Validation-based caching in Rest API - Step-by-Step Execution

Concept Flow - Validation-based caching

Client sends request

↓

Check cache for response

↓

Has validation info?

↓

Send validation request to server

↓

Fetch fresh data

↓

Return cached response

↓

Client receives response

The client first checks if a cached response exists. If yes, it validates with the server if the cached data is still fresh. Depending on validation, it either returns cached data or fetches fresh data.

Execution Sample

Rest API

GET /resource
Check cache for ETag
If ETag exists:
  Send If-None-Match header
If 304 Not Modified:
  Return cached data
Else:
  Update cache and return new data

This code shows how a client uses ETag validation to decide whether to use cached data or fetch fresh data.

Execution Table

Step	Action	Cache State	Request Sent	Server Response	Client Action
1	Client sends GET /resource	Empty	GET /resource	200 OK with ETag: "abc123"	Cache response with ETag "abc123" and data
2	Client sends GET /resource again	Has cached data with ETag "abc123"	GET /resource with If-None-Match: "abc123"	304 Not Modified	Return cached data without fetching new data
3	Client sends GET /resource again	Has cached data with ETag "abc123"	GET /resource with If-None-Match: "abc123"	200 OK with ETag: "def456"	Update cache with new ETag "def456" and new data, return new data
4	Client sends GET /resource again	Has cached data with ETag "def456"	GET /resource with If-None-Match: "def456"	304 Not Modified	Return cached data
5	Client sends GET /resource again	Has cached data with ETag "def456"	GET /resource with If-None-Match: "def456"	404 Not Found	Remove cache, return 404 error
6	Client sends GET /resource again	Cache empty	GET /resource	200 OK with ETag: "xyz789"	Cache new data with ETag "xyz789", return data

💡 Execution stops when client receives final response or error; cache updates accordingly.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
Cache Data	Empty	Data with ETag "abc123"	Data with ETag "abc123"	Data with ETag "def456"	Data with ETag "def456"	Empty	Data with ETag "xyz789"
ETag	None	"abc123"	"abc123"	"def456"	"def456"	None	"xyz789"
Last Server Response	None	200 OK	304 Not Modified	200 OK	304 Not Modified	404 Not Found	200 OK

Key Moments - 3 Insights

Why does the client send the If-None-Match header with the cached ETag?

The client sends If-None-Match with the cached ETag to ask the server if the cached data is still valid. If the server replies 304 Not Modified (see execution_table step 2), the client knows it can safely use the cached data without downloading it again.

What happens when the server responds with 200 OK and a new ETag?

When the server sends 200 OK with a new ETag (execution_table step 3), it means the cached data is outdated. The client updates its cache with the new data and ETag, then returns the fresh data to the user.

Why is the cache emptied when the server returns 404 Not Found?

A 404 means the resource no longer exists. The client removes the cached data (execution_table step 5) because it is no longer valid and returns the error to the user.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What does the client do after receiving 304 Not Modified?

A Clear the cache

B Fetch fresh data from server

C Return cached data without fetching new data

D Send another request without If-None-Match

💡 Hint

Check the 'Client Action' column at step 2 in execution_table.

At which step does the client update the cache with a new ETag?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look for 'Update cache' in the 'Client Action' column in execution_table.

If the server never returned 304 Not Modified, how would the cache state change?

A Cache would never update

B Cache would always update with new data

C Cache would be cleared every time

D Cache would remain empty

💡 Hint

Refer to variable_tracker and execution_table steps where 304 Not Modified is missing.

Concept Snapshot

Validation-based caching uses server validation (like ETag) to check if cached data is fresh.
Client sends cached ETag in If-None-Match header.
Server replies 304 Not Modified if data is fresh, else 200 OK with new data.
Client updates cache only when data changes.
This saves bandwidth and speeds up responses.

Full Transcript

Validation-based caching helps clients avoid downloading data again if it hasn't changed. The client stores data with a validation token called ETag. When requesting the same resource, the client sends the ETag to the server using the If-None-Match header. The server checks if the data changed. If not, it replies 304 Not Modified, so the client uses cached data. If changed, the server sends new data with a new ETag, and the client updates its cache. If the resource is gone, the server sends 404, and the client clears the cache. This process saves time and bandwidth by reusing valid cached data.

