Why do some page requests hang when fetching Javascript/image assets using Safari and Apache 2.2.3?

Question

Some of the users of our Ruby on Rails app have complained that page requests occasionally hang indefinitely under Safari (a couple have noticed it under Firefox, but it's overwhelmingly Safari users). After some investigation it seems that these requests are being served correctly by our Rails application and the hang occurs when fetching image assets (which are hosted on the same server) which are referenced in the HTML.

We have configured Apache to serve the image assets directly and bypass the Rails app for performance. We have also enabled gzip compression on text/javascript/css assets. Below are the relevant settings from our Apache Virtual Host configuration -- perhaps we have configured this in such a way which might explain these arbitrary hanging requests?

RewriteEngine On

# Correct behaviour of IE under SSL
SetEnvIf User-Agent ".*MSIE.*" \
    nokeepalive ssl-unclean-shutdown \
    downgrade-1.0 force-response-1.0

SSLEngine On
SSLCertificateFile /etc/httpd/conf/ssl/_.mycert.com.crt
SSLCertificateKeyFile /etc/httpd/conf/ssl/_. mycert.com.key
SSLCertificateChainFile /etc/httpd/conf/ssl/gd_bundle.crt

RequestHeader set X_ORIGINAL_PROTOCOL 'https'
RequestHeader set X_FORWARDED_PROTO 'https'

# Rewrite index to check for static
RewriteRule ^/$ /index.html [QSA] 
RewriteRule "^/(images|stylesheets|javascripts|system)/?(.*)" "$0" [L]

# Rewrite to check for Rails cached page
RewriteRule ^([^.]+)$ $1.html [QSA]

# Deflate
AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript application/x-javascript
BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4\.0[678] no-gzip
BrowserMatch \bMSIE !no-gzip !gzip-only-text/html

ExpiresActive On
<FilesMatch "\.(ico|gif|jpe?g|png|js|css)$">
  ExpiresDefault "access plus 1 year"
  Header append Cache-Control "public"
</FilesMatch>

Has anyone experienced a similar problem before?

Our Ruby on Rails web application runs using mod_rails and Apache 2.2.3 on RedHat Enterprise Linux 5.

Update: I have now tried removing the following block and the problem still persists, so it looks like we can exclude the expires header from being the problem:

ExpiresActive On
<FilesMatch "\.(ico|gif|jpe?g|png|js|css)$">
  ExpiresDefault "access plus 1 year"
  Header append Cache-Control "public"
</FilesMatch>

Answer 1

I was having a spectacularly good time debugging a similar issue on our site today. Safari users--but seemingly only those on a Mac--would complain that our site would "hang" randomly while loading a page. It usually appeared to be hanging on an image--2 of 3 items completed, etc.--but then I disabled the cache in Safari, JavaScript, and CSS, and guess what? The page still hung.

First, a note on Safari caching. Even if you have "Disable Caching" selected in the "Develop" menu and have selected "Empty Cache" from the "Safari" menu, you still have a RAM-based cache. This means that if you REALLY want to simulate an empty cache, you have to quit Safari with each request or hold down the Option + Shift + Prayer-To-Deity-of-Choice keys while pressing the Reload button. This took me a while to figure out, and until I figured it out, I had a hard time consistently reproducing the issue.

So! Without JavaScript, CSS, or images, what do you have left? Not much, aside from the actual HTML. This is what turned me onto the fact that is was probably compression related. And sure enough, turning off compression in IIS 6.0 resulted in the page loading instantaneously. Since IIS 6.0 doesn't have a convenient way of turning off compression based on the user agent, I used IIRF (an ISAPI filter that rewrites URLs) to rewrite the Accept-Encoding header when it comes from Safari:

# Safari doesn't handle gzip compression properly; we turn it off by setting 
# the q value to zero for all agents identifying themselves as Safari
RewriteCond %{HTTP_USER_AGENT} Safari
RewriteHeader Accept-Encoding: .* *;q=0

The issue seems to be that if Safari is receiving static compressed content (that is, the server sends the Content-Length header), then Safari deals with it just fine. But if Safari is receiving dynamic compressed content (that is, the server is serving a response being rendered by ASP.NET and the content length is not known until it's done, so the server sends Transfer-Encoding: chunked) then Safari goes into Flaky Sir-Hangs-A-Lot Mode.

Why? I have no idea. But this is how I work around it.

Answer 2

Olly,

This may or may not be related, but browsers have a limit of 2 (by default) simultaneous connections that they can make. If there are connections kept open for communication, and you are also fetching images, the call to the image may not go through till the browser has one of its stipulated connections free. The hang during image fetch may actually be triggered by some other server connections which are not completing or are being held open by the server and browser. So you may actually be hunting in the wrong place.

If you are able to reproduce the error, try switching to HTTP 1.0 on your dev server and see if it fixes the issue. Also try moving some of the assets to another domain/subdomain and fetch from there.

Hope that gives you another angle.

Regards, Narayan