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From "CR Hota (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-13522) Support observer node from Router-Based Federation
Date Thu, 02 May 2019 17:53:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-13522?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16831823#comment-16831823

CR Hota commented on HDFS-13522:

[~elgoiri] [~ayushtkn] [~csun] Thanks a lot for the discussion and [~surendrasingh] many
thanks for your initial patch. Great to see more interest in this work.

Based on understanding, below is the problem statement and design exit criteria we may want
to look at. Attached is a file that represents the problem statement pictorially.
h1. Problem statement 

Figure 1 in attached Router+Observer RPC clogging.png shows a typical RPC mechanism with respect
to active namenode and observer namenode. In this case, observer namenodes are strictly processing
read only requests from clients. Since there is no global lock or contention, RPC queue wait
times are lower and processing times are significantly faster in Observer namenodes when compared
to active namenode. With router based federation, a proxy layer is introduced that serves
incoming client traffic on behalf of the client and performs the actual action against the
downstream namenode. _This server proxy layer inherits the same server implementation that
name nodes utilize_. With a single RPC queue in router, all read and writes will get intermingled
again thus substantially diminishing the benefits of Observer namenode. Figure 2 illustrates
this behavior. This is particularly problematic for rpc latency sensitive real time engines
such as Presto.
h1. Design criteria 

On a high level, the design of this feature should help achieve below 2 key objectives.
 # Separation of read vs write queuing in routers to begin with to fundamental continue maintaining
a fast lane access for read calls. This work should create foundations to help to eventually
separate read vs write per nameservice thus helping solve HDFS-14090.
 # Honor existing client configurations around whether or not to access Observer NN for certain
use cases. For ex: a client can currently continue using ConfiguredFailOverProxy without connecting
to Observer. If a client maintains such preference, routers should honor it and connect to
Active Namenode for read calls as well.



> Support observer node from Router-Based Federation
> --------------------------------------------------
>                 Key: HDFS-13522
>                 URL: https://issues.apache.org/jira/browse/HDFS-13522
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: federation, namenode
>            Reporter: Erik Krogen
>            Assignee: Chao Sun
>            Priority: Major
>         Attachments: HDFS-13522.001.patch, Router+Observer RPC clogging.png
> Changes will need to occur to the router to support the new observer node.
> One such change will be to make the router understand the observer state, e.g. {{FederationNamenodeServiceState}}.

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