Practice

1. What is the main purpose of validation-based caching in REST APIs?

easy

A. To encrypt data during transmission

B. To speed up the server response by skipping validation

C. To store data permanently on the client side

D. To check if the data has changed before downloading it again

Solution

1. Step 1: Understand validation-based caching concept

   It is used to avoid downloading data again if it has not changed.

2. Step 2: Identify the main purpose

   The main goal is to check if data changed before downloading again to save bandwidth and time.

3. Final Answer:

   To check if the data has changed before downloading it again -> Option D

4. Quick Check:

   Validation-based caching = check data change before download [OK]

Hint: Remember: caching avoids re-downloading unchanged data [OK]

Common Mistakes:

• Thinking caching stores data permanently
• Confusing caching with encryption
• Assuming caching skips validation

2. Which HTTP header does the client send to ask the server if the cached data is still valid?

easy

A. If-Modified-Since

B. ETag

C. If-None-Match

D. Cache-Control

Solution

1. Step 1: Identify client headers for validation

   The client uses headers to tell the server what version it has.

2. Step 2: Match header for ETag validation

   The If-None-Match header is sent with the ETag value to check if data changed.

3. Final Answer:

   If-None-Match -> Option C

4. Quick Check:

   Client asks with If-None-Match header [OK]

Hint: Client sends If-None-Match with ETag to validate cache [OK]

Common Mistakes:

• Confusing If-Modified-Since with If-None-Match
• Using Cache-Control header for validation
• Thinking ETag is sent by client

3. Given this server response to a client request with If-None-Match: "abc123" header:

HTTP/1.1 304 Not Modified
ETag: "abc123"

What does this response mean?

medium

A. The data has changed and new data is sent

B. The data has not changed; client should use cached data

C. The server does not support caching

D. The client must resend the request without ETag

Solution

1. Step 1: Understand HTTP 304 status meaning

   304 means Not Modified, so data is unchanged since last fetch.

2. Step 2: Interpret ETag match

   ETag matches client's value, so server confirms data is still valid.

3. Final Answer:

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

4. Quick Check:

   304 Not Modified = use cached data [OK]

Hint: 304 status means no new data, use cache [OK]

Common Mistakes:

• Thinking 304 means new data sent
• Confusing 304 with 200 OK
• Assuming server rejects caching

4. A client sends a request with header If-None-Match: "xyz789". The server responds with:

HTTP/1.1 200 OK
ETag: "abc123"
{ "data": "new content" }

What is the likely problem in this interaction?

medium

A. The server sent a different ETag, indicating data changed, so 200 OK is correct

B. The server should respond with 304 Not Modified instead of 200 OK

C. The client sent an invalid ETag header

D. The server response is missing the If-None-Match header

Solution

1. Step 1: Compare client and server ETags

   Client sent "xyz789", server responded with "abc123" which is different.

2. Step 2: Understand server response code

   Since ETags differ, data changed, so server correctly sends 200 OK with new data.

3. Final Answer:

   The server sent a different ETag, indicating data changed, so 200 OK is correct -> Option A

4. Quick Check:

   Different ETag means data changed, send 200 OK [OK]

Hint: Different ETag means new data, so 200 OK is correct [OK]

Common Mistakes:

• Expecting 304 when ETags differ
• Thinking client ETag must match server's
• Confusing If-None-Match with If-Match

5. You want to implement validation-based caching in your REST API. Which sequence correctly describes the interaction between client and server?

hard

A. Client sends request with If-None-Match header; server replies 304 if data unchanged or 200 with new data and ETag if changed

B. Server sends ETag in request; client replies 304 if data unchanged or 200 with new data if changed

C. Client sends ETag in response; server sends If-None-Match header to validate cache

D. Server always sends 200 OK with data and never uses ETag

Solution

1. Step 1: Identify correct client-server header roles

   Client sends If-None-Match with ETag to ask if data changed.

2. Step 2: Understand server response logic

   Server replies 304 if data unchanged, or 200 with new data and new ETag if changed.

3. Final Answer:

   Client sends request with If-None-Match header; server replies 304 if data unchanged or 200 with new data and ETag if changed -> Option A

4. Quick Check:

   Client sends If-None-Match; server replies 304 or 200 [OK]

Hint: Client sends If-None-Match; server replies 304 or 200 with ETag [OK]

Common Mistakes:

• Mixing up who sends ETag and If-None-Match
• Thinking server sends If-None-Match header
• Assuming server never sends 304