Calculate same etag in javascript as nginx

Question

I have a setup where images are rendered on a just-in-time fashion in nodejs. Afterwards the images are cached in a directory from where nginx will fetch them directly on subsequent requests.

How do I produce the same etag in nodejs/javascript as nginx will produce on the second request?

Answer 1

I had a thought. You could setup nginx to use try_files to first check if the file exists in the static directory and if not, proxy to node for creation. This is not exactly what you are looking for, but it will allow the browser to cache a consistent URL and then etag isn't as important.

location / {
    root /path/to/root/of/static/files;
    try_files $uri $uri/ @nodeserver;

    expires max;
    access_log off;
}

location @nodserver {
    proxy_set_header X-Real-IP  $remote_addr;
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_pass http://127.0.0.1:3000;
}

Answer 2

Here's the answer outlining how nginx calculates etag: https://serverfault.com/questions/690341/algorithm-behind-nginx-etag-generation

basically

etag = sprintf("%xT-%xO", file.last_modified_time, content_length)

You can repeat that in js using sprintf-js, but you'll need to know the file's last_modified_time before nginx caches it, and this is IMO impossible.