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From Jim Kellerman <...@powerset.com>
Subject RE: HBase UnknownScannerException
Date Mon, 29 Oct 2007 23:12:16 GMT
UnknownScannerException is thrown by the region server if a scanner request (next or close)
is called with either a bogus scanner id (unlikely since the scanner id is hidden from the
client application) or if the scanner's lease has expired.

Every client request is given a lease by the region server. If the lease times out, the region
server thinks that the client has gone away and it can clean up any resources being held for
that request. Lease timeouts almost always occur on operations related to scanners. If a scanner's
lease times out, UnknownScannerException will be thrown if the client issues a scanner request
after the timeout because the region server has already cleaned up the resources associated
with the scanner. (The default lease timeout for client requests is 30 seconds, so it is very
likely that this is the situation you are running into).

If you attempt to do an update to the same table that you have a scanner open on, the update
will stall waiting for the scanner to complete. So a loop which does

while (scanner.next(key, val)) {
  if (key.equals(something) || val.equals(somethingElse)) {

will not work because the update will wait for the scanner to finish, (which it won't), and
both the update and the next scanner.next should both fail.

Hope that helps.
Jim Kellerman, Senior Engineer; Powerset

> -----Original Message-----
> From: Cedric Ho [mailto:cedric.ho@gmail.com]
> Sent: Thursday, October 25, 2007 7:58 PM
> To: hadoop-user@lucene.apache.org
> Subject: HBase UnknownScannerException
> Hi,
> I was trying HBase from the 0.15 branch. And was doing:
> HScannerInterface s = table.obtainScanner(...)
> while(s.next(key, val)) {
>     ....
> }
> And encounter the following exception whenever I spent too
> much time in the while loop, (e.g. > 30 ~ 120 seconds). I am
> also doing insert in the same table in another process. But
> the records they are working on are definitely non-overlapping.
> org.apache.hadoop.ipc.RemoteException:
> org.apache.hadoop.hbase.UnknownScannerException: Name:
> -7220522571873774180
>         at
> org.apache.hadoop.hbase.HRegionServer.next(HRegionServer.java:1075)
>         at
> sun.reflect.GeneratedMethodAccessor19.invoke(Unknown Source)
>         at
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMeth
> odAccessorImpl.java:25)
>         at java.lang.reflect.Method.invoke(Method.java:585)
>         at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:379)
>         at org.apache.hadoop.ipc.Server$Handler.run(Server.java:596)
>         at org.apache.hadoop.ipc.Client.call(Client.java:482)
>         at org.apache.hadoop.ipc.RPC$Invoker.invoke(RPC.java:184)
>         at $Proxy1.next(Unknown Source)
>         at
> org.apache.hadoop.hbase.HTable$ClientScanner.next(HTable.java:831)
>         ...
> there are totally 6 machines:
> one running namenode, secondarynamenode, hbase master five
> running datanode, regionserver
> Cheers,
> Cedric

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