When the nginx proxy_pass command address is a domain name, nginx starts to report to host not found in upstream

when I want to visit the specified uri of the local website, I actually visit another website, so I use nginx as the proxy

nginx related configuration is as follows
server {

listen 80;
server_name www.example.com;
.....

location /foo/ {
    proxy_store off;
    proxy_redirect off;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header Referer "no-referrer-when-downgrade";
    proxy_set_header User-Agent "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebkit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.62 Safari/537.36";
    proxy_pass https://api.example.com/foo/;
}

}

nginx startup Times error host not found in upstream "api.example.com"

Linux nginx
Apr.11,2021

resolved 

location /foo/ {
    resolver 8.8.8.8; 
    set $backend "api.example.com";
    proxy_pass https://$backend;
}

there is no 'api.example.com',' in your upstream configuration. Can you post your upstream configuration

?

something like this 

http {
    upstream myapp1 {
        server srv1.example.com;
        server srv2.example.com;
        server srv3.example.com;
    }

    server {
        listen 80;

        location / {
            proxy_pass http://myapp1;
        }
    }
}

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3627f-2b76b.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3627f-2b76b.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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