Capturing SQL Exceptions

[docyes]

Does a mechanism exist for capturing SQL exceptions and returning the error message in a http response?

[agentzh]

It's possible to use nginx's "error_page" to override the default 500/411 exceptions.

For more complicated cases, please check out the ngx_lua module: http://github.com/chaoslawful/lua-nginx-module

Basically you can issue a subrequest from within your Lua code to an internal nginx location configured by ngx_postgres, and then process the subrequest responses (like sql exceptions) the way you like in Lua ;)

-agentzh

[PiotrSikora]

If you want to pass error message returned from PostgreSQL server, then no, it's not possible right now.

I'll add this to the TODO list (but there is no ETA).

[docyes]

Just following up if you have any suggested patch paths I could take in the the source tree. My use case is failures for columns that are unique and detecting the error message on the consumer end (eg., username, email fields).

[agentzh]

I think Piotr Sikora is working on $postgres_error_code and $postgres_error_message variables for this. Basically you can output these nginx variables via ngx_headers_more's more_set_headers, and capture these headers in Lua by inspecting the return values of ngx.location.capture or ngx.location.capture_multi in the context of ngx_lua :)

[PiotrSikora]

What @agentzh meant is that I have this on my imminent (current or next week) TODO list :)

I want to add $postgres_error and $postgres_error_oneline (for pretty printing), so that you would be able to add those variables as headers using ngx_headers_more. Would that work for your use case or would you prefer to have errors printed in the response body?

[docyes]

The $postgres_error and $postgres_error_oneline would definitely be useful and powerful. I do like your idea about having errors printed in the response body as I suspect a lot of users would love that feature including myself.

I've been using your module for www.geobacon.com and it's been rock solid.

[agentzh]

I wonder if it's feasible to encode such error code and error messages into the RDS output?

[PiotrSikora]

@agentzh: error code & message are already present in the RDS header.

[agentzh]

@PiotrSikora Nice :)

[agentzh]

@docyes when PiotrSikora implemented the $postgres_error varialbe, you can append the error message to your main response body by using echo_after_body: http://wiki.nginx.org/HttpEchoModule#echo_after_body

[PiotrSikora]

@agentzh: I thought about using ngx_echo for this, but how would echo_after_body differentiate error from non-error response?

[agentzh]

@docyes alternatively you can use ngx_lua + Lua + lua-rds-parser to generate a nice error page all by yourself now: see http://wiki.nginx.org/HttpLuaModule and https://github.com/agentzh/lua-rds-parser

[agentzh]

On Tue, Sep 13, 2011 at 5:28 PM, Piotr Sikora
reply@reply.github.com
wrote:

@agentzh: I thought about using ngx_echo for this, but how would echo_after_body differentiate error from non-error response?

That's a very good question :) We can do something like this as long
as the response status code can differentiate these two cases at
least:

location /foo {
    set $footer '';
    postgres_query ...;
    postgres_pass ...;
    header_filter_by_lua '
        if ngx.status ~= ngx.HTTP_OK then
            ngx.var.footer = "\\n\\n" .. ngx.var.postgres_error
        end
    ';
    echo_after_body $footer;
}

That is, appending empty string (in $footer) is a no-op for echo_after_body.

-agentzh