It used to be that RHQ's availability scanning was pretty rigid. RHQ scanned every resource on a fixed 5 minute schedule. Though this was configurable, it was only configurable on an agent-wide scale (meaning, you could not say that you want to check resource A every 5 minutes, but resource B every 1 minute).
In addition, when the agent would go down (for whatever reason), it took a long time (around 15 minutes by default) for the RHQ Server to consider the agent to be "missing in action" and mark all of the resources that agent was managing as "down". Prior to those 15 minutes (what RHQ calls the "Agent Max Quiet Time Allowed"), you really had no indication there was a problem and some users found this to be too long before alerts started firing. And even though this Agent Max Quiet Time Allowed setting is configurable, under many situations, setting it to something appreciably lower than 15 minutes would cause the RHQ Server to think agents were down when they weren't, triggering false alerts.
RHQ has since addressed these issues. Now, you will notice all server and service resources have a common "Availability" metric in their Monitoring>Schedules subtab in the GUI. This works just like other metric schedules - you can individually configure these as you wish just like normal metrics. By default, all server resources will have their availability checked every 1 minutes (for example, see the server resource snapshot in figure 1). By default, all service resources will have their availability checked every 10 minutes (for example, see the service resource snapshot in figure 2).
|Figure 1 - A server resource and its Availability metric, with default interval of 1 minute|
|Figure 2 - A service resource and its Availability metric, with default interval of 10 minutes|
Another change that was made is that when an agent gracefully shuts down, it sends one final message to the RHQ Server to indicate it is shutting down and at that point the RHQ Server immediately marks that agent's platform as "down" rather than wait for the agent to go quiet for a period of 15 minutes (the Agent Max Quiet Time Allowed period) before being marked "down". Additionally, although the platform is marked "down", that platform's child resources (all of its servers and services) are now marked as "unknown". This is because, technically, RHQ doesn't really know what's happening on that box now that the agent is no longer reporting in. Those managed resources that the agent was monitoring (the servers and services) could be down, but RHQ can't make that determination - they could very well still be up. Therefore, the RHQ Server places all the servers and services in the new availability state known as "unknown" as indicated by the question-mark-within-a-gray-circle icon. See figure 3 as an example.
|Figure 3 - Availability data for a service that has been flagged as being in an "unknown" state|