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Date: Wed, 27 Feb 2013 11:46:31 +0100
From: =?ISO-8859-15?Q?Jochen_Wi=DFmann?= <jochen.wissmann@kepol.com>
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Subject: Re: mod_jk how to add JK_WORKER_NAME to http-header

On 27.02.2013 09:59, Andr� Warnier wrote:
> Rainer Jung wrote:
>> On 26.02.2013 19:41, Jochen Wi�mann wrote:
>>> Hello,
>>>
>>> i am trying to find an easy way to determine from the http-client side,
>>> which  AJP13-worker handled my request.
>>> So my basic idea is to use mod_header to add mod_jk`s env-variable
>>> JK_WORKER_NAME to the Header of the http-response.
>>> I tried to get something like this working:
>>>
>>> httpd.conf:
>>> ...
>>> Header add TC-Server "%{JK_WORKER_NAME}e"
>>> ...
>>>
>>> All my previous attempts resulted in http-responses looking like this:
>>>
>>> < HTTP/1.1 200 OK
>>> < Set-Cookie: ARPT=LRWQXVS110.160.11.26CKMLQ; path=/
>>> < Date: Tue, 26 Feb 2013 18:25:31 GMT
>>> < Server: Apache/2.2.22 (Unix) mod_jk/1.2.36 mod_ssl/2.2.22
>>> OpenSSL/0.9.8x DAV/2
>>> < Set-Cookie: JSESSIONID=A4052BF60BA2007F0B0F47E2699AFDE.liferay0; 
>>> Path=/
>>> < Set-Cookie: GUEST_LANGUAGE_ID=en_US; Expires=Wed, 26-Feb-2014 
>>> 18:25:31
>>> GMT; Path=/
>>> < Set-Cookie: COOKIE_SUPPORT=true; Expires=Wed, 26-Feb-2014 18:25:31
>>> GMT; Path=/
>>> < Liferay-Portal: Liferay Portal Community Edition 6.0.6 CE (Bunyan /
>>> Build 6006 / February 17, 2011)
>>> < ETag: "838b4ae2"
>>> < Content-Length: 8043
>>> < X-Server-Ip: 110.160.11.26
>>> < TC-Server: (null)
>>> < Content-Type: text/html;charset=utf-8
>>>
>>> Why do i get "TC-Server: (null)" ?
>>> Using the Variable JK_WORKER_NAME to log the worker's name via
>>> mod_log_config works fine.
>>>
>>> Am i missing something? Is it even possible to access the env-variable
>>> JK_WORKER_NAME with module mod_header?
>>> Could anyone please help?
>>> Thanks!
>>
>> mod_jk respects the environment variable JK_WORKER_NAME when choosing a
>> target worker, but it does not set it.
>>
>> mod_jk only sets a note with name JK_WORKER_NAME, which is different
>> from an env var. The "n" in your log format means "note".
>>
>> Unfortunately most variable use supported in Apache config is for env
>> vars, not for notes, so you can't directly set the header from a note.
>>
>> There are two things you can experiment with:
>>
>> a) using an undocumented mod_rewrite feature to try to copy the note
>> into an env var and then using that env var to set the header
>>
>> or
>>
>> b) not using the normal JkMount or uriworkermap.properties way of
>> defining the workers responsible for handling the URIs, but instead
>> using mod_rewrite via setting the env var JK:WORKER_NAME
>>
>> Ad a)
>>
>> Works roughly like that (untested):
>>
>> RewriteCond %{ENV:JK_WORKER_NAME} (.*)
>> RewriteRule . - [ENV=WORKER:%1]
>>
>> Note 1: the undocumented feature of mod_rewrite is, that
>> %{ENV:JK_WORKER_NAME} first looks for a note named "JK_WORKER_NAME" and
>> only if it doesn't find it for an env var.
>>
>> Note 2: The RewriteRule now sets a new env var "WORKER" to the captured
>> value of the note. lease choose a new variable name here, not
>> "JK_WORER_NAME".
>>
>> Now we add
>>
>> Header add TC-Server-Ip "%{WORKER}e"
>>
>> If you choose another name as "WORKER" for the new env var in the
>> RewriteRule, please adjust here as well.
>>
>> Finally: if it doesn't work, try again by placing the Rewrite lines in a
>> <Directory></Directory> block. Don't forget to switch
>>
>> RewriteEngine On
>>
>> Ad b)
>>
>> You can use mod_rewrite to set the env var JK_WORKER_NAME (now it is
>> important to use that name). mod_jk will then route the request to that
>> worker and since you had set the env var yourself, you can also copy it
>> to a header.
>>
>> RewriteRule ^/c - [ENV=JK_WORKER_NAME:liferay]
>> RewriteRule ^/c/.* - [ENV=JK_WORKER_NAME:liferay]
>> RewriteRule ^/group - [ENV=JK_WORKER_NAME:liferay]
>> RewriteRule ^/group/.* - [ENV=JK_WORKER_NAME:liferay]
>>
>> etc.
>>
>> and then
>>
>> Header add TC-Server-Ip "%{JK_WORKER_NAME}e"
>>
>> Final remarks:
