Mailing List Archive

Perhaps I didn't explain myself clearly enough... My problem wasn't
that the mod_backhand was selecting itself from the list. The problem
was that it didn't have any servers to select from because 'byAge 3'
had removed every single entry from the list.

In the case of a null candidacy list, mod_backhand attempts to handle
the request itself (ignoring the 'BackhandSelfRedirect' directive)
which is different than proxying it to the mod_perl server on the same
machine. byChooseMeOverLocal will eliminate this possibility by
converting a null list to a one element list containing the local
mod_perl server.

-Blake

> That looks pretty cool. But why not use byHostname?
> Say you've got the following server setup:
>
> Proxies:
> proxy1
> proxy2
> proxy3
>
> mod_perl boxes:
> app1
> app2
> app3
>
> You could use the following setup on your proxies:
>
> <Directory /path/to/docroot>
> Backhand byAge 3
> BackhandFromSO libexec/byHostname.so byHostname app
> Backhand byRandom
> Backhand byLoad -1000
> </Directory
>
> This will make all requests go off to the mod_perl application boxes and
> should take the proxy boxes out of the loop for filling requests.
>
> Just a thought... :)
> --Steve Nickels
>
> -----Original Message-----
> From: Blake D. Mills IV
> To: backhand-users@lists.backhand.org
> Sent: 8/1/00 12:53 AM
> Subject: [mod_backhand-users] Another New Candidacy Function:
> byChooseMeOverLocal
>
> OK, this one deserves a bit of explaining.... I'm running mod_backhand
> as a proxy server for heavy mod_perl servers. I had the bulk of my
> docroot proxied with the following rules:
>
> BackhandSelfRedirect On
> <Directory /path/to/docroot>
> Backhand byAge 3
> Backhand byRandom
> BackhandFromSO libexec/byChooseN.so byChooseN 2
> Backhand byLoad -1000
> </Directory>
>
> Which worked exceptionally well 99.99% of the time. However,
> occasionally (very occasionally) byAge 3 would eliminate all of my
> server candidates. The default behavior for mod_proxy in this case is
> is to attempt to handle the request itself. Since my mod_backhand is
> supposed to proxy *all* requests, this results in a "File does not
> Exist" error. My logs for these cases look like:
>
> Func executed for (null) [byAge(3)] (512 -> 0)
> Func executed for (null) [byRandom(NULL)] (0 -> 0)
> Func executed for (null) [libexec/byChooseN.so::byChooseN(2)] (0 ->
> 0)
> Func executed for (null) [byLoad(-1000)] (0 -> 0)
> All funcs executed -> local
> [error] File does not exist: /path/to/docroot/myfile.html
>
> Apparently in this case 'BackhandSelfRedirect On' doesn't step in, and
> the proxy server attempts to handle the request intended for the
> mod_perl server. This is probably a bug, but I didn't feel like
> hacking the internals to get self redirection to take effect in this
> case.
>
> So, I wrote a new candidacy function, which simply inserts "ME" into
> the candidacy list if the list empty. My new configuration looks
> like:
>
> <Directory /path/to/docroot>
> Backhand byAge 3
> Backhand byRandom
> BackhandFromSO libexec/byChooseN.so byChooseN 2
> Backhand byLoad -1000
> BackhandFromSO libexec/byChooseMeOverLocal.so byChooseMeOverLocal
> </Directory>
>
> And the logs for a request that would have caused the above error read:
>
> Func executed for (null) [byAge(3)] (512 -> 0)
> Func executed for (null) [byRandom(NULL)] (0 -> 0)
> Func executed for (null) [libexec/byChooseN.so::byChooseN(2)] (0 ->
> 0)
> Func executed for (null) [byLoad(-1000)] (0 -> 0)
> Func executed for (null) [libexec/byChooseMeOverLocal.so::
> byChooseMeOverLocal(NULL)] (0 -> 1)
> All funcs executed -> myhost.mydomain.com
>
> This essentially avoids the situation of a null candidacy list, which
> solves the immediate problem. A better solution would be to follow
> the BackhandSelfRedirect setting in this case, but the quick solution
> seems to work well enough for me.
>
> I didn't try to optimize the new code, as the meat of it is only
> invoked on about 1 out of 20,000 requests. Though, the new list ist
> trivial, so I'm sure there is a better way to generate it. Feel free
> to distribute/modify at will.
>
> --------------- BEGIN byChooseMeOverLocal.c ----------
> #include "httpd.h"
> #include "http_log.h"
> #include "mod_backhand.h"
> int byChooseMeOverLocal(request_rec *r, int *servers, int *n, char *arg)
> {
> int size=*n, i;
> if(size == 0) {
> for(i=0; i<MAXSERVERS; i++) {
> if(servers[i] == 0)
> servers[0]=servers[i];
> }
> size = 1;
> *n = 1;
> }
> return size;
> }
> --------------- END byChooseMeOverLocal.c ----------
>
>
> On a separate note, the one remaining issue I have with using
> mod_backhand as a proxy for mod_perl is the lack of an analogue to
> mod_proxy's ProxyPassReverse directive.
>
> Under my current setup mod_backhand is on port 80 and mod_perl is on
> port 81. If mod_perl issues a redirect (for instance from
> 'http://mydomain.com' to 'http://mydomain.com/') the resulting url is
> passed directly back to the client. I'd like to be able to munge the
> headers a bit, so I can force the client to come back through
> mod_backhand.
>
> I'd like to be able to modify the headers at step #4 below.
>
> 1.) client->backhand: GET http://mydomain.com
> 2.) backhand->mod_perl: forwards to mod_perl
> 3.) mod_perl->backhand: redirect to http://mydomain.com:81/
> 4.) backhand->client: redirect to http://mydomain.com:81/
> (would like this to be http://mydomain.com/)
>
> Has anyone thought about or done anything along these lines?
>
> -Blake
>
>
>
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