Metadata refresh downloads every time even though source not changing

[shibboleth655 at lewenberg.com]

On 12/9/2016 5:33 AM, Tom Scavo wrote:
> On Fri, Dec 9, 2016 at 8:04 AM,  <shibboleth655 at lewenberg.com> wrote:
>>
>> In the case of the InCommon metadata, the backing file only changes when the
>> data at the other end of the InCommon URL changes.
>
> This is because the InCommon metadata server supports HTTP Conditional
> GET. Under normal circumstances, you end up downloading new metadata
> just once every business day. Most of the requests issued by your
> client result in HTTP 304.

It appears that my Apache server supports HTTP Conditional GET. You can 
see that when I do a HEAD request with the "If-Modified-Since" header 
set to a time _after_ the last-modified date of the file I am requesting:

% curl -I --header 'If-Modified-Since: Tue, 15 Dec 2016 16:23:24 GMT' 
https://spdb.mysite.com/spmetadata/metadata.xml

HTTP/1.1 304 Not Modified
Date: Thu, 15 Dec 2016 16:44:22 GMT
Server: Apache
Connection: close
ETag: "2803d9-543b4a26d6a64"

So why is Shibboleth IdP insisting on downloading even when there is no 
change?


>
>> In the case of the local
>> metadata, however, the backing file is changing *every* time the IdP is
>> scheduled to check for new metadata, even the if the data at the other end
>> of the URL has *not* changed.
>>
>> This is unexpected and worrisome.
>
> Does the server support HTTP Conditional GET? If not, that explains
> the behavior you are seeing.
>
> Tom
>