[jira] Commented: (HBASE-2408) Add envelope around client<->server communication so can pass state along w/ data during interchange

["Andrew Purtell (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-2408?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12853463#action_12853463 ] 

Andrew Purtell commented on HBASE-2408:
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I like this idea. We also need a channel for flagging data as perhaps returned during degraded operation (e.g. temporarily read only due to riding over HDFS fail over). 

> Add envelope around client<->server communication so can pass state along w/ data during interchange
> ----------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-2408
>                 URL: https://issues.apache.org/jira/browse/HBASE-2408
>             Project: Hadoop HBase
>          Issue Type: Improvement
>            Reporter: stack
>
> This issue is about adding a dimension along which we can pass metadata on changes in client-server communications.  What i mean by this is that that while an HTable#get will return a RowResult, we need to also be able to also convey, messages like "I got the result for this row from a region other than the one you asked for -- update your cache with this new location".
> I can think of two examples where this mechanism could be useful.
> 1. HBASE-72 "'Normal' operation should not depend on throwing of exceptions (e.g. NotServingRegionException)".  Rather than have the server throw a NotServingRegionException as we do now as signal to client to go look elsewhere for the wanted data, we could instead signal the client to look elsewhere by setting a state in the envelope.
> 2. If a client asks for a row and meantime the region has split, if the regionserver queried is hosting the daughter that is carrying the wanted row, it could save the client hops by passing back the wanted row with a message in the envelope that client should update its cache removing parent and replacing with daughter location.
> AVRO rpc carries headers?  We could stuff our enveloping stuff there?  Or, shudder, if we used AVRO HTTP for RPC, we could do our messages as HTTP headers.

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