Document postgres_set functionality in control flow

[Downchuck]

The eval module has been deprecated, "production use is discouraged"; is the postgres_set feature usable at all in the nginx control flow? There does not seem to be a means of passing the value of postgres_set into an if statement, redirect or error page; the value seems to be opaque until sending it to the client.

[agentzh]

You're recommended to use the ngx_lua module with ngx_postgres, which supports all the magic of ngx_eval (and much more!) and is considered stable for long: http://wiki.nginx.org/HttpLuaModule

[PiotrSikora]

You can use it with ngx_lua (http://wiki.nginx.org/HttpLuaModule), ngx_http_auth_request_module (http://mdounin.ru/hg/ngx_http_auth_request_module/) or anything else that uses subrequests.

You cannot use it as a conditional in an if statement (without using subrequests), because if statements are evaluated at the rewrite phase, long time before SQL query is actually sent to the database.

[Downchuck]

Does that include postgres_rewrite? I'm currently using postgres_rewrite + error_page; not currently using postgres_set.

[PiotrSikora]

What are you trying to do?

[Downchuck]

I'm currently just using "select true from table where" for authentication before passing over to the push stream module. I'd like to select a value from the table to check if the user should be redirect to another server.

This is working well: postgres_rewrite rows 410; postgres_rewrite no_rows 403; error_page 410 = /json/subscribe?$args;

At this point, I'm checking if this will work: postgres_set $value 0 0 optional; error_page 410 = /json/subscribe?value=$value&$args;

[PiotrSikora]

I'd recommend using either auth request module or ngx_lua's access_by_lua to do that, i.e.:

location = /auth {
    internal;

    postgres_escape   $user $remote_user;
    postgres_escape   $pass $remote_passwd;

    postgres_pass     database;
    postgres_query    "SELECT optional FROM users WHERE login=$user AND pass=$pass";
    postgres_rewrite  no_rows 403;
    postgres_output   none;
    postgres_set      $postgres_optional 0 0 required;
}

location / {
    auth_request /auth;
    auth_request_set $optional $postgres_optional;

    if ($optional != "") {
        # do something
    }

    # do something else
}