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From Ted Dunning <ted.dunn...@gmail.com>
Subject Re: Topology is stuck
Date Thu, 10 Apr 2014 10:05:03 GMT
Jason,

A single client is guaranteed to have strict consistency in its own reads
and writes.  If the write has occurred according to the client, then all
subsequent reads by that client will show it.  This applies even if the
client does a write, is disconnected from ZK, reconnects automatically and
then reads.  The only place that the weaker consistency applies is that if
A successfully writes to ZK and then sends an out-of-band message to B and
B looks at ZK upon receiving the notification from A.  In that case, B may
not see the update from A right away.

The most common source of hangs such as you describe that I know about are
cases where change notification handlers are not coded correctly and they
lose the watcher on a status variable by forgetting to reset it when
handling a change.  This can happen due to exceptions.  The best way to
avoid such problems is to use a higher level library such as Curator.  I
forget if Storm already uses Curator, but I seem to remember not.




On Thu, Apr 10, 2014 at 1:39 AM, Jason Jackson <jasonjckn@gmail.com> wrote:

> My idea for the bug was that trident expects to read from zookeeper what
> was recently written zookeeper for the same zknode, and due to sequential
> consistency it sometimes reads an older value even though it just wrote a
> newer value. I could be way off the mark though, it's just an idea to
> explore more.
>
>
> On Thu, Apr 10, 2014 at 1:36 AM, Jason Jackson <jasonjckn@gmail.com>wrote:
>
>> Hi Ted, thanks for clearing up the language, I intended to express
>> sequential consistency then.
>>
>> Yes you could do a forced sync too, that would be another way good test.
>>
>> Taylor, the bug that I witnessed only occurs after you leave a trident
>> topology running for at least a day. One day it'll just stop making
>> progress and re-attempt the same batch forever.  Unfortunately I can't send
>> the particular trident code to you, but I don't think there's anything
>> unique about it. I suspect any trident topology could reproduce the bug if
>> ran for a week. Other correlated factors may include that the trident
>> topology has to occasionally fail batches, the zookeeper cluster has to be
>> under significant load from other applications beyond trident. I don't many
>> much details unfortunately.
>>
>> -Jason
>>
>>
>>
>>
>> On Wed, Apr 9, 2014 at 3:03 PM, Ted Dunning <ted.dunning@gmail.com>wrote:
>>
>>>
>>> In what sense do you mean when you say that reads in ZK are eventually
>>> consistent?
>>>
>>> You may get a slightly old value, but you are guaranteed to see a
>>> consistent history.  That is, if a value has values (which include version
>>> numbers) v_1 ... v_n, then if you see v_i, you will never see v_j where j<i.
>>>
>>> You can also guarantee that you don't even see delayed values by using
>>> sync.
>>>
>>> Normally when people say "eventually consistent" they mean that two
>>> participants can see inconsistent histories under partition.  That isn't
>>> possible in ZK.  As I understand it, ZK would be better described as
>>> providing sequential consistency since all observers will see all updates
>>> in the same order.
>>>
>>>
>>>
>>>
>>> On Wed, Apr 9, 2014 at 2:50 PM, Jason Jackson <jasonjckn@gmail.com>wrote:
>>>
>>>> I have one theory that because reads in zookeeper are eventually
>>>> consistent, this is a necessary condition for the bug to manifest. So one
>>>> way to test this hypothesis is to run a zookeeper ensemble with 1 node, or
>>>> a zookeeper ensemble configured for 5 nodes, but take 2 of them offline,
so
