It happened in YARN nodemanger scenario. But it could happen to any long running service that use cached instance of DistrbutedFileSystem.
1. Active NN is under heavy load. So it became unavailable for 10 minutes; any DFSClient request will get ConnectTimeoutException.
2. YARN nodemanager use DFSClient for certain write operation such as log aggregator or shared cache in
YARN-1492. DFSClient used by YARN NM's renewLease RPC got ConnectTimeoutException.
3. After DFSClient is in Aborted state, YARN NM can't use that cached instance of DistributedFileSystem.
We can make YARN or DFSClient more tolerant to temporary NN unavailability. Given the callstack is YARN -> DistributedFileSystem -> DFSClient, this can be addressed at different layers.
- YARN closes the DistributedFileSystem object when it receives some well defined exception. Then the next HDFS call will create a new instance of DistributedFileSystem. We have to fix all the places in YARN. Plus other HDFS applications need to address this as well.
- DistributedFileSystem detects Aborted DFSClient and create a new instance of DFSClient. We will need to fix all the places DistributedFileSystem calls DFSClient.
- After DFSClient gets into Aborted state, it doesn't have to reject all requests , instead it can retry. If NN is available again it can transition to healthy state.