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https://issues.apache.org/jira/browse/HBASE-10679?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13924492#comment-13924492
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stack commented on HBASE-10679:
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+1 on patch. When the AtomicLong hits the max, it goes negative which should
be fine since we are toString the value. It then goes down all the ways around
the zero so all should be good. Nice one Honghua. I know its only a few lines
but probably took a lot longer than that to figure it out.
> 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: Feng Honghua
> Assignee: Feng Honghua
> Priority: Critical
> Attachments: HBASE-10679-trunk_v1.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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