App Engine not caching static content despite expiration set in app.yaml

Question

I am using Google App Engine Standard Environment for a node.js app.

I have a folder named public for my static files. This contains files such as chunk-XIAWB55V.js. I want these files to be heavily cached as the hashed filename will change if the contents change.

I added a static file handler to my app.yaml to redirect requests starting /static to my public folder, and I confirmed it worked by checking the logs: requests to e.g. https://<redacted>.com/static/javascript/build/prod/chunk-Z4M5HAC7.js are in the request log and are not hitting my app. The docs imply that a default cache of 10 minutes should be applied. However the browser devtools show that the actual headers are cache-control: no-cache, must-revalidate and a request (which gets a 304) is sent every time.

I have tried adding a expiration param to my app.yaml but it did not make a difference. I also tried setting the headers in app.yaml but it did not make a difference. My current file looks like:

handlers:
  - url: /static
    static_dir: public
    secure: always
    expiration: "1d"
    http_headers:
      Cache-Control: public
      Vary: Accept-Encoding

  - url: /.*
    secure: always
    script: auto

I am still getting responses like:

I am using Identity Aware Proxy in front of App Engine and I know from e.g. this question that IAP and caching can mix badly but that seems to refer simply to possible IAP bypass. My static files are not sensitive and I am happy to accept that risk.

Answer 1

There's some documentation of this at : This page describes best practices for using Identity-Aware Proxy (IAP) https://cloud.google.com/iap/docs/concepts-best-practices -- TL;DR, App Engine does some caching for static_files that interacts poorly with IAP. That page has some instructions you can apply if you want to protect your static_files.

Answer 2

GCP support have suggested this is an edge case due to the use of IAP. A workaround I have implemented which I can recommend for anyone else in this situation is to remove the static route and serve the files from your app instead. While this increases load on the app, the high max age means they will not be requested often.