Max value for cache control header in HTTP

问题:

I'm using Amazon S3 to serve static assets for my website. I want to have browsers cache these assets for as long as possible. What meta-data headers should I include with my assets

Cache-Control: max-age=???

回答1:

Generally one year is advised as a standard max value. See RFC 2616:

To mark a response as "never expires," an origin server sends an Expires date approximately one year from the time the response is sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future.

Although that applies to the older expires standard, it makes sense to apply to cache-control too in the absence of any explicit standards guidance. It's as long as you should generally need anyway and picking any arbitrarily longer value could break some user-agents. So:

Cache-Control: max-age=31536000

回答2:

Consider not storing it for "as long as possible," and instead settling for as long as reasonable. For instance, it's unlikely you'd need to cache it for longer than say 10 years...am I right?

The RFC discusses max-age here: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.3

Eric Lawrence says that prior to IE9, Internet Explorer would treat as stale any resource with a Cache-Control: max-age value over 2147483648 (2^31) seconds, approximately 68 years (http://blogs.msdn.com/b/ie/archive/2010/07/14/caching-improvements-in-internet-explorer-9.aspx).

Other user agents will of course vary, so...try and choose a number that is unlikely (rather than likely!) to cause an overflow. Max-age greater than 31536000 (one year) makes little sense, and informally this is considered a reasonable maximum value.

回答3:

The people who created the recommendation of maximum 1 year caching did not think it through properly.

First of all, if a visitor is being served an outdated cached file, then why would it provide any benefit to have it suddenly load a fresh version after 1 year? If a file has 1 year TTL, from a functional perspective, it obviously means that the file is not intended to be changed at all.

So why would one need more than 1 year?

1) Why not? It doesn't server any purpose to tell the visitors browser "hey, this file is 1 year old, it might be an idea to check if it has been updated".

2) CDN services. Most content delivery networks use the cache header to decide how long to serve a file efficiently from the edge server. If you have 1 year cache control for the files, it will at some point start re-requesting non-changed files from the origin server, and edge-cache will need to be entirely re-populated, causing slower loads for client, and unnecessary calls to the origin.

What is the point of having max 1 year? What browsers will choke on an amount set higher than 31536000?