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From Harsh J <ha...@cloudera.com>
Subject Re: Lease does not exist exceptions
Date Thu, 20 Sep 2012 13:55:02 GMT
Hi Daniel,

That sounds fine to do (easier a solution, my brain's gotten complex today ha).

We should classify the two types of error in the docs for users the
way you have here, to indicate what the issue is in each of the error
cases - UnknownScannerException and LeaseException. Mind filing a
JIRA? :)

On Thu, Sep 20, 2012 at 7:21 PM, Daniel Iancu <daniel.iancu@1and1.ro> wrote:
> Thaaank you! I was waiting for this email for months. I've read all the
> posts regarding lease timeouts and see that people usually have them for 2
> reasons. One, the normal case where the client app does not process the row
> fast enough so they get UnknownScannerException and some had the issue below
> and get LeaseException instead.
>
> How about using a try/catch for the
>
> // Remove lease while its being processed in server; protects against case
>       // where processing of request takes > lease expiration time.
>       lease = this.leases.removeLease(scannerName);
>
> and re-throw an IllegalStateException or log a warning message because a
> client with and active scanner but no lease does not seem to be in the right
> state?
>
> Just an idea but you know  better.
> Daniel
>
> On 09/20/2012 03:42 PM, Harsh J wrote:
>
> Hi,
>
> I hit this today and got down to investigate it and one of my
> colleagues discovered this thread. Since I got some more clues, I
> thought I'll bump up this thread for good.
>
> Lucian almost got the issue here. The thing we missed thinking about
> is the client retry. The client of HBaseRPC seems to silently retry on
> timeouts. So if you apply Lucian's theory below and apply that a
> client retry calls next(ID, Rows) yet again, you can construct this
> issue:
>
> - Client calls next(ID, Rows) first time.
> - RS receives the handler-sent request, removes lease (to not expire
> it during next() call) and begins work.
> - RS#next hangs during work (for whatever reason we can assume - large
> values or locks or whatever)
> - Client times out after a minute, retries (due to default nature).
> Retry seems to be silent though?
> - New next(ID, Rows) call is invoked. Scanner still exists so no
> UnknownScanner is thrown. But when next() tries to remove lease, we
> get thrown LeaseException (and the client gets this immediately and
> dies) as the other parallel handler has the lease object already
> removed and held in its stuck state.
> - A few secs/mins later, the original next() unfreezes, adds back
> lease to the queue, tries to write back response, runs into
> ClosedChannelException as the client had already thrown its original
> socket away. End of clients.
> - Lease-period expiry later, the lease is now formally removed without
> any hitches.
>
> Ideally, to prevent this, the rpc.timeout must be > lease period as
> was pointed out. Since in that case, we'd have waited for X units more
> for the original next() to unblock and continue itself and not have
> retried. That is how this is avoided, unintentionally, but can still
> happen if the next() still takes very long.
>
> I haven't seen a LeaseException in any other case so far, so maybe we
> can improve that exception's message to indicate whats going on in
> simpler terms so clients can reconfigure to fix themselves?
>
> Also we could add in some measures to prevent next()-duping, as that
> is never bound to work given the lease-required system. Perhaps when
> the next() stores the removed lease, we can store it somewhere global
> (like ActiveLeases or summat) and deny next() duping if their
> requested lease is already in ActiveLeases? Just ends up giving a
> better message, not a solution.
>
> Hope this helps others who've run into the same issue.
>
> On Mon, Oct 24, 2011 at 10:52 PM, Jean-Daniel Cryans
> <jdcryans@apache.org> wrote:
>
> So you should see the SocketTimeoutException in your *client* logs (in
> your case, mappers), not LeaseException. At this point yes you're
> going to timeout, but if you spend so much time cycling on the server
> side then you shouldn't set a high caching configuration on your
> scanner as IO isn't your bottle neck.
>
> J-D
>
> On Mon, Oct 24, 2011 at 10:15 AM, Lucian Iordache
> <lucian.george.iordache@gmail.com> wrote:
>
> Hi,
>
