The following is a http response header from a image on our company's website.
HTTP/1.1 200 OK
Content-Type: image/png
Last-Modified: Thu, 03 Dec 2009 15:51:57 GMT
Accept-Ranges: bytes
ETag: "1e61e38a3074ca1:0"
Date: Wed, 06 Jan 2010 22:06:23 GMT
Content-Length: 9140
Is there anyway to know if this image is publicly cacheable in some proxy server? The RFC definition seems to be ambiguous http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.1 and http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.4.
Run RED on your URL and it'll tell you whether the response is cacheable, among other information.
The headers you show appear to be cacheable.
If you would like to control the caching behavior of correctly configured proxies and web browsers, you might investigate using the Cache-Control and Expires headers to gain additional control.
Here is a webpage I had bookmarked that has one person's opinion of how to intepret the specifications you list (plus some other ones):
http://www.web-caching.com/mnot_tutorial/how.html
If you need to guarantee that someone sees a completely new image each time (even with misconfigured devices between you and them), you may want to consider using a randomized or GUID value as part of the URL.
Here is a tutorial on setting headers for proxy caching. Be sure to read the part about setting cookies!
Related
I have a server serving static files with an expire of 1 year but my browsers still get the file and receive a 304 - not modified. I want to prevent the browser from even attempting the connection. I realize that that happens in several different setup (Ubuntu Linux) with Chrome and Firefox.
My test is as follows:
$ wget -S -O /dev/null http://trepalchi.it/static/img/logo-trepalchi-black.svg
--2016-03-14 19:56:14-- http://trepalchi.it/static/img/logo-trepalchi-black.svg
Risoluzione di trepalchi.it (trepalchi.it)... 213.136.85.40
Connessione a trepalchi.it (trepalchi.it)|213.136.85.40|:80... connesso.
Richiesta HTTP inviata, in attesa di risposta...
HTTP/1.1 200 OK
Server: nginx/1.2.1
Date: Mon, 14 Mar 2016 18:55:29 GMT
Content-Type: image/svg+xml
Content-Length: 25081
Last-Modified: Sun, 13 Mar 2016 23:03:53 GMT
Connection: keep-alive
Expires: Tue, 14 Mar 2017 18:55:29 GMT
Cache-Control: max-age=31536000
Cache-Control: public
Accept-Ranges: bytes
Lunghezza: 25081 (24K) [image/svg+xml]
Salvataggio in: "/dev/null"
100%[==================================================================================================================================================================>] 25.081 --.-K/s in 0,07s
2016-03-14 19:56:14 (328 KB/s) - "/dev/null" salvato [25081/25081]
That shows correctly providing expires and cache control (via nginx).
If I go to the browser and enable cache and open diagnostic tools, the first hit I notice a 200 return code, then I refresh the page (Control-r) and find a connection with 304 - not modified return code.
Inspecting firefox cache (about:cache) I found it with correct expire and clicking on the link in that page I was able to see it w/o hitting the remote server.
I also tested pages where the images are loaded from image tags (as opposed as directly called as in the example above).
All the letterature I read state that with such an expire the browser should not even try a connection. What's wrong? RFC 2616 states:
HTTP caching works best when caches can entirely avoid making requests
to the origin server. The primary mechanism for avoiding requests is
for an origin server to provide an explicit expiration time in the
future, indicating that a response MAY be used to satisfy subsequent
requests. In other words, a cache can return a fresh response without
first contacting the server.
Note another question addresses the problem of how 304 is generated, I just want to prevent the connection to be made
Sandro
Thanks
I’m using a custom framework for bundling stylesheets and scripts. (I.e., these are dynamically generated responses, not static files.)
The response for the initial request, when the response is being generated for the very first time, includes these headers:
HTTP/1.1 200 OK
Cache-Control: public, no-transform, max-age=31536000
Content-Type: text/css; charset=utf-8
Content-Encoding: gzip
Last-Modified: Mon, 25 Aug 2014 18:15:50 GMT
Vary: Accept-Encoding
Date: Tue, 09 Sep 2014 16:19:36 GMT
Content-Length: 3126
Now that the response above has been generated and cached by the server, subsequent requests for the same stylesheet are responded to with these headers:
HTTP/1.1 200 OK
Cache-Control: public, no-transform, max-age=31536000
Content-Type: text/css; charset=utf-8
Content-Encoding: gzip
Last-Modified: Mon, 25 Aug 2014 18:15:50 GMT
Date: Tue, 09 Sep 2014 16:20:00 GMT
Content-Length: 3126
Ignoring the new Date value, the headers are identical with the obvious exception of the missing Vary header.
One nasty consequence that I’ve seen in the wild is that if the very first response generated for a given asset is not compressed (due to the corresponding client not supporting compression), then the server caches that non-compressed response and serves it for all subsequent requests to all clients.
Any idea how to have the server retain the Vary header for cached responses?
I’m using HttpCacheability.Public for these responses. I can avoid the issue by using HttpCacheability.Private instead, but I’d prefer to allow the server and proxies to cache responses.
