Calling external api in Nginx location section - nginx

Question

I am trying to resolve proxy_pass value dynamically (through web api) in nginx.
I need something like below;
Example taken from: https://www.nginx.com/resources/wiki/start/topics/depth/ifisevil/
location /proxy-pass-uri {
set $urlToProxy = CallWebAPI("http://localhost:8081/resolver?url=" + $url);
proxy_pass $urlToProxy;
}
So, my question is that, is it possible to make HTTP request or to write method such as CallWebAPI?
I know it might be a bad practice, but the website I am dealing with has thousands of web urls, which are mapped as key-value pairs, and 90% of them does not obey any specific regex rules. So I have content mapped database, and I need to fetch incoming url with content dynamically.
I am trying to use a very light web service to look up URLs from redis, and return proxy url.
Would this be a valid scenario, or is there any other built in solution in nginx like this?

Answer 1

I doubt this can be done with "pure" nginx, but this definitely can be done with openresty or ngx_http_lua_module with the help of ngx.location.capture method. For example:
resolver 8.8.8.8;
location ~/proxy-pass-uri(/.*)$ {
set $url $1;
set $proxy "";
access_by_lua_block {
res = ngx.location.capture("http://localhost:8081/resolver?url=" .. ngx.var.url)
ngx.var.proxy = res.body
}
proxy_pass $proxy$url;
}
There is also an ngx_http_js_module (documentation, GitHub) which have an ability to do subrequests (example), but I never used it and cannot tell if it can be used this way.
Important update
After almost a three years since this answer was written, it comes that I needed the similar functionality myself, and it turns out that the above answer is completely broken and unworkable. You can't do a subrequest via ngx.location.capture to anything else but to some other nginx location. So the correct (checked and confirmed to be workable) example for the above question is
resolver 8.8.8.8;
location /resolver {
internal;
proxy_pass http://localhost:8081;
}
location ~ ^/proxy-pass-uri(/.*)$ {
set $url $1;
set $proxy "";
access_by_lua_block {
res = ngx.location.capture("/resolver?url=" .. ngx.var.url)
if res.status == ngx.HTTP_OK then
ngx.var.proxy = res.body
else
ngx.exit(res.status)
end
}
proxy_pass $proxy$url$is_args$args;
}
The above example assumes that the proxy resolution service is really expecting request in a /resolver?url=<uri> form. The location /resolver { ... } while being internal behaves like any other prefix location, so if the /resolver prefix for that location cannot be used for some reason, the same can be written as
resolver 8.8.8.8;
location /get_proxy {
internal;
proxy_pass http://localhost:8081/resolver;
}
location ~ ^/proxy-pass-uri(/.*)$ {
set $url $1;
set $proxy "";
access_by_lua_block {
res = ngx.location.capture("/get_proxy?url=" .. ngx.var.url)
if res.status == ngx.HTTP_OK then
ngx.var.proxy = res.body
else
ngx.exit(res.status)
end
}
proxy_pass $proxy$url$is_args$args;
}