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https://issues.apache.org/jira/browse/HBASE-15576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16022592#comment-16022592
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Phil Yang commented on HBASE-15576:
-----------------------------------
Keep the isCursor() logic but don't use getRow in Result, users should
getCursor() to get the Cursor object.
> Scanning cursor to prevent blocking long time on ResultScanner.next()
> ---------------------------------------------------------------------
>
> Key: HBASE-15576
> URL: https://issues.apache.org/jira/browse/HBASE-15576
> Project: HBase
> Issue Type: New Feature
> Reporter: Phil Yang
> Assignee: Phil Yang
> Fix For: 2.0.0, 1.4.0
>
> Attachments: HBASE-15576.v01.patch, HBASE-15576.v02.patch
>
>
> After 1.1.0 released, we have partial and heartbeat protocol in scanning to
> prevent responding large data or timeout. Now for ResultScanner.next(), we
> may block for longer time larger than timeout settings to get a Result if the
> row is very large, or filter is sparse, or there are too many delete markers
> in files.
> However, in some scenes, we don't want it to be blocked for too long. For
> example, a web service which handles requests from mobile devices whose
> network is not stable and we can not set timeout too long(eg. only 5 seconds)
> between mobile and web service. This service will scan rows from HBase and
> return it to mobile devices. In this scene, the simplest way is to make the
> web service stateless. Apps in mobile devices will send several requests one
> by one to get the data until enough just like paging a list. In each request
> it will carry a start position which depends on the last result from web
> service. Different requests can be sent to different web service server
> because it is stateless.
> Therefore, the stateless web service need a cursor from HBase telling where
> we have scanned in RegionScanner when HBase client receives an empty
> heartbeat. And the service will return the cursor to mobile device although
> the response has no data. In next request we can start at the position of
> cursor, without the cursor we have to scan from last returned result and we
> may timeout forever. And of course even if the heartbeat message is not empty
> we can still use cursor to prevent re-scan the same rows/cells which has beed
> skipped.
> Obviously, we will give up consistency for scanning because even HBase client
> is also stateless, but it is acceptable in this scene. And maybe we can keep
> mvcc in cursor so we can get a consistent view?
> HBASE-13099 had some discussion, but it has no further progress by now.
> API:
> In Scan we need a new method setNeedCursorResult(true) to get the cursor row
> key when there is a RPC response but client can not return any Result. In
> this mode we will not block ResultScanner.next() longer than this timeout
> setting.
> {code}
> while (r = scanner.next() && r != null) {
> if(r.isCursor()){
> // scanning is not end, it is a cursor, save its row key and close scanner
> if you want, or
> // just continue the loop to call next().
> } else {
> // just like before
> }
> }
> // scanning is end
> {code}
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