nginx proxy_pass fallback location

Question

I'm using nginx as a reverse proxy for my web application with proxy_pass for the api. This is my current config, where I've nginx directive location /data/ where images are being displayed from the /data directory of my docker container.

location /data/ {
    alias /data/;
    error_page 404 = @api;
    recursive_error_pages on;
    log_not_found off;
    autoindex off;
    include cache.conf;
}

location /api/ {
    error_page 418 = @api;
    return 418;
}

location @api {
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-NginX-Proxy true;
    proxy_buffering off;
    proxy_request_buffering off;

    proxy_http_version 1.1;
    proxy_pass http://api:1338;
    proxy_cache_bypass $http_upgrade;
}

How to add fallback location to the same named location nginx directive /data/ where I need to add public s3 bucket's url in proxy_pass and at first tries to search for that image inside that s3 bucket using proxy_pass, if found then serves that image from there, if returns 403 or 404 then use alias directive /data/ where it will serve that image from the /data directory of container.

I've tried with this config file, but it fails the nginx config rules check when I validate the nginx configuration using nginx -t command, and shows error nginx: [emerg] the "alias" directive cannot be used inside the named location

server {
    location /data/ {
        proxy_pass http://bucket_url;
        error_page 404 500 502 503 504 = @fallback;
    }

    location @fallback {
        alias /data/;
    }
}

Answer 1

If your /data/ strings from location /data/ { ... } and alias /data/ are equal, you can use root directive in the named location:

location @fallback {
    root /;
}

If those string are different, lets say /data1/ and /data2/, the following workaround is possible:

location @fallback {
    root /;
    rewrite ^/data1(/.*) /data2$1 break;
}