow - Retrieving a document by ID

Start: Receive document ID

↓

Send GET request to Elasticsearch

↓

Elasticsearch searches index for ID

↓

Document found?

No→Return 'not found' response

Yes↓

Return document data

↓

End

The process starts with a document ID, sends a GET request to Elasticsearch, which searches the index. If found, it returns the document; otherwise, it returns a not found response.

Execution Sample

Elasticsearch

GET /my_index/_doc/1

This request asks Elasticsearch to get the document with ID '1' from the index 'my_index'.

Execution Table

Step	Action	Request Sent	Response	Result
1	Receive document ID	ID = '1'	N/A	Ready to send request
2	Send GET request	GET /my_index/_doc/1	Waiting for response	Request sent to Elasticsearch
3	Elasticsearch searches index	N/A	Searching for ID '1'	Searching document
4	Check if document exists	N/A	Document found	Proceed to return document
5	Return document data	N/A	{"_index":"my_index","_id":"1","_source":{"field":"value"}}	Document data returned
6	End	N/A	Response complete	Process finished

💡 Document with ID '1' found and returned successfully

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
document_id	N/A	'1'	'1'	'1'	'1'	'1'	'1'
request	N/A	N/A	GET /my_index/_doc/1	GET /my_index/_doc/1	GET /my_index/_doc/1	GET /my_index/_doc/1	GET /my_index/_doc/1
response	N/A	N/A	Waiting	Searching	Found document	{"_index":"my_index","_id":"1","_source":{"field":"value"}}	Returned

Key Moments - 3 Insights

What happens if the document ID does not exist in the index?

If the document is not found, Elasticsearch returns a response indicating 'not found'. This is shown in the execution_table at step 4 where the check fails and the process returns a not found response instead of document data.

Why do we send a GET request with the document ID in the URL?

The GET request URL includes the index and document ID to tell Elasticsearch exactly which document to retrieve. This direct path is efficient and is shown in the execution_table at step 2.

Is the document data returned immediately after sending the request?

No, the document data is returned only after Elasticsearch searches and finds the document. This delay is shown between steps 2 and 5 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the response at step 4?

A Waiting for response

B Document found

C Request sent to Elasticsearch

D Response complete

💡 Hint

Check the 'Response' column in execution_table row for step 4

At which step does Elasticsearch actually search for the document?

A Step 3

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Action' column in execution_table to find when searching happens

If the document ID was '2' instead of '1', which variable would change in variable_tracker?

A response

B request

C document_id

D All variables remain the same

💡 Hint

Check the 'document_id' row in variable_tracker for the value that holds the ID

Concept Snapshot

Syntax: GET /index/_doc/document_id
Behavior: Sends a GET request to retrieve a document by its ID from the specified index.
Key rule: If the document exists, Elasticsearch returns its data; if not, it returns a not found response.
Use the document ID in the URL path to specify which document to get.

Full Transcript

This visual trace shows how retrieving a document by ID works in Elasticsearch. First, the document ID is received. Then a GET request is sent to Elasticsearch with the index and document ID in the URL. Elasticsearch searches the index for the document. If found, it returns the document data. If not found, it returns a not found response. Variables like document_id, request, and response change step by step. Key moments include understanding what happens if the document is missing, why the ID is in the URL, and when the document data is returned.

Practice

1. What is the fastest way to retrieve a single document in Elasticsearch?

easy

A. Retrieve the document by its ID using the GET API

B. Search all documents with a match_all query

C. Use a scroll API to iterate over documents

D. Run an aggregation query on the index

Solution

1. Step 1: Understand retrieval methods

   Retrieving by ID directly targets one document, making it the fastest method.

2. Step 2: Compare with other methods

   Search queries and scroll APIs scan multiple documents, which is slower than direct ID retrieval.

