Google Cloud CDN `stale-while-revalidate` doesn't work as expected for some requests

Google Cloud Compute Infrastructure
1

Question Title

Google Cloud CDN stale-while-revalidate doesn’t work as expected for some requests

Content

Background

We are currently using Google Cloud CDN with the following cache configuration:

  • Cache-Control Header:
    public, max-age=300, stale-while-revalidate=86400

  • Cache expiration is set to 300 seconds.

  • After expiration, the cache should serve stale content for up to 86400 seconds (1 day) while asynchronously fetching fresh content from the backend.

  • Cache Mode:
    The Cloud CDN cache mode is set to USE_ORIGIN_HEADERS.

Expected Behavior

When the cache expires, we expect the following behavior:

  1. The client (browser) receives the stale cache immediately.
  2. Cloud CDN fetches fresh content asynchronously from the backend to update the cache.

Observed Behavior

During testing, we observed the following behaviors:

  1. Case where it works as expected:
    When a request is sent after the cache expiration (300 seconds), Cloud CDN correctly serves the response from the cache (response_from_cache). Immediately afterward, the logs show that Cloud CDN asynchronously fetches new content from the backend (a request with the user agent Cloud-CDN-Google (GFE/2.0)).

  2. Case where it does not work as expected:
    Approximately 3 hours later, another request from the same browser results in Cloud CDN fetching content directly from the backend (response_sent_by_backend).
    In this case, no asynchronous backend request (with the Cloud-CDN-Google (GFE/2.0) user agent) was recorded.

Investigations

  • Client Request Headers:
    The request headers sent from the browser were inspected, and there was no Cache-Control: no-cache or any other header that could bypass the cache.
    Additionally, no cache-bypass actions (e.g., Shift + Reload) were performed during testing.

  • Cloud CDN Cache Logs:
    The logs for the problematic request include the following cacheDecision entries: [ “REQUEST_CACHE_CONTROL_NO_CACHE”, “RESPONSE_HAS_CACHE_CONTROL”, “RESPONSE_CACHE_CONTROL_PUBLIC”, “RESPONSE_HAS_CONTENT_TYPE”, “CACHE_MODE_USE_ORIGIN_HEADERS” ]

  • Cloud CDN Cache State:
    During the problematic request, cacheLookup: true indicates that the cache was present. However, the response was logged as response_sent_by_backend.

Questions

  • Why does stale-while-revalidate not work as expected for some requests, resulting in a direct fetch from the backend?
  • What could be potential conditions or misconfigurations that lead to this behavior? Are there any additional points we should check to identify the root cause?

Additional Information

  • Service: Google Cloud CDN
  • Cache Mode: USE_ORIGIN_HEADERS
  • Cache-Control Header: public, max-age=300, stale-while-revalidate=86400

I can provide additional logs or details if needed. Thank you for your help!

2

Hi @yatsuda_retbit ,

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It appears that a more thorough investigation for your project is necessary. For more detailed insights you may reach out to Google Cloud Support for assistance. Our 1:1 support team has specialized expertise in diagnosing underlying problems. When contacting them, provide all your comprehensive details and screenshots. This will help our 1:1 support team better understand and address your issue.

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