# Pointers on writing an Nginx module/application which needs to talk to multiple backends (sometimes **URL:** \ **Category:** NGINX\ **Created:** [March 8, 2012, 10:19am UTC](https://www.ruby-forum.com/t/pointers-on-writing-an-nginx-module-application-which-needs-to-talk-to-multiple-backends-sometimes/217674 "2012-03-08T10:19:58Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![luislavena](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/0ea827/32.png) [@luislavena](https://www.ruby-forum.com/u/luislavena)\ **Post date:** [March 8, 2012, 10:19am UTC](https://www.ruby-forum.com/t/pointers-on-writing-an-nginx-module-application-which-needs-to-talk-to-multiple-backends-sometimes/217674/1 "2012-03-08T10:19:58Z")
Hi, I am trying to do something similar to what is described in this older thread: [http://nginx.2469901.n2.nabble.com/How-can-a-content-handler-block-on-certain-events-before-sending-a-response-td5764856.html](http://nginx.2469901.n2.nabble.com/How-can-a-content-handler-block-on-certain-events-before-sending-a-response-td5764856.html) Below is what i am trying to achieve (my use-case): When my nginx process gets a request, i want to do the following things in order: ``` 1. Connect to a postgres database, retrieve some ``` information based on a subset of the original request args 2. Connect to a memcache server with subset of info from request. If possible, do 1) and 2) in parallel. 3. Construct an “upstream request” to one of my (http based) backend service with info from original request + memcache + postgres ( I wrote this upstream service by modifying the “upstream proxy” standard module) 4. Do some additional processing (filtering, ranking etc) on the response from my backend 5. Send final response to the caller. 6. Update cache if needed. Is there some sample code for modules that might have similarities to the above, that i can follow and learn? For #1 above, I am playing around with postgres module code [FRiCKLE Labs / nginx / ngx\_postgres](http://labs.frickle.com/nginx_ngx_postgres/) to communicate with postgres DB. Currently, for the sake of learning/experiments, i defined a separate upstream block for postgres in my config, and also defined a separate location block for db query, with the above postgres module hooked up to it, and in my own module i just do a subrequest to this postgres block. I followed the eval module code to do the subrequest (based on suggestions in this older thread [architecture of a module - processing a response from a subrequest](http://forum.nginx.org/read.php?29,114237)), but figured that eval does an in-memory subrequest, while Piotr’s postgres module above doesn’t support in-memory subrequest. Has anyone modified postgres module code to work with in-memory sub-requests? I also experimented with a simple case of firing in-memory subrequests from my module, to fetch a static file (instead of calling postgres location above), before i do my upstream backend call. Similar to the code in eval module ([https://github.com/vkholodkov/nginx-eval-module/blob/master/ngx\_http\_eval\_module.c](https://github.com/vkholodkov/nginx-eval-module/blob/master/ngx_http_eval_module.c)), in my ctx object, i have a “done”, and “in\_progress” flags. During the initial call to my handler, ctx-\>done is not set, and i create and fire a in-memory subrequest to a url that points to the static file, and return NGX\_DONE. And in my sub-request handler, i set the ctx-\>done, and return NGX\_OK. In my main request handler, if ctx-\>done is set, i have code which will do a call to my upstream backend (step #3 in my list) . However, the call never returns from sub-request handler to my main handler. What might i be doing wrong? I am currently not looking into nginx\_lua and other extensions, until i have a certain level of comfort with stock nginx module development. I’ve also tried suggestions and code from Piotr and Shaun, from the following thread: [Parallel subrequests for multi-source long polling? - NGINX - Ruby-Forum](http://www.ruby-forum.com/topic/199392) (Parallel subrequests for multi-source long polling) for creating independent sub-requests for step #1, Basically, i create an independent subrequest during the content phase of my module’s handler, and was hoping that i could (when i get response), setup the hooks for upstream functionality (create request etc) for my own module. (maybe i was wrong here) I had to call the ngx\_indep\_subreq\_postconf in the sample code, during the post-configuration phase of MY module, to get around some initial segfaults. But now i am stuck at another sigsegv, with backtrace below: Program received signal SIGSEGV, Segmentation fault. 