Manage maintenance pages

Maintenance pages are used for two use cases:

• For applications which are temporarily unavailable, e.g., due to a software update.
• For nevisProxy instances which shall become disabled within an upstream load balancer using a heartbeat URL to check the availability of a nevisProxy.

Maintenance pages are managed by the MaintenanceFilter of nevisProxy. This filter is able to poll a pre-defined maintenance periodically and reads the HTML meta information out of the file. If the meta information tells the filter to show the maintenance page instead of letting an HTTP request pass the proxy, the filer answers the request by returning the maintenance page.