Some reading has led me to believe that IIS can’t do “kernel caching” if you vary by encoding. But I’m not sure if that means I can’t cache on the server at all or if it just prevents a special kind of server-side caching.
Update:
I was originally using the following to set the Vary header:
response.AppendHeader("Vary", "Accept-Encoding");
I tried a different method of specifying it:
response.Cache.SetVaryByCustom("Accept-Encoding");
That caused Vary to never be emitted. Not even on the very first response.
As a last resort, I’m also considering using:
response.Cache.SetNoServerCaching();
That causes the Cache-Control header to still specify public (so that proxies can still cache), but prevents the server from caching.
In line with the update I made to the question, I tried yet another way of specifying the Vary header:
response.Cache.VaryByHeaders["Accept-Encoding"] = true;
…And it fixed the problem. Responses now retain the Vary header across requests from multiple clients and are also cached by the server.
In the document "Optimize Cache - Make the Web Faster - Google Developers", Google states that
It is important to specify ONE of Expires or Cache-Control
max-age, AND ONE of Last-Modified or ETag, for all cacheable
resources. It is redundant to specify both Expires and Cache-Control:
max-age, or to specify both Last-Modified and ETag.
I'm using the classes in Microsoft.WindowsAzure.StorageClient to upload images to a blob container, pratically the same code as can be seen in the open source project Azure Storage Explorer.
The resulting image is served with BOTH Last-Modified and ETag:
ETag: 0x8CFED5D3384112F
Last-Modified: Tue, 12 Mar 2013 17:21:43 GMT
So the next browser requests sends HTTP headers:
If-Modified-Since: Tue, 12 Mar 2013 17:21:43 GMT
If-None-Match: 0x8CFED5D3384112F
How can I force Azure Storage to use only one of the two directives to eliminate this redudancy?
The short answer is you can't.
When thinking about this it's important to remember that when you access blob storage you not accessing a file on a web server, you're using a rest API that happens to return files.
Microsoft offer no way to remove headers that they deem as essential to the storage API.
If you're worried about excessive headers, the response also includes several x-ms-... headers which are intended for clients of the API that aren't browsers.
Personally I would not worry that much about both tags being send back as this is actually recommended by RFC 2616.
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates
...
HTTP/1.1 origin servers:
...
... the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity tag and a Last-Modified value.
An HTTP 1.1 client MUST use the Entity Tags in any cache-conditional requests, and if both an Entity Tags and Last-Modified are present, it SHOULD use both.
I hope that will clarify why both tags are sent back from the Azure Storage server.
I pushed some changes to one of our images to web server, but I found it's not updated on IE and firefox web browsers due to browser caching, since I saw the browsers didn't even request for image from server.
I have two choice. One is to update the image file name and re-deploy. The other is to let our users' browsers think the image exipires.
I haven't set anything in header or in code, here is the response:
Content-Length 32042
Content-Type image/png
Last-Modified Wed, 09 Nov 2011 18:41:50 GMT
Accept-Ranges bytes
Server Microsoft-IIS/6.0
X-Powered-By ASP.NET
Date Fri, 09 Dec 2011 01:26:03 GMT
Connection keep-alive
So I am wondering how soon it can expire?
Ideally, if your last-modified time stamp changes, everything should take of by itself. You can set up cache-control http://blogs.msdn.com/b/rahulso/archive/2006/02/02/suppress-caching-of-certain-mime-types-like-gif-jpg-etc.aspx to avoid it.
I'm using valid expires and no-cache headers for my static files and they stay cached for as long as I keep browsing, but when I close my browser and use it back after a while I see the static files loading again, even when not refreshing with ctrl (+ shift) + r
I'm using Firefox, cache size set to 250MB and I don't let it remove any private or cached data.
Headers:
Accept-Ranges: bytes
Cache-Control: max-age=29030400, public
Content-Length: 142061
Content-Type: image/png
Date: Tue, 08 Dec 2009 19:18:43 GMT
Expires: Tue, 09 Nov 2010 19:18:43 GMT
Last-Modified: Sun, 18 Jan 2009 18:33:48 GMT
Server: Apache/2.2.14 (EL)
Which version of Firefox? Is the server sending Etags for the static files? You can view details about Firefox cache by going to the address about:cache and poking around. That will give you an idea of what Firefox is caching.
Update: After looking at your header tags, it seems as if the max-age value is set to a date that is way in the past and that is overriding the the value being set in the Expires header. See the HTTP 1.1 protocol definition at: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.3.
If a response includes both an Expires
header and a max-age directive, the
max-age directive overrides the
Expires header, even if the Expires
header is more restrictive. This rule
allows an origin server to provide,
for a given response, a longer
expiration time to an HTTP/1.1 (or
later) cache than to an HTTP/1.0
cache. This might be useful if certain
HTTP/1.0 caches improperly calculate
ages or expiration times, perhaps due
to desynchronized clocks.
You will have to modify your Cache-Control header being sent by the server.