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

Introduction

Response headers like Cache-Control and ETag help browsers and servers work together to save time and data by reusing stored information.

When you want to make your website load faster by telling browsers to reuse saved pages.

When you want to check if a file on the server has changed before sending it again.

When you want to reduce the amount of data sent over the internet to save bandwidth.

When you want to improve user experience by loading pages quickly.

When you want to control how long browsers keep a copy of your content.

Syntax

Rest API

Cache-Control: max-age=seconds, no-cache, no-store, public, private
ETag: "unique-identifier"

Cache-Control tells the browser how long to keep a copy of the response.

ETag is a unique tag that changes when the content changes, helping browsers check if they need to update.

Examples

This tells the browser to keep the response for 1 hour (3600 seconds) before asking again.

Rest API

Cache-Control: max-age=3600

This tells the browser to always check with the server before using the cached response.

Rest API

Cache-Control: no-cache

This is a unique tag that the server sends. The browser can send it back to check if the content changed.

Rest API

ETag: "abc123xyz"

This means the response is for a single user only, kept for 10 minutes, with a version tag to check changes.

Rest API

Cache-Control: private, max-age=600
ETag: "v1.0-2024"

Sample Program

This small web server sends data with Cache-Control and ETag headers. If the client already has the same ETag, it returns a 304 status to save data.

Rest API

from flask import Flask, request, make_response

app = Flask(__name__)

@app.route('/data')
def data():
    content = 'Hello, this is some data.'
    etag = 'abc123xyz'

    # Check if client sent ETag
    if_none_match = request.headers.get('If-None-Match')
    if if_none_match == etag:
        # Content not changed
        response = make_response('', 304)  # 304 Not Modified
    else:
        # Send content with headers
        response = make_response(content)
        response.headers['Cache-Control'] = 'max-age=60'
        response.headers['ETag'] = etag
    return response

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Always use ETag with Cache-Control to make caching smarter.

Setting Cache-Control: no-store means the browser should never save the response.

304 status means 'Not Modified' and tells the browser to use its saved copy.

Summary

Cache-Control tells browsers how long to keep data.

ETag helps check if data changed to avoid sending it again.

Using these headers makes websites faster and saves data.