[GitHub] [hadoop] SanQiMax commented on pull request #4663: HDFS-16699:Router Update Observer NameNode state to Active when failo…

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SanQiMax commented on PR #4663:
URL: https://github.com/apache/hadoop/pull/4663#issuecomment-1200967858

   > @SanQiMax Thank you very much for your contribution, but in JIRA, your description needs to be clearer, you can refer to @ZanderXu's Jira description, he wrote very well.
   
   The router will obtain all nn lists (randomly sorted) before forwarding a read request each time. If it is a write request, it will obtain the Active and Standby、OBserver nn lists, and access them in turn until the request is processed successfully.
   From the theoretical analysis, if the router processes the read request and the first one in the nn list is the observer nn, and the observer nn  throw a ConnectTimeoutException to the router when processing the request, and the second time it will try to connect to the second nn from the nn list  , if the second nn connects  observer nn and returns successfully, the logic shown in the following figure will be executed, and the state of nn will be set to ACTIVE, then if the write request is processed, the router will obtain the list of active states (at this time it may be It will get multiple NNs in the ACTIVE state), and the request will be forwarded to the slave NN.
   This logic will  be abnormal when there are only active and standby nn in the cluster. Even if the status of standby nn is set to ACTIVE and the request is transferred to standby nn, the standby nn will not process read and write requests, and then the router will Forwarded to the Active nn for processing; if the observer nn is added, the state of the slave nn may be set to ACTIVE


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