Problem: Service Temporarily Unavailable is showing on every page of my website after upgrade or adding an extension:

Solution:

Magento 2 maintenance mode file has the .maintenance.flag (with the leading dot) and is located in the var folder.
But HTTP 503 code response can have different causes. For example, the cause could be a webserver misconfiguration or file/permissions/ownership errors.

In lots of cases, after installing a new extension and recompiling Magento 2 code, you need to set file and folder permissions again.

After running these ...