Express - ETag fix for multiple servers

express-etag-fix.md

Express Etag fix (Multiple servers)

Etag uses information from file system stat. Including the modified time similar to nginx & apache. This can't calculate the hash of the contents. Doing so that would require reading the file from the file system twice for every single request scope.

Files usually have slightly different modification timestamp between multiple servers. Similarly apache/nginx would do the same.

Simple way to calculate ETag for the static files:

app.use(express.static(path.join(__dirname, 'public', {
  setHeaders: setStaticETag
})));

function setStaticETag(res, path, stat){
  const etag = ''; // Custom logic to calculate ETag
  res.setHeader('ETag', etag);
}

Basic curl command to verify before and after the fix

curl -I http://<SERVER_IP_1>/static/img.jpeg
ETag: "1234-2323453245"

curl -I http://<SERVER_IP_2>/static/img.jpeg
ETag: "1234-2323453245"