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From Steve Loughran <ste...@apache.org>
Subject Re: different modes of inter process communication
Date Fri, 25 Sep 2009 15:24:15 GMT
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, 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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