What is the purpose of http_cache_hosts entry in app/etc/env.php?

To make Magento aware of the Varnish servers, add the following section to app/etc/env.php. For Example:

'http_cache_hosts' =>
  array (
    0 =>
    array (
      'host' => 'varnish-0',
      'port' => '80',
    ),
    1 =>
    array (
      'host' => 'varnish-1',
      'port' => '80',
    ),
  ),

In order to ensure the Varnish cache can be cleared, you must also add a node to your env.php to tell Magento how to communicate with Varnish

I hope this will help