How to configure nginx proxying of requests according to the table of correspondences?

Question

Nginx

I have 2 Apache servers: a_1 and a_2
There is a correspondence table
Server | Address
a_1 | 1
a_1 | 2
a_1 | 3
a_2 | 4
a_2 | 5
a_2 | 6
At the front, the address looks like this:
/a/1
, where a is the server attribute
, 1 is the page address
How to make nginx redirect to the required apache when specifying the page address.
The number of apache servers and pages varies, but it is known exactly from the correspondence table on which of the servers which page is located.
The same page cannot be located on different servers.

Answer 1

Sketch, not checked for validity.

server {
  map $uri $apache_host {
  ~ ^/a/1 a_1
  ~ ^/a/2 a_1
  ~ ^/a/3 a_1
  ~ ^/a/4 a_2
  ~ ^/a/5 a_2
  ~ ^/a/6 a_2
  }
  if ($apache_host) {
    proxy_pass $apache_host;
  }
}