HTTP Cache-Control: max-age with must-revalidate

The Cache-Control: max-age=<seconds>, must-revalidate directive is used to specify the duration a resource should be cached while ensuring the client revalidates the resource with the server after the caching period expires. This provides a balance between performance and content freshness.

For other Cache-Control combination guides, see Cache-Control: max-age — Complete Guide, Cache-Control: max-age for Dynamic Content, and Gzip and Vary: Accept-Encoding.

1. Time-Sensitive Content: Content that changes frequently but does not need to be revalidated for every request during its max-age period.
2. Examples: News articles, API responses, event schedules.
3. Reliable Freshness: Ensures users always get up-to-date content after the cache duration expires.
4. Intermediate Cache Control: Prevents stale content from being served by intermediary caches (e.g., CDNs, proxies) without revalidation.

1. Within the Cache Period (max-age): The browser or cache serves the resource without contacting the server. Example: Cache-Control: max-age=600 (10 minutes).
2. After max-age Expires: The client sends a conditional request (using If-Modified-Since or If-None-Match) to check if the resource has changed. If unchanged, the server responds with 304 Not Modified. If changed, the server sends the updated resource.

Add the following configuration to your .htaccess file or Apache configuration:

<IfModule mod_headers.c>

    <FilesMatch "\.(html|php)$">

        Header set Cache-Control "max-age=600, must-revalidate"

    </FilesMatch>

</IfModule>

This sets a 10-minute cache (max-age=600) for HTML and PHP files and requires revalidation after that.

Restart Apache to apply changes:

sudo systemctl restart apache2

Edit your Nginx configuration file to include the Cache-Control header for specific file types:

location ~* \.(html|php)$ {

    add_header Cache-Control "max-age=600, must-revalidate";

}

Reload Nginx:

sudo systemctl reload nginx

Dynamically set caching headers for resources generated by PHP:

<?php

// Set a 10-minute cache duration with revalidation

header("Cache-Control: max-age=600, must-revalidate");



// Generate dynamic content

echo "This content is cached for 10 minutes but requires revalidation after that.";

?>

Set Cache-Control headers for an API endpoint:

app.get('/api/data', (req, res) => {

    res.set('Cache-Control', 'max-age=300, must-revalidate');

    res.json({ message: 'This is temporary API data.' });

});

1. Using cURL: Run the following command to inspect the headers:

curl -I https://yourdomain.com/resource

Expected Output:

Cache-Control: max-age=600, must-revalidate

2. Using Browser Developer Tools:
   • Open Developer Tools (F12).
   • Go to the Network tab.
   • Reload the page and inspect the headers of the requested resource.
3. Using Online Tools: Use WebPageTest or GTmetrix to analyze caching headers.

HTTP/1.1 200 OK

Content-Type: text/html

Cache-Control: max-age=300, must-revalidate

ETag: "12345"

Last-Modified: Tue, 19 Jan 2025 16:00:00 GMT

By using Cache-Control: max-age with must-revalidate, you can optimize caching for temporary or dynamic content while ensuring users receive up-to-date information after the caching period expires.