[web-devel] Content-Length on sendFile

Greg Weber greg at gregweber.info
Wed Jun 15 04:57:14 CEST 2011

On Tue, Jun 14, 2011 at 7:29 PM, Michael Snoyman <michael at snoyman.com>wrote:

> Let me point out one other distinction: sendFile versus yesod-static.
> The former is a function you would call from a normal handler, while
> yesod-static is the "magical" package which would actually know all of
> the stuff about your files at compile time. For the sendFile case, the
> only options for getting the file size are (1) the programmer manually
> adding the header and (2) Yesod automatically doing a system call to
> get it.
> As for the behavior of Warp...  while I agree Kazu that we should
> reduce system call overhead, it might make sense for Warp to perform
> the system call to get file size *if* no content-length header is
> present.
> And I'm still very uncomfortable setting the content-length header for
> static files based on compile-time information. In this case, having
> the wrong value (e.g., someone modified a CSS file after compiling)
> will completely break things and corrupt an open HTTP connection.

Then it should probably not be the default. Lets add a big scary warning to
such a setting and tell users the files should be set to read-only

The most efficient technique for file serving static files that could be
changed would actually be to setup a file notifier (that uses an efficient
OS listener, like inotify on linux) that listens to static assets and knows
when they are changed or a new one is added and would stat them just when
changed. But there would be a race condition if you had to wait for the
notification, so you would actually have to have the old file revision on
hand- perhaps having a symlink convention for adding new files, but meaning
the system could still get screwed up by someone careless.

> Michael
> On Wed, Jun 15, 2011 at 5:22 AM, Kazu Yamamoto <kazu at iij.ad.jp> wrote:
> > Greg,
> >
> >> I apologize for the confusing terminology. I am not differentiating
> between
> >> sending a static file with sendfile and a streaming response. I
> >> am differentiating between 2 different use cases for sending static
> files
> >> (with sendfile). For all of my web applications, I know what all the
> static
> >> files are and they will never change until I deploy another web
> application.
> >> That means I can stat the files once when the application is deployed
> and keep
> >> that information in memory. So I already have the file length
> information to
> >> include in the header, even though I don't do a file stat when the file
> is
> >> requested. wai-app-static and yesod-static supports these techniques.
> >
> > Thanks. I think I understand. :)
> >
> > So, do you support to *not* change the API (apps should add CL: by
> > themselves)?
> >
> > --Kazu
> >
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