Investigation has found that a cause of the ArcGIS Server Web Adaptor being unavailable is due to the webadaptor.config file being blank. To understand why this is happening, here is some background to what the Web Adaptor does.
The Web Adaptor checks back to the ArcGIS Server site every 60 seconds by default. There are two reasons for this:
- To make sure that the Web Adaptor is still registered with the site
- To maintain a list of GIS Server machines registered with the site
Before version 10.2.2, when the Web Adaptor does these checks, it actually modifies the webadaptor.config file and updates the file, even if no changes are made.
There can be occasional incidents when there is a write failure, for example, if there is a power cut during modification or if the Web Adaptor application pool crashes. When this happens the webadaptor.config file gets wiped or becomes corrupted.
At 10.2.2, the Web Adaptor was improved to not write to the webadaptor.config file unless it has actually changed, so a write failure is less likely.
At 10.3, there are further modifications to better handle the check made by the Web Adaptor for whether it still registered with the site. During this check ArcGIS Server examines the config-store for the presence of a file with the name of the GUID associated with the Web Adaptor. If the file check fails, for example, if there is a network issue or a read failure, the server would have responded that the Web Adaptor was no longer registered and the webadaptor.config file would have been wiped. At 10.3, ArcGIS Server now responds with an HTTP 500 Internal Server Error and does not unregister itself.