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Minor Description
While executing a mapreduce job in an environment utilizing Router-Based Federation with Observer read enabled, there is an estimated 1% chance of encountering the following error.
"java.io.IOException: Resource hdfs://XXXX/user/XXXX/.staging/job_XXXXXX/.tez/application_XXXXXX/tez-conf.pb changed on src filesystem - expected: \"2023-07-07T12:41:16.801+0900\", was: \"2023-07-07T12:41:16.822+0900\", current time: \"2023-07-07T12:41:22.386+0900\"",
This error happens in function verifyAndCopy inside FSDownload.java when nodemanager tries to download a file right after the file has been written to the HDFS. The write operation runs on active namenode and read operation runs on observer namenode as expected.
The edits file and hdfs-audit files indicate that the expected timestamp mentioned in the error message aligns with the OP_CLOSE MTIME of the 'tez-conf.pb' file (which is correct). However, the actual timestamp retrieved from the read operation corresponds to the OP_ADD MTIME of the target 'tez-conf.pf' file (which is incorrect). This inconsistency suggests that the observer namenode responds to the client before its edits file is updated with the latest stateId.
Further troubleshooting has revealed that during write operations, the router responds to the client before receiving the latest stateId from the active namenode. Consequently, the outdated stateId is then used in the subsequent read operation on the observer namenode, leading to inaccuracies in the information provided by the observer namenode.
To resolve this issue, it is essential to ensure that the router sends a response to the client only after receiving the latest stateId from the active namenode.
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