ETag for conditional requests in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an ETag in the context of REST APIs?

An ETag (Entity Tag) is a unique identifier assigned by a server to a specific version of a resource. It helps clients know if the resource has changed since the last time they accessed it.

beginner

How does a client use an ETag for conditional requests?

The client sends the ETag value in the 'If-None-Match' header when requesting a resource. The server compares it with the current ETag and returns the resource only if it has changed; otherwise, it returns a 304 Not Modified status.

beginner

What HTTP status code indicates that the resource has not changed when using ETag?

The server returns HTTP status code 304 Not Modified to tell the client that the resource has not changed and the cached version can be used.

intermediate

Why are ETags useful for web performance?

ETags reduce bandwidth and improve performance by allowing clients to avoid downloading unchanged resources. This saves time and network data by using cached versions when possible.

intermediate

What is the difference between a strong and a weak ETag?

A strong ETag changes whenever the resource changes in any way. A weak ETag may not change for minor differences and is prefixed with 'W/'. Weak ETags allow some flexibility in caching but may not detect all changes.

What header does a client send to use an ETag for conditional GET requests?

A If-None-Match

B If-Match

C ETag

D Cache-Control

The client sends the ETag value in the 'If-None-Match' header to ask the server if the resource has changed.

What does the server return if the resource has not changed when using ETag?

A 304 Not Modified

B 404 Not Found

C 200 OK with resource

D 500 Internal Server Error

The server returns 304 Not Modified to indicate the cached resource is still valid.

Which of the following best describes a weak ETag?

A An ETag that is always a hash of the resource content

B An ETag that changes with every minor change

C An ETag used only for POST requests

D An ETag prefixed with 'W/' that may not change for minor differences

Weak ETags are prefixed with 'W/' and allow some flexibility by not changing for minor differences.

Why are ETags important for caching?

A They force the client to always download the resource

B They help the client know if the cached resource is still valid

C They encrypt the resource data

D They replace the URL of the resource

ETags help clients check if their cached copy is still valid, avoiding unnecessary downloads.

Which HTTP method commonly uses ETags for conditional requests?

A PUT

B DELETE

C GET

D OPTIONS

ETags are most commonly used with GET requests to check if the resource has changed.

Explain how ETags help improve web application performance.

Think about how caching works with version checks.

Describe the difference between strong and weak ETags and when you might use each.

Consider strictness of change detection.

Practice

1.

What is the main purpose of an ETag in REST APIs?

easy

A. To help clients check if the resource has changed since last fetch

B. To encrypt the data sent from server to client

C. To authenticate the client making the request

D. To specify the content type of the response

Solution

1. Step 1: Understand ETag role in REST

   ETags are unique identifiers for resource versions, used to detect changes.

2. Step 2: Match purpose with options

   Only To help clients check if the resource has changed since last fetch describes checking if resource changed, which is the ETag's purpose.

3. Final Answer:

   To help clients check if the resource has changed since last fetch -> Option A

4. Quick Check:

   ETag purpose = freshness check [OK]

Hint: ETags track changes, not encrypt or authenticate [OK]

Common Mistakes:

• Confusing ETag with authentication tokens
• Thinking ETag encrypts data
• Assuming ETag sets content type

2.

Which HTTP header should a client send to make a conditional GET request using an ETag?

easy

A. If-Modified-Since

B. If-None-Match

C. ETag

D. Cache-Control

Solution

1. Step 1: Identify client header for ETag conditional requests

   The client sends If-None-Match header with the ETag value to check freshness.

2. Step 2: Compare options

   If-Modified-Since uses dates, ETag is a response header, Cache-Control controls caching rules. Only If-None-Match fits.

3. Final Answer:

   If-None-Match -> Option B

4. Quick Check:

   Client uses If-None-Match for ETag check [OK]

Hint: Client sends If-None-Match with ETag to check freshness [OK]

Common Mistakes:

• Using ETag header in request instead of response
• Confusing If-Modified-Since with ETag usage
• Sending Cache-Control instead of conditional header

3.

Given this HTTP interaction:
GET /resource HTTP/1.1
Host: example.com
If-None-Match: "abc123"
Server responds:
HTTP/1.1 304 Not Modified
What does this response mean?

medium

A. The resource has changed and new data is sent

B. The server encountered an error

C. The resource has not changed; client should use cached data

D. The request is unauthorized

Solution

1. Step 1: Understand 304 Not Modified status

   304 means resource is unchanged since client's ETag, so no new data sent.

2. Step 2: Match meaning with options

   The resource has not changed; client should use cached data correctly states client should use cached data because resource is unchanged.

3. Final Answer:

   The resource has not changed; client should use cached data -> Option C

4. Quick Check:

   304 = Not Modified, use cache [OK]

Hint: 304 means no change; use cached copy [OK]

Common Mistakes:

• Thinking 304 means new data sent
• Confusing 304 with error or unauthorized status
• Ignoring ETag and caching role

4.

Consider this server code snippet handling ETags:

etag = generate_etag(resource)
if request.headers.get('If-None-Match') == etag:
    return Response(status=304)
else:
    return Response(data=resource, headers={'ETag': etag})

What is a potential bug in this code?

medium

A. It compares ETag strings without quotes, causing mismatch

B. It returns 304 even if ETag does not match

C. It does not handle If-Modified-Since header

D. It always returns data ignoring conditional requests

Solution

1. Step 1: Analyze ETag comparison

   ETags in headers often include quotes, so direct string equality may fail if quotes differ.

2. Step 2: Identify mismatch cause

   If client sends "abc123" but server etag is abc123 without quotes, comparison fails incorrectly.

3. Final Answer:

   It compares ETag strings without quotes, causing mismatch -> Option A

4. Quick Check:

   ETag quotes matter in comparison [OK]

Hint: ETag strings must match exactly including quotes [OK]

Common Mistakes:

• Ignoring quotes around ETag values
• Assuming If-Modified-Since is required for ETag logic
• Returning 304 without checking ETag

5.

You want to optimize your REST API to reduce bandwidth by using ETags. Which approach correctly implements conditional GET requests with ETags?

1. Server generates ETag for resource.
2. Server sends ETag in response header.
3. Client stores ETag.
4. Client sends GET with If-None-Match header.
5. Server compares ETag and returns 304 if unchanged.
6. Otherwise, server sends new data with new ETag.

Which step is missing or incorrect?

hard

A. Step 6: Server should always send 200 status even if unchanged

B. Step 4: Client should send If-Modified-Since instead of If-None-Match

C. Step 3: Client should not store ETag

D. Step 5: Server must compare ETag including quotes exactly

Solution

1. Step 1: Review each step for correctness

   Steps 1-4 are correct standard flow for ETag conditional requests.

2. Step 2: Identify subtle issue in step 5

   Server must compare ETag values exactly, including quotes, or else conditional check fails.

3. Step 3: Confirm other options are incorrect

   Client should store ETag (step 3), If-None-Match is correct header (step 4), and 304 status is correct for unchanged (step 6).

4. Final Answer:

   Step 5: Server must compare ETag including quotes exactly -> Option D

5. Quick Check:

   Exact ETag match including quotes is required [OK]

Hint: ETag comparison must be exact including quotes [OK]

Common Mistakes:

• Ignoring quotes in ETag comparison
• Using wrong conditional header
• Sending 200 instead of 304 for unchanged resource