Feng Honghua created HBASE-10679:
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Summary: Both clients operating on a same region will get wrong
scan results if the first scanner expires and the second scanner is created
with the same scannerId
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
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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