How can I set the Nginx upstream port using a variable?

Setting the Nginx upstream port using a variable allows for more dynamic configurations, especially when dealing with multiple applications or environments. Here's how you can achieve this:

1. Using Nginx Variables: Nginx supports the use of variables within its configuration files. These can be set based on different conditions or values, including environment variables.

2. Setting the Variable: You can define a variable with the port number. This can be done in the `http` block or inside a `server` block, depending on the scope needed. For example:

http {
  map $http_host $backend_port {
    default 8080;
    example.com 8081;
    api.example.com 8082;
  }
... }

In this example, the `$backend_port` variable is set based on the `Host` header. If it is example.com then the port will be 8081.

Alternatively, you can use environment variables, for example:

http {
  env PORT;
  ... }

Then use the variable as follows:

server {
  ...
  location / {
    proxy_pass http://127.0.0.1:$PORT;
    ...
  }
  ...
}

3. Using the Variable in `upstream` Block: The variable can now be incorporated directly into the `upstream` definition.

upstream backend {
  server 127.0.0.1:$backend_port;
}

4. Proxying with the `upstream`: When proxying requests, use the defined `upstream` block:

server {
  listen 80;
  server_name example.com;
  location / {
    proxy_pass http://backend;
    ...
  }
}

5. Important Considerations:

- Scope: Variables have scope. Ensure that the variable is accessible in the context where you use it.

- Variable Validation: Be cautious about using variables from external sources without proper validation to prevent security issues.

- Reload Nginx: After making changes, reload Nginx for the configurations to take effect: `sudo nginx -s reload`

By using variables, your Nginx configurations become more versatile, making it easier to handle changing port numbers and different application environments.

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