leverage browser caching - expires or max-age, last-modified or etag

Is this correct?

Yes, Expires and max-age do the same thing, but in two different ways. Same thing with Last-Modified and Etag

If so, should I do expires or max-age?

Expires depends on accuracy of user's clock, so it's mostly a bad choice (as most browsers support HTTP/1.1). Use max-age, to tell the browser that the file is good for that many seconds. For example, a 1 day cache would be:

Cache-Control: max-age=86400

Note that when both Cache-Control and Expires are present, Cache-Control takes precedence. read

If I have to also do Last-Modified or ETag, which one of those? I think I get Last-Modified

You're right, Last-Modified should be better. Although it's a time, it's sent by the server. Hence no issue with user's clock. It's the reason why Last-Modified is better than Expires. The browser sends the Last-Modified the server sent last time it asked for the file, and if it's the same, the server answsers with an empty response «304 Not Modified»

Also, you must note that ETag can be useful too, because Last-Modified has a time window of one second. So you can't distinguish two different sources with the same Last-Modified value. [2]

Etag needs some more computing than Last-Modified, as it's a signature of the current state of the file (similar to a md5 sum or a CRC32).

Also, which files should I enable browser caching for?

All files can benefit caching. You've got two different approaches:

• with max-age: useful for files that never change (images, CSS, javascript). For as long as the max-age value, the browser won't send any request to the server. So you'll see the page loading really fast on the second load. If you need to update files, just append a question mark, and the