Understanding the interaction between fastcgi_cache_valid and inactive in fastcgi_cache_path

1. fastcgi_cache_valid Directive:

• Purpose:

  • The fastcgi_cache_valid directive controls how long a specific type of response (e.g., 200, 301, 302) should be considered valid in the cache. After this period, the cached response is considered “stale.”

• Example:

  • fastcgi_cache_valid 200 301 302 12h; means that any response with a 200, 301, or 302 status code will be considered valid and served from the cache for 12 hours after it is first cached.

• Effect:

  • After 12 hours, the cached response becomes stale. However, it doesn’t mean the cached file is immediately deleted; rather, it is marked as stale, and the next time the content is requested, Nginx will need to regenerate the content by forwarding the request to the upstream server.

2. inactive in fastcgi_cache_path Directive:

• Purpose:

  • The inactive parameter in the fastcgi_cache_path directive determines how long a cached item should be kept in the cache if it has not been accessed.

• Example:

  • inactive=24h means that if a cached item has not been accessed for 24 hours, it will be removed from the cache.

• Effect:

  • If a cached file is not accessed within the inactive period (24 hours in this case), it is considered inactive and is removed from the cache.

How These Two Work Together:

• Expiration and Regeneration (via fastcgi_cache_valid):

  • The fastcgi_cache_valid directive governs how long a cached response is considered fresh. Once this period expires, the response becomes stale, and the next request for that resource will cause Nginx to regenerate the content from the origin server.
  • Regeneration occurs when a stale cache item is requested, and Nginx fetches a fresh copy from the upstream server.

• Deletion (via inactive in fastcgi_cache_path):

  • The inactive parameter determines how long a cached item remains on disk if it hasn’t been accessed. If a cached item is not accessed for 24 hours, it will be deleted from the cache.
  • Deletion occurs when the cached item is not accessed for the specified inactive period.

Which Happens First?

1. Staleness (fastcgi_cache_valid):

   • The cached item becomes stale after the fastcgi_cache_valid period expires (12 hours in your example).
   • After 12 hours, the cached content is still in the cache, but it is stale. The next time a user requests the content, Nginx will bypass the cache and regenerate the content by fetching it from the upstream server.

2. Inactivity (inactive in fastcgi_cache_path):

   • If a cached item is not accessed for 24 hours, it is removed from the cache, regardless of whether it is still valid or stale.
   • If a cached item was not accessed at all during the 24-hour inactive period, it would be deleted, even if the fastcgi_cache_valid period has not yet expired.

Practical Example:

• Scenario:

  • You have a page cached at 00:00.
  • fastcgi_cache_valid is set to 12 hours, so the page will be considered fresh until 12:00.
  • inactive is set to 24 hours.

• Timeline:

  • From 00:00 to 12:00: The cached page is fresh and served from the cache.
  • After 12:00: The cached page becomes stale, and the next request will cause Nginx to regenerate the content.
  • If the page is not accessed between 12:00 and 24:00, it remains in the cache as stale but not deleted.
  • At 24:00: If the page has not been accessed for 24 hours, it is deleted from the cache.

Which Value to Adjust:

• Longer fastcgi_cache_valid: If you rarely update content, you might want to increase fastcgi_cache_valid to a longer period (e.g., 24 hours, 48 hours, or even longer). This reduces the frequency of content regeneration.

• Longer inactive: If you want to keep infrequently accessed content cached longer, increase the inactive value (e.g., to 48 hours or 7 days). This prevents the cache from being cleared out too quickly.

Optimized Configuration Example:

If you seldom update your pages and want to optimize caching:

fastcgi_cache_path /var/cache/nginx levels=1:2 keys_zone=WPCACHE:1g max_size=20G inactive=48h use_temp_path=off;
fastcgi_cache_valid 200 301 302 48h;
fastcgi_cache_valid any 1h;

• fastcgi_cache_valid 200 301 302 48h;: Cached pages remain valid for 48 hours before needing regeneration.
• inactive=48h;: Cached content remains on disk for 48 hours after its last access.

Conclusion:

• fastcgi_cache_valid controls how long content remains fresh in the cache before it needs to be regenerated.
• inactive controls how long content remains in the cache if it’s not accessed.
• The staleness of content (controlled by fastcgi_cache_valid) typically prompts regeneration, while inactivity (controlled by inactive) leads to deletion if the content isn’t accessed.

Adjust these settings based on how frequently your content changes and how long you want to keep cached content available without frequent regeneration.