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From "Hudson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10679) Both clients get wrong scan results if the first scanner expires and the second scanner is created with the same scannerId on the same region
Date Thu, 13 Mar 2014 03:34:44 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10679?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13932840#comment-13932840
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Hudson commented on HBASE-10679:
--------------------------------

FAILURE: Integrated in hbase-0.96-hadoop2 #236 (See [https://builds.apache.org/job/hbase-0.96-hadoop2/236/])
HBASE-10679 Both clients get wrong scan results if the first scanner expires and the second
scanner is created with the same scannerId on the same region (stack: rev 1576930)
* /hbase/branches/0.96/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java


> Both clients get wrong scan results if the first scanner expires and the second scanner
is created with the same scannerId on the same region
> ---------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-10679
>                 URL: https://issues.apache.org/jira/browse/HBASE-10679
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver
>            Reporter: Honghua Feng
>            Assignee: Honghua Feng
>            Priority: Critical
>             Fix For: 0.96.2, 0.98.1, 0.99.0
>
>         Attachments: HBASE-10679-trunk_v1.patch, HBASE-10679-trunk_v2.patch, HBASE-10679-trunk_v2.patch,
HBASE-10679-trunk_v2.patch
>
>
> The scenario is as below (both Client A and Client B scan against Region R)
> # A opens a scanner SA on R, the scannerId is N, it successfully get its first row "a"
> # SA's lease expires and it's removed from scanners
> # B opens a scanner SB on R, the scannerId is N too. it successfully get its first row
"m"
> # A issues its second scan request with scannerId N, regionserver finds N is valid scannerId
and the region matches too. (since the region is always online on this regionserver and both
two scanners are against it), so it executes scan request on SB, returns "n" to A -- wrong!
(get data from other scanner, A expects row something like "b" that follows "a")
> # B issues its second scan request with scannerId N, regionserver also thinks it's valid,
and executes scan on SB, return "o" to B -- wrong! (should return "n" but "n" has been scanned
out by A just now)
> The consequence is both clients get wrong scan results:
> # A gets data from scanner created by other client, its own scanner has expired and removed
> # B misses data which should be gotten but has been wrongly scanned out by A
> The root cause is scannerId generated by regionserver can't be guaranteed unique within
regionserver's whole lifecycle, *there is only guarantee that scannerIds of scanners that
are currently still valid (not expired) are unique*, so a same scannerId can present in scanners
again after a former scanner with this scannerId expires and has been removed from scanners.
And if the second scanner is against the same region, the bug arises.
> Theoretically, the possibility of above scenario should be very rare(two consecutive
scans on a same region from two different clients get a same scannerId, and the first expires
before the second is created), but it does can happen, and once it happens, the consequence
is severe(all clients involved get wrong data), and should be extremely hard to diagnose/debug



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