> The servers have been restarted (I have this configuration for more than a
> month, so this is not the problem).
> About the stack traces, they show exactly the same, a lot of
> ClosedChannelConnections and LeaseExceptions.
>
> But I found something that could be the problem: hbase.rpc.timeout . This
> defaults to 60 seconds, and I did not modify it in hbase-site.xml. So it
> could happen the next way:
> - the mapper makes a scanner.next call to the region server
> - the region servers needs more than 60 seconds to execute it (I use
> multiple filters, and it could take a lot of time)
> - the scan client gets the timeout and cuts the connection
> - the region server tries to send the results to the client ==>
> ClosedChannelConnection
>
> I will get a deeper look into it tomorrow. If you have other suggestions,
> please let me know!
>
> Thanks,
> Lucian
>
> On Mon, Oct 24, 2011 at 8:00 PM, Jean-Daniel Cryans
> <jdcryans@apache.org>wrote:
>
> Did you restart the region servers after changing the config?
>
> Are you sure it's the same exception/stack trace?
>
> J-D
>
> On Mon, Oct 24, 2011 at 8:04 AM, Lucian Iordache
> <lucian.george.iordache@gmail.com> wrote:
>
> Hi all,
>
> I have exactly the same problem that Eran had.
> But there is something I don't understand: in my case, I have set the
>
> lease
>
> time to 240000 (4 minutes). But most of the map tasks that are failing
>
> run
>
> about 2 minutes. How is it possible to get a LeaseException if the task
>
> runs
>
> less than the configured time for a lease?
>
> Regards,
> Lucian Iordache
>
> On Fri, Oct 21, 2011 at 12:34 AM, Eran Kutner <eran@gigya.com> wrote:
>
> Perfect! Thanks.
>
> -eran
>
>
>
> On Thu, Oct 20, 2011 at 23:27, Jean-Daniel Cryans <jdcryans@apache.org
>
> wrote:
>
> hbase.regionserver.lease.period
>
> Set it bigger than 60000.
>
> J-D
>
> On Thu, Oct 20, 2011 at 2:23 PM, Eran Kutner <eran@gigya.com> wrote:
>
> Thanks J-D!
> Since my main table is expected to continue growing I guess at some
>
> point
>
> even setting the cache size to 1 will not be enough. Is there a way
>
> to
>
> configure the lease timeout?
>
> -eran
>
>
>
> On Thu, Oct 20, 2011 at 23:16, Jean-Daniel Cryans <
>
> jdcryans@apache.org
>
> wrote:
>
> On Wed, Oct 19, 2011 at 12:51 PM, Eran Kutner <eran@gigya.com>
>
> wrote:
>
> Hi J-D,
> Thanks for the detailed explanation.
> So if I understand correctly the lease we're talking about is a
>
> scanner
>
> lease and the timeout is between two scanner calls, correct? I
>
> think
>
> that
>
> make sense because I now realize that jobs that fail (some jobs
>
> continued
>
> to
> fail even after reducing the number of map tasks as Stack
>
> suggested)
>
> use
>
> filters to fetch relatively few rows out of a very large table,
>
> so
>
> they
>
> could be spending a lot of time on the region server scanning
>
> rows
>
> until
>
> it
>
> reached my setCaching value which was 1000. Setting the caching
>
> value
>
> to
>
> 1
>
> seem to allow these job to complete.
> I think it has to be the above, since my rows are small, with
>
> just
>
> a
>
> few
>
> columns and processing them is very quick.
>
> Excellent!
>
>
> However, there are still a couple ofw thing I don't understand:
> 1. What is the difference between setCaching and setBatch?
>
> * Set the maximum number of values to return for each call to
>
> next()
>
> VS
>
> * Set the number of rows for caching that will be passed to
>
> scanners.
>
> The former is useful if you have rows with millions of columns and
>
> you
>
> could
> setBatch to get only 1000 of them at a time. You could call that
>
> intra-row
>
> scanning.
>
>
> 2. Examining the region server logs more closely than I did
>
> yesterday
>
> I
>
> see
>
> a log of ClosedChannelExceptions in addition to the expired
>
> leases
>
> (but
>
> no
>
> UnknownScannerException), is that expected? You can see an
>
> excerpt
>
> of
>
> the
>
> log from one of the region servers here:
>
> http://pastebin.com/NLcZTzsY
>
> It means that when the server got to process that client request
>
> and
>
> started
> reading from the socket, the client was already gone. Killing a
>
> client
>
> does
>
> that (or killing a MR that scans), so does SocketTimeoutException.
>
> This
>
> should probably go in the book. We should also print something
>
> nicer
>
> :)
>
> J-D
>
>
>
> --
> Harsh J
>
>



-- 
Harsh J

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