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.