>>>> that every write operation only succeeds if every member of the ensemble
>>>> sees the write. This should produce strong consistent reads. If you run
>>>> this test, let me know what the results are. (Clearly this isn't a good
>>>> production system though as you're making a tradeoff for lower availability
>>>> in favor of greater consistency, but the results could help narrow down the
>>>> bug)
>>>>
>>>>
>>>> On Wed, Apr 9, 2014 at 2:43 PM, Jason Jackson <jasonjckn@gmail.com>wrote:
>>>>
>>>>> Yah it's probably a bug in trident. It would be amazing if someone
>>>>> figured out the fix for this. I spent about 6 hours looking into, but
>>>>> couldn't figure out why it was occuring.
>>>>>
>>>>> Beyond fixing this, one thing you could do to buy yourself time is
>>>>> disable batch retries in trident. There's no option for this in the API,
>>>>> but it's like a 1 or 2 line change to the code. Obviously you loose exactly
>>>>> once semantics, but at least you would have a system that never falls
>>>>> behind real-time.
>>>>>
>>>>>
>>>>>
>>>>> On Wed, Apr 9, 2014 at 1:10 AM, Danijel Schiavuzzi <
>>>>> danijel@schiavuzzi.com> wrote:
>>>>>
>>>>>> Thanks Jason. However, I don't think that was the case in my stuck
>>>>>> topology, otherwise I'd have seen exceptions (thrown by my Trident
>>>>>> functions) in the worker logs.
>>>>>>
>>>>>>
>>>>>> On Wed, Apr 9, 2014 at 3:02 AM, Jason Jackson <jasonjckn@gmail.com>wrote:
>>>>>>
>>>>>>> An example of "corrupted input" that causes a batch to fail would
be
>>>>>>> for example if you expected a schema to your data that you read
off kafka,
>>>>>>> or some queue, and for whatever reason the data didn't conform
to your
>>>>>>> schema and the function that you implement that you pass to stream.each
>>>>>>> throws an exception when this unexpected situation occurs. This
would cause
>>>>>>> the batch to be retried, but it's deterministically failing,
so the batch
>>>>>>> will be retried forever.
>>>>>>>
>>>>>>>
>>>>>>> On Mon, Apr 7, 2014 at 10:37 AM, Danijel Schiavuzzi <
>>>>>>> danijel@schiavuzzi.com> wrote:
>>>>>>>
>>>>>>>> Hi Jason,
>>>>>>>>
>>>>>>>> Could you be more specific -- what do you mean by "corrupted
>>>>>>>> input"? Do you mean that there's a bug in Trident itself
that causes the
>>>>>>>> tuples in a batch to somehow become corrupted?
>>>>>>>>
>>>>>>>> Thanks a lot!
>>>>>>>>
>>>>>>>> Danijel
>>>>>>>>
>>>>>>>>
>>>>>>>> On Monday, April 7, 2014, Jason Jackson <jasonjckn@gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> This could happen if you have corrupted input that always
causes a
>>>>>>>>> batch to fail and be retried.
>>>>>>>>>
>>>>>>>>> I have seen this behaviour before and I didn't see corrupted
>>>>>>>>> input. It might be a bug in trident, I'm not sure. If
you figure it out
>>>>>>>>> please update this thread and/or submit a patch.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Mon, Mar 31, 2014 at 7:39 AM, Danijel Schiavuzzi <
>>>>>>>>> danijel@schiavuzzi.com> wrote:
>>>>>>>>>
>>>>>>>>> To (partially) answer my own question -- I still have
no idea on
>>>>>>>>> the cause of the stuck topology, but re-submitting the
topology helps --
>>>>>>>>> after re-submitting my topology is now running normally.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Wed, Mar 26, 2014 at 6:04 PM, Danijel Schiavuzzi <
>>>>>>>>> danijel@schiavuzzi.com> wrote:
>>>>>>>>>
>>>>>>>>> Also, I did have multiple cases of my IBackingMap workers
dying
>>>>>>>>> (because of RuntimeExceptions) but successfully restarting
afterwards (I
>>>>>>>>> throw RuntimeExceptions in the BackingMap implementation
as my strategy in
>>>>>>>>> rare SQL database deadlock situations to force a worker
restart and to
>>>>>>>>> fail+retry the batch).
>>>>>>>>>
