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From Raghu Angadi <rang...@yahoo-inc.com>
Subject Re: different modes of inter process communication
Date Fri, 25 Sep 2009 18:58:45 GMT
Steve Loughran wrote:
> Raghu Angadi wrote:
>>
>> A heartBeat is also an RPC. When you pause Namenode for 30 sec the 
>> datanode's heartbeat thread just waits for 30 sec for its heartbeat 
>> RPC to return. Note that when you pause Namenode, the RPCs to it don't 
>> fail immediately. During this wait, DNs can perform other transactions 
>> like serving data to clients.
> 
> If the heartbeat were just telling the NN that the DN is alive, 

Thats not the case in Hadoop. Central servers don't actively contact 
their slaves. It's been a long standing problem. For e.g. anything that 
NN want to tell a DN, it has to be in the form of response to a 
heartbeat or another RPC.

Raghu.

> you 
> could do it with a UDP that didn't block the DN. If, however, the DN 
> needs to know/care that the NN is up, then you do need to care about the 
> state of the namenode. But you don't have to do it blocking; looking for 
> a UDP back some time later is all you need to do.
> 
> 


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