nginx map directive: why is it allowed only on http level?

Question

There is a very useful directive in Nginx map.

But it is possible to use it only on the http level (see docs here http://nginx.org/en/docs/http/ngx_http_map_module.html#map).

For example, I have a server defined and I would like to use some conditional redirects in this server using $url variable. It would be very handy to use this directive inside server but it is not possible. Why?

Yes, I can do it also on the http level but there may be different servers defined on the http level and I would like to keep these conditions inside server section they are defined for.

Answer 1

Pretty old post but I realy want to bring some light into the darkness. The answer itself is quite simple.

DR;TL Variables in NGINX are always global and once defined accessable from anywhere in the configration. Therfore it would not make any sense to define a map in a server or location block.

map creates a new variable whose value depends on values of one or more of the source variables specified in the first parameter.

example configuration:

map $host $myvar { example.com "test"; foo.com     "for";  } 

As variables in NGINX are ALWAYS global and once defined available anywhere else in the configuration. So it wouldn't make any sense to move the map into a location or server block. The interesting fact with our map directive is when the variable myvar will receive its value or when it will be assigned?

map assigns the value to the variable once the variable will be used in your configuration

That means you can define the map in the http context but the value will be assigned at the point you are accessing $myvar in your nginx configuration.

Back to your question: As NGINX variables are always global having a map per server block would make sense as they would be global anyway.