>>>>>>>>> From the logs, one such IBackingMap worker death (and
subsequent
>>>>>>>>> restart) resulted in the Kafka spout re-emitting the
pending tuple:
>>>>>>>>>
>>>>>>>>>     2014-03-22 16:26:43 s.k.t.TridentKafkaEmitter [INFO]
>>>>>>>>> re-emitting batch, attempt 29698959:736
>>>>>>>>>
>>>>>>>>> This is of course the normal behavior of a transactional
topology,
>>>>>>>>> but this is the first time I've encountered a case of
a batch retrying
>>>>>>>>> indefinitely. This is especially suspicious since the
topology has been
>>>>>>>>> running fine for 20 days straight, re-emitting batches
and restarting
>>>>>>>>> IBackingMap workers quite a number of times.
>>>>>>>>>
>>>>>>>>> I can see in my IBackingMap backing SQL database that
the batch
>>>>>>>>> with the exact txid value 29698959 has been committed
-- but I suspect that
>>>>>>>>> could come from another BackingMap, since there are two
BackingMap
>>>>>>>>> instances running (paralellismHint 2).
>>>>>>>>>
>>>>>>>>> However, I have no idea why the batch is being retried
>>>>>>>>> indefinitely now nor why it hasn't been successfully
acked by Trident.
>>>>>>>>>
>>>>>>>>> Any suggestions on the area (topology component) to focus
my
>>>>>>>>> research on?
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>>
>>>>>>>>> On Wed, Mar 26, 2014 at 5:32 PM, Danijel Schiavuzzi <
>>>>>>>>> danijel@schiavuzzi.com> wrote:
>>>>>>>>>
>>>>>>>>> Hello,
>>>>>>>>>
>>>>>>>>> I'm having problems with my transactional Trident topology.
It has
>>>>>>>>> been running fine for about 20 days, and suddenly is
stuck processing a
>>>>>>>>> single batch, with no tuples being emitted nor tuples
being persisted by
>>>>>>>>> the TridentState (IBackingMap).
>>>>>>>>>
>>>>>>>>> It's a simple topology which consumes messages off a
Kafka queue.
>>>>>>>>> The spout is an instance of storm-kafka-0.8-plus
>>>>>>>>> TransactionalTridentKafkaSpout and I use the trident-mssql
transactional
>>>>>>>>> TridentState implementation to persistentAggregate()
data into a SQL
>>>>>>>>> database.
>>>>>>>>>
>>>>>>>>> In Zookeeper I can see Storm is re-trying a batch, i.e.
>>>>>>>>>
>>>>>>>>>      "/transactional/<myTopologyName>/coordinator/currattempts"
is
>>>>>>>>> "{"29698959":6487}"
>>>>>>>>>
>>>>>>>>> ... and the attempt count keeps increasing. It seems
the batch
>>>>>>>>> with txid 29698959 is stuck, as the attempt count in
Zookeeper keeps
>>>>>>>>> increasing -- seems like the batch isn't being acked
by Trident and I have
>>>>>>>>> no idea why, especially since the topology has been running
successfully
>>>>>>>>> the last 20 days.
>>>>>>>>>
>>>>>>>>> I did rebalance the topology on one occasion, after which
it
>>>>>>>>> continued running normally. Other than that, no other
modifications were
>>>>>>>>> done. Storm is at version 0.9.0.1.
>>>>>>>>>
>>>>>>>>> Any hints on how to debug the stuck topology? Any other
useful
>>>>>>>>> info I might provide?
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> Danijel Schiavuzzi
>>>>>>>>>
>>>>>>>>> E: danijel@schiavuzzi.com
>>>>>>>>> W: www.schiavuzzi.com
>>>>>>>>> T: +385989035562
>>>>>>>>> Skype: danijel.schiavuzzi
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> Danijel Schiavuzzi
>>>>>>>>>
>>>>>>>>> E: danije
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>> Danijel Schiavuzzi
>>>>>>>>
>>>>>>>> E: danijel@schiavuzzi.com
>>>>>>>> W: www.schiavuzzi.com
>>>>>>>> T: +385989035562
>>>>>>>> Skype: danijels7
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Danijel Schiavuzzi
>>>>>>
>>>>>> E: danijel@schiavuzzi.com
>>>>>> W: www.schiavuzzi.com
>>>>>> T: +385989035562
>>>>>> Skype: danijels7
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>

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