[web-devel] Content-Length on sendFile

[Michael Snoyman]

On Tue, Jun 14, 2011 at 9:55 PM, David Pollak
<feeder.of.the.bears at gmail.com> wrote:
>
>
> On Tue, Jun 14, 2011 at 11:50 AM, Michael Snoyman <michael at snoyman.com>
> wrote:
>>
>> I think the reason we didn't include the header is because, when I
>> wrote it, I didn't know of a cross-platform way to get file sizes. Now
>> that I know about unix-compat, this seems like a reasonable thing to
>> add. Anyone know a reason we *shouldn't* do this?
>
> In Lift, we do not automatically populate the Content-Length for generic
> BLOB responses because if the response is streaming, then we have to
> strictly evaluate the stream to determine the length.  For returning large
> items (e.g., a 2 GB AVI), this can cause issues.  We break the Response
> types into "In Memory" which are strict and we include the Content-Length
> header, but for responses that are not strict, it's up to the coder.
>
> My 2 cents.

Nubis was referring specifically to sending files. Yesod has a similar
breakdown where some datatypes (strict ByteStrings for instance) have
a content-length attached, while others (lazy ByteStrings) do not. We
achieve this via the optional Int argument in the ContentBuilder
constructor[1]. Various instances of ToContent simply handle this
differently.

In the case of files, we would either use the length specified by the
FilePart parameter, or simply get the file size from the file system.

Michael

[1] http://hackage.haskell.org/packages/archive/yesod-core/0.8.2/doc/html/Yesod-Content.html

>>
>> Michael
>>
>> On Tue, Jun 14, 2011 at 8:16 PM, Nubis <nubis at woobiz.com.ar> wrote:
>> > Hi guys,
>> > Just wanted to give you a heads up that yesod's sendFile is not setting
>> > the
>> > Content-Length header, which 'breaks' some proxies and load balancers.
>> > (in
>> > our case, amazon elastic load balancer).
>> > I don't know if that's by design, I'm not 100% sure but not setting the
>> > header would be only useful for 'long pooling' of binary files for doing
>> > some sort of hackish http streaming, which I don't think is yesod's
>> > purpose
>> > anyways :)
>> > A workaround is setting the Content-Length header before calling
>> > sendFile, I
>> > guess it may be something easy to add, I was going to propose a patch
>> > that
>> > just wraps the current implementation in a do block that sets the
>> > header,
>> > but then I noticed a number of optimizations being made that hint there
>> > should be a 'better' way.
>> >
>> > cheers!
>> > ----nubis :)
>> >
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>> >
>> >
>>
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>
>
>
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> Simply Lift http://simply.liftweb.net
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