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https://issues.apache.org/jira/browse/HADOOP-15707?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16603492#comment-16603492
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Botong Huang commented on HADOOP-15707:
---------------------------------------

Looks good to me overall. Only one small ask: can we change 
IsActiveServlet.PATH_SPEC to "/isactive" to avoid upper/lower case confusion in 
the future? 

> Add IsActiveServlet to be used for Load Balancers
> -------------------------------------------------
>
>                 Key: HADOOP-15707
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15707
>             Project: Hadoop Common
>          Issue Type: New Feature
>          Components: common
>            Reporter: Lukas Majercak
>            Assignee: Lukas Majercak
>            Priority: Major
>         Attachments: HADOOP-15707.000.patch, HADOOP-15707.001.patch, 
> HADOOP-15707.002.patch, HADOOP-15707.003.patch, HADOOP-15707.004.patch
>
>
> Hadoop has a few services with HA setups and it is common to set them behind 
> Load Balancers.
> We should add a way for the Load Balancers to understand what should be the 
> UI to show.
> For example, the standby RM just redirects the requests to the active RM.
> However, if both RMs are behind a Load Balancer the IP might not be reachable.
> Most Load balancers have probes to check if a server reports HTTP code 200:
> * 
> [Azure|https://docs.microsoft.com/en-us/azure/load-balancer/load-balancer-custom-probe-overview]
> * 
> [AWS|https://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-healthchecks.html]
> Components in Hadoop (e.g., NN, RM, Router,...) should have a unified way to 
> report if they are active.



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