Response headers (Cache-Control, ETag) in Rest API - Step-by-Step Execution

Concept Flow - Response headers (Cache-Control, ETag)

Client sends request

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Server processes request

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Server adds Cache-Control header

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Server adds ETag header

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Server sends response with headers

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Client caches response based on Cache-Control

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Client sends conditional request with If-None-Match

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Server compares ETag

Match No↓

Send 304

The client requests data; the server adds Cache-Control and ETag headers to control caching. The client uses ETag to check if data changed, reducing data transfer.

Execution Sample

Rest API

GET /data HTTP/1.1
Host: example.com

HTTP/1.1 200 OK
Cache-Control: max-age=60
ETag: "abc123"

{ "value": 42 }

Client requests data; server responds with Cache-Control and ETag headers and JSON data.

Execution Table

Step	Action	Header Added	Header Value	Client Behavior
1	Client sends GET request	None	None	Request sent to server
2	Server processes request	None	None	Preparing response
3	Server adds Cache-Control	Cache-Control	max-age=60	Client can cache for 60 seconds
4	Server adds ETag	ETag	"abc123"	Client stores ETag for validation
5	Server sends response	Cache-Control, ETag	max-age=60, "abc123"	Client receives data and headers
6	Client caches response	None	None	Stores data and ETag
7	Client sends conditional GET with If-None-Match	If-None-Match	"abc123"	Asks if data changed
8	Server compares ETag	None	None	Checks if ETag matches
9	ETag matches	None	None	Server sends 304 Not Modified
10	Client uses cached data	None	None	No new data downloaded
11	After max-age expires, client sends new GET	None	None	Starts cycle again

💡 Client stops sending conditional requests when cache expires or data changes.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 6	After Step 7	After Step 9
Cache-Control	None	max-age=60	max-age=60	max-age=60	max-age=60	max-age=60
ETag	None	None	"abc123"	"abc123"	"abc123"	"abc123"
Client Cache	Empty	Empty	Empty	Cached data + ETag	Cached data + ETag	Cached data + ETag
Client Request Header	None	None	None	None	If-None-Match: "abc123"	If-None-Match: "abc123"

Key Moments - 3 Insights

Why does the client send the If-None-Match header?

The client sends If-None-Match with the stored ETag to ask the server if the data has changed. If it matches, the server replies 304 to save bandwidth (see execution_table step 7 and 9).

What happens when Cache-Control max-age expires?

When max-age expires, the client must ask the server for fresh data again, starting a new request cycle (see execution_table step 11).

Why is ETag value quoted like "abc123"?

ETag values are quoted strings to uniquely identify the version of the resource, ensuring exact matching (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution_table at step 7, what header does the client send?

A Cache-Control: max-age=60

B If-None-Match: "abc123"

C ETag: "abc123"

D Authorization: Bearer token

💡 Hint

Check the 'Header Added' and 'Header Value' columns at step 7 in execution_table.

At which step does the server decide to send a 304 Not Modified response?

A Step 5

B Step 8

C Step 9

D Step 11

💡 Hint

Look for the step where the ETag matches and server sends 304 in execution_table.

If the Cache-Control max-age was set to 0, what would change in the client behavior?

A Client would always revalidate immediately

B Client would never cache data

C Client would cache data longer

D Client would ignore ETag

💡 Hint

Refer to variable_tracker Cache-Control values and client caching behavior.

Concept Snapshot

Response headers control caching:
Cache-Control sets how long client caches data (e.g., max-age=60 means 60 seconds).
ETag is a unique tag for resource version.
Client sends If-None-Match with ETag to check freshness.
Server replies 304 Not Modified if data unchanged.
This saves bandwidth and speeds up responses.

Full Transcript

When a client requests data from a server, the server can include response headers like Cache-Control and ETag. Cache-Control tells the client how long it can keep the data before asking again. ETag is a unique identifier for the data version. The client stores the ETag and later sends it back in an If-None-Match header to ask if the data changed. If the server sees the ETag matches, it sends a 304 Not Modified response, telling the client to use its cached data. This process reduces data transfer and speeds up loading. When the cache time expires, the client requests fresh data again, repeating the cycle.