Alerting on memory use and instance crash

1)If we want to have alerting on any node/instance that gets crashed :- In other databases like Oracle the catalog Views like "GV$Instance" used to give information on whether the instances are currently active/down or not. But in postgres it seems all the pg_* views are instance specific and are not showing information on the global/cluster level but are restricted to instance level only. So is there any other way to query the pg_* views to have alerts on the specific instance crash?
2)Is there a way to fetch the data from pg_* view to highlight the specific connection/session/sqls which is using high memory in postgres?