3. Final Answer:

   Retrieve the document by its ID using the GET API -> Option A

4. Quick Check:

   Fastest retrieval = GET by ID [OK]

Hint: Use GET API with document ID for fastest retrieval [OK]

Common Mistakes:

• Thinking search queries are faster than direct ID retrieval
• Using scroll API for single document retrieval
• Confusing aggregation with document retrieval

2. Which of the following is the correct syntax to retrieve a document by ID in Elasticsearch?

easy

A. GET /index/_doc/123

B. GET /index/_search {"query": {"term": {"_id": "123"}}}

C. POST /index/_doc/123/_get

D. GET /index/_search/123

Solution

1. Step 1: Recall the GET by ID syntax

   The correct syntax uses GET with the index and document ID: GET /index/_doc/123

2. Step 2: Identify incorrect options

   GET /index/_search {"query": {"term": {"_id": "123"}}} uses a search query, not direct GET by ID. POST /index/_doc/123/_get uses POST and an invalid path. GET /index/_search/123 has an invalid URL structure.

3. Final Answer:

   GET /index/_doc/123 -> Option A

4. Quick Check:

   Correct syntax = GET /index/_doc/ID [OK]

Hint: Use GET /index/_doc/{id} to retrieve document by ID [OK]

Common Mistakes:

• Using search API instead of GET by ID
• Wrong HTTP method like POST
• Incorrect URL path structure

3. Given the following request, what will Elasticsearch return if the document with ID 'abc123' exists?

GET /products/_doc/abc123

medium

A. HTTP 404 Not Found

B. {"_index":"products","_id":"abc123","found":false}

C. {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}}

D. {"error":"Document not found"}

Solution

1. Step 1: Understand the GET by ID response

   If the document exists, Elasticsearch returns found: true with the document source.

2. Step 2: Match the response format

   {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}} shows found: true and the document data, which matches expected output.

3. Final Answer:

   {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}} -> Option C

4. Quick Check:

   Document exists = found: true with _source [OK]

Hint: Found true means document exists with _source data [OK]

Common Mistakes:

• Expecting HTTP 404 instead of found: false
• Confusing error message with standard response
• Ignoring the _source field in response

4. You run this request to get a document by ID but get an error:

GET /store/_doc

What is the problem and how to fix it?

medium

A. The _doc endpoint does not support GET requests

B. Wrong HTTP method; use POST instead of GET

C. Index name is incorrect; change 'store' to '_store'

D. Missing document ID in URL; add the ID after _doc

Solution

1. Step 1: Identify the request format

   The GET by ID request requires the document ID after _doc in the URL.

2. Step 2: Fix the URL

   Add the document ID after _doc, e.g., GET /store/_doc/123

3. Final Answer:

   Missing document ID in URL; add the ID after _doc -> Option D

4. Quick Check:

   GET by ID needs document ID in URL [OK]

Hint: Always include document ID after _doc in GET URL [OK]

Common Mistakes:

• Omitting document ID in GET request URL
• Using wrong HTTP method
• Misnaming the index with underscore

5. You want to retrieve a document by ID but also handle the case when the document is missing without causing an error. Which approach is best?

hard

A. Use a search query with term _id and expect zero hits if missing

B. Use GET /index/_doc/id and check if 'found' is true in the response

C. Use GET /index/_doc/id and catch HTTP 404 error to handle missing

D. Use scroll API to find the document and handle empty results

Solution

1. Step 1: Understand GET by ID response behavior

   GET by ID returns a JSON with 'found': false if the document is missing, no error thrown.

2. Step 2: Compare with other methods

   Search queries return hits but are slower; catching HTTP 404 is unnecessary; scroll API is for multiple docs.

3. Final Answer:

   Use GET /index/_doc/id and check if 'found' is true in the response -> Option B

4. Quick Check:

   Check 'found' field to detect missing document [OK]

Hint: Check 'found' in GET response to detect missing docs safely [OK]

Common Mistakes:

• Expecting HTTP 404 error for missing documents
• Using search queries instead of GET by ID
• Using scroll API for single document retrieval