0x080607a6 in ngx\_event\_connect\_peer (pc=0x9fbf1c4) at src/event/ngx\_event\_connect.c:30 30 s = ngx\_socket(pc-\>sockaddr-\>sa\_family, SOCK\_STREAM, 0); (gdb) where #0 0x080607a6 in ngx\_event\_connect\_peer (pc=0x9fbf1c4) at src/event/ngx\_event\_connect.c:30 #1 0x08080333 in ngx\_http\_upstream\_connect (r=0x9fd0eac, u=0x9fbf1bc) at src/http/ngx\_http\_upstream.c:1104 #2 0x08080da4 in ngx\_http\_upstream\_init\_request (r=0x9fd0eac) at src/http/ngx\_http\_upstream.c:632 #3 0x0808116b in ngx\_http\_upstream\_init (r=0x9fd0eac) at src/http/ngx\_http\_upstream.c:433 #4 0x080aa4c5 in ngx\_indep\_subreq\_init\_upstream (r=0x9fd0eac, url=0xbf965e60, callback=0x80a34cb \, callback\_data=0x9fd0360, upstream\_extensions=0xbf965d88) #5 0x080aa57f in ngx\_indep\_subreq\_fetch (r=0x9fc9698, pool=0x9fcfe30, url=0xbf965e60, callback=0x80a34cb \, callback\_data=0x9fd0360, upstream\_extensions=0xbf965d88) #6 0x080a51d3 in ngx\_http\_my\_broker\_handler (r=0x9fc9698) #7 0x0806e4c4 in ngx\_http\_core\_content\_phase (r=0x9fc9698, ph=0x9fd846c) at src/http/ngx\_http\_core\_module.c:1367 #8 0x0806a075 in ngx\_http\_core\_run\_phases (r=0x9fc9698) at src/http/ngx\_http\_core\_module.c:862 … I am trying to see if either sub-requests, or independent subrequests (with some modifications to the postgres module code) will suffice for my use-case, or will i need to work directly with nginx events/nginx\_event\_connect\_peer etc. Any help is appreciated. Thanks! Ashish ---
**Author:** ![Ashish\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/35a633/32.png) [@Ashish\_S](https://www.ruby-forum.com/u/Ashish_S)\ **Post date:** [March 8, 2012, 2:15pm UTC](https://www.ruby-forum.com/t/pointers-on-writing-an-nginx-module-application-which-needs-to-talk-to-multiple-backends-sometimes/217674/2 "2012-03-08T14:15:24Z")
On 08/03/2012 09:19, Ashish S wrote: > Hi, > > I am trying to do something similar to what is described in this older > thread: > [http://nginx.2469901.n2.nabble.com/How-can-a-content-handler-block-on-certain-events-before-sending-a-response-td5764856.html](http://nginx.2469901.n2.nabble.com/How-can-a-content-handler-block-on-certain-events-before-sending-a-response-td5764856.html) > > Below is what i am trying to achieve (my use-case): Does it need to be an nginx module, or could you use CGI of some sort? eg you could knock this up in your favourite scripting language very easily? Good luck Ed W ---
**Author:** ![Ashish\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/35a633/32.png) [@Ashish\_S](https://www.ruby-forum.com/u/Ashish_S)\ **Post date:** [March 8, 2012, 2:31pm UTC](https://www.ruby-forum.com/t/pointers-on-writing-an-nginx-module-application-which-needs-to-talk-to-multiple-backends-sometimes/217674/3 "2012-03-08T14:31:40Z")
Hi, > but figured that eval does > an in-memory subrequest, while Piotr’s postgres module above doesn’t > support in-memory subrequest. Has anyone modified postgres module > code to work with in-memory sub-requests? agentzh forked eval module and made it work with regular subrequests (so also with ngx\_postgres): > **[GitHub - openresty/nginx-eval-module: A module for evaluating memcached or proxy...](https://github.com/openresty/nginx-eval-module)** > > A module for evaluating memcached or proxy response into variable Alternatively, you could use ngx.location.capture() feature from ngx\_lua module to make the subrequests to PostgreSQL location. I also believe that the first test case from [ngx\_postgres/t/eval.t at master · FRiCKLE/ngx\_postgres · GitHub](https://github.com/FRiCKLE/ngx_postgres/blob/master/t/eval.t) might be of interest to you. Best regards, Piotr S. \< [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) \> ---
**Author:** ![Ashish\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/35a633/32.png) [@Ashish\_S](https://www.ruby-forum.com/u/Ashish_S)\ **Post date:** [March 9, 2012, 3:27am UTC](https://www.ruby-forum.com/t/pointers-on-writing-an-nginx-module-application-which-needs-to-talk-to-multiple-backends-sometimes/217674/4 "2012-03-09T03:27:38Z")
Thank you Piotr. Agentzh’s fork of eval module works for me. I am now trying to implement #2 - #6 now. Are there sample 3rd party modules which parallely talk to postgres and some other backend service (rather, more than one backend services in parallel), then merge results from the two, followed by an upstream call to another backend service? Thanks, Ashish