Critical Description
When mapping hosts concurrently with reading information from a cluster, there was a deadlock between the building the cluster health report and mapping the new hosts.
A few changes to note here:
- ClustersImpl uses concurrent maps; there's really no need to keep the internal lock. I removed it in several places where the cluster is guaranteed to be available (such as when using the ID to retrieve the cluster). The concurrent maps guard against concurrent modifications.
- The Ambari Event Publisher was actually synchronous. This not only caused bottlenecks, but also contributed to a secondary deadlock detected while fixing the original issue. It was changed into a single-threaded asynchronous bus. Consumers of this bus should never rely on it to perform its actions in order to perform their own logic, so changing the behavior seemed correct
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