>>
>> Your config does not really warrant this effort: you only have one
>> worker name, so what do you expect in the header? I'm asking, because if
>> you are using a load balancer worker (assumed name "lb") in your real
>> config, and it has sub workers for example "node1" and "node2" then
>> JK_WORKER_NAME will always be "lb". To actually get the chosen sub
>> worker, only recipe 1 has a chance to work. You would there replace
>> JK_WORJER_NAME in the RewriteCond by JK_WORKER_ROUTE. Unfortunately I
>> think that the route note is not set before the request actually gets
>> forwarded and that is to late for mod_rewrite to be able to copy it in
>> an env var.
>>
>> In that case (load balancer and you want to know the chosen lb sub
>> worker) we would either need to add something to mod_jk, or allow
>> mod_headers to also support notes, or write a special small module.
>>
>> Having written all that, I think the following patch to mod_headers
>> would be best. It simply adds support for notes to mod_headers (%{xxx}n
>> notation):
>>
>> Index: modules/metadata/mod_headers.c
>> ===================================================================
>> --- modules/metadata/mod_headers.c      (revision 1449529)
>> +++ modules/metadata/mod_headers.c      (working copy)
>> @@ -200,6 +200,16 @@
>>          return "(null)";
>>  }
>>
>> +static const char *header_request_note(request_rec *r, char *a)
>> +{
>> +    const char *s = apr_table_get(r->notes,a);
>> +
>> +    if (s)
>> +        return unwrap_header(r->pool, s);
>> +    else
>> +        return "(null)";
>> +}
>> +
>>  static const char *header_request_ssl_var(request_rec *r, char *name)
>>  {
>>      if (header_ssl_lookup) {
>> @@ -853,6 +863,7 @@
>>      register_format_tag_handler("D", (const void
>> *)header_request_duration);
>>      register_format_tag_handler("t", (const void 
>> *)header_request_time);
>>      register_format_tag_handler("e", (const void 
>> *)header_request_env_var);
>> +    register_format_tag_handler("n", (const void 
>> *)header_request_note);
>>      register_format_tag_handler("s", (const void 
>> *)header_request_ssl_var);
>>
>>      return OK;
>>
>> You should be able to apply it to a 2.2 mod_headers, recompile and be
>> done. Not a standard feature though.
>>
>> Regards,
>>
>> Rainer
>>
>> P.S.: Gr��e von Bonn nach Bonn
>>
>
> And from Schwaben too...
> I feel a bit naive after all the sophisticated technical stuff above, 
> in suggesting the following, but how about :
> If I understand the original post correctly, the whole point would be 
> to know, at the httpd level, which "worker" (Tomcat) actually 
> processed this request, right ?
> If so, why not have the desired response header added at the Tomcat 
> level ?
> The more or less standard workhorsse URLRewriteFilter could be used 
> here for that.
>     <rule>
>         <from>.*</from>
>         <set type="response-header" name="TC-worker">name of this 
> tomcat</set>
>     </rule>
> and then check this on the way back in Apache..
> That would also overcome Rainer's note about load-balancing above.
> As a matter of fact, I am quite sure that Tomcat itself already adds 
> some kind of response header to indicate that it was involved in this, 
> so it might just be a matter of proper Tomcat configuration.
>
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>

Hi Andr�,

thanks for that hint. I initially tried a similar approach as the 
URLRewriteFilter, by writing my own Filter-class.
The reason i would like to see, which ajp-worker handled my requests is, 
that i am working on an automated cluster-test, which should be able to 
detect failover-events between workers. So changing the web.xml will 
work, i am sure. But it would alter my test-subject (something i'd like 
to avoid), and i would have to add the filter to approximately 30 
web.xmls (something i would also like to avoid, i am lazy) ...
I will try both approaches (Filter vs. mod_rewrite) and let you know my 
results.

Thanks to you and Rainer!

Tsch�ss ins L�ndle

Jochen


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