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From Connor Woodson <cwoodson....@gmail.com>
Subject Re: Problem Events
Date Sat, 03 Aug 2013 01:27:07 GMT
As another option to solve the problem of having a bad event in a channel:
using a fail-over sink processor, log all bad events to a local file. And
to be extra cautious, add a third failover of a null sink. This will mean
that events will always flow through your channel. The file sink should
almost never fail, so you shouldn't be losing events in the process. And
then you can re-process everything in the file if you still want those
events for something.

For the system of having Flume detect bad events, I think implementing
something like above is better than discarding events that fail X times.
For instance, if you have an Avro sink -> Avro source, and you're
restarting your source, Flume would end up discarding events unnecessarily.
Instead, how about implementing the above system and then go a step
further: Flume will attempt to re-send the bad events itself. And then if a
bad event isn't able to be sent after X attempts, it is can be discarded.

I envision this system as an extension to the current File Channel; when an
event fails, it is written to a secondary File Channel from which events
can be pulled when the main channel isn't in use. It would add headers like
"lastAttempt" and "numberOfAttempts" to events. Then it can be configurable
for a "min time between attempts" and "maximum attempts." When an event
fails the second time, those headers are updated and it goes back into the
fail-channel. If it comes out of the fail-channel but the lastAttempt is
too recent, it goes back in. If it fails more times than the maximum, it is
written to a final location (perhaps its just sent to another sink; maybe
this would have to be in a sink processor). Assuming all of those steps are
error-free, then all messages are preserved, and the badly-formatted
eventually get stored somewhere else. (This system could be hacked together
with current code - fail over sink processor -> avro sink -> avro source on
same instance, but that's a little too hacky).

Just some thoughts.

- Connor

On Thu, Aug 1, 2013 at 3:25 PM, Arvind Prabhakar <arvind@apache.org> wrote:

> This sounds like a critical problem that can cause pipelines to block
> permanently. If you find yourself in this situation, a possible work around
> would be to decommission the channel, remove its data and route the flow
> with a new empty channel. If you have the ability to identify which
> component is causing the problem and see if you can remove it temporarily
> to let the problem events pass through another peer component.
> I have also created FLUME-2140 [1] which will eventually allow the
> pipelines to identify and divert such bad events. If you have any logs,
> data, configurations that can be shared and will help provide more details
> for this problem, it will be great if you could attach them to this jira
> and provide your comments.
> [1] https://issues.apache.org/jira/browse/FLUME-2140
> Regards,
> Arvind Prabhakar
> On Thu, Aug 1, 2013 at 10:33 AM, Paul Chavez <
> pchavez@verticalsearchworks.com> wrote:
>> **
>> There's no way to deal with a bad event once it's in the channel, but you
>> can mitigate future issues by having a timestamp interceptor bound to the
>> source feeding the channel. There is a parameter 'preserve existing' that
>> will only add the header if it doesn't exist. If you don't want to have
>> 'bad' time data in there you could try a static interceptor with a specific
>> past date so that corrupt events fall into a deterministic path in HDFS.
>> I use this technique to prevent stuck events for both timestamp headers
>> as well as some of our own custom headers we use for tokenized paths. The
>> static interceptor will insert an arbitrary header if it doesn't exist so I
>> have a couple that put in the value 'Unknown' so that I can still send the
>> events through the HDFS sink but I can also find them later if need be.
>> hope that helps,
>> Paul Chavez
>>  ------------------------------
>> *From:* Roshan Naik [mailto:roshan@hortonworks.com]
>> *Sent:* Thursday, August 01, 2013 10:27 AM
>> *To:* user@flume.apache.org
>> *Subject:* Re: Problem Events
>>  some questions:
>> - why is the sink unable to consume the event ?
>> - how would you like to identify such an event ? by examining its content
>> ? or by the fact that its ping-pong-ing between channel and sink ?
>> - what would you prefer to do with such an event ? merely drop it ?
>> On Thu, Aug 1, 2013 at 9:26 AM, Jeremy Karlson <jeremykarlson@gmail.com>wrote:
>>>  To my knowledge (which is admittedly limited), there is no way to deal
>>> with these in a way that will make your day.  I'm happy if someone can say
>>> otherwise.
>>> This is very similar to a problem I had a week or two ago.  I fixed it
>>> by restarting Flume with debugging on, connecting to it with the debugger,
>>> and finding the message in the sink.  Discover a bug in the sink.
>>>  Downloaded Flume, fixed bug, recompiled, installed custom version, etc.
>>> I agree that this is not a practical solution, and I still believe that
>>> Flume needs some sort of "sink of last resort" option or something, like
>>> JMS implementations.
>>> -- Jeremy
>>> On Thu, Aug 1, 2013 at 2:42 AM, Anat Rozenzon <anat@viber.com> wrote:
>>>>  The message is already in the channel.
>>>> Is there a way to write an interceptor to work after the channel? or
>>>> before the sink?
>>>> The only thing I found is to stop everything and delete the channel
>>>> files, but I won't be able to use this approach in production :-(
>>>> On Thu, Aug 1, 2013 at 11:13 AM, Ashish <paliwalashish@gmail.com>wrote:
>>>>>  On Thu, Aug 1, 2013 at 1:29 PM, Anat Rozenzon <anat@viber.com>
>>>>>>   Hi,
>>>>>> I'm having the same problem with HDFS sink.
>>>>>> A 'poison' message which doesn't have timestamp header in it as the
>>>>>> sink expects.
>>>>>> This causes a NPE which ends in returning the message to the channel
>>>>>> , over and over again.
>>>>>> Is my only option to re-write the HDFS sink?
>>>>>> Isn't there any way to intercept in the sink work?
>>>>> You can write a custom interceptor and remove/modify the poison
>>>>> message.
>>>>> Interceptors are called before message makes it way into the channel.
>>>>> http://flume.apache.org/FlumeUserGuide.html#flume-interceptors
>>>>> I wrote a blog about it a while back
>>>>> http://www.ashishpaliwal.com/blog/2013/06/flume-cookbook-implementing-custom-interceptors/
>>>>>> Thanks
>>>>>> Anat
>>>>>> On Fri, Jul 26, 2013 at 3:35 AM, Arvind Prabhakar <arvind@apache.org>wrote:
>>>>>>> Sounds like a bug in ElasticSearch sink to me. Do you mind filing
>>>>>>> Jira to track this? Sample data to cause this would be even better.
>>>>>>> Regards,
>>>>>>> Arvind Prabhakar
>>>>>>> On Thu, Jul 25, 2013 at 9:50 AM, Jeremy Karlson <
>>>>>>> jeremykarlson@gmail.com> wrote:
>>>>>>>> This was using the provided ElasticSearch sink.  The logs
were not
>>>>>>>> helpful.  I ran it through with the debugger and found the
source of the
>>>>>>>> problem.
>>>>>>>> ContentBuilderUtil uses a very "aggressive" method to determine
>>>>>>>> the content is JSON; if it contains a "{" anywhere in it,
it's considered
>>>>>>>> JSON.  My body contained that but wasn't JSON, causing the
JSON parser to
>>>>>>>> throw a CharConversionException from addComplexField(...)
(but not the
>>>>>>>> expected JSONException).  We've changed addComplexField(...)
to catch
>>>>>>>> different types of exceptions and fall back to treating it
as a simple
>>>>>>>> field.  We'll probably submit a patch for this soon.
>>>>>>>> I'm reasonably happy with this, but I still think that in
>>>>>>>> bigger picture there should be some sort of mechanism to
>>>>>>>> detect and toss / skip / flag problematic events without
them plugging up
>>>>>>>> the flow.
>>>>>>>> -- Jeremy
>>>>>>>> On Wed, Jul 24, 2013 at 7:51 PM, Arvind Prabhakar <
>>>>>>>> arvind@apache.org> wrote:
>>>>>>>>> Jeremy, would it be possible for you to show us logs
for the part
>>>>>>>>> where the sink fails to remove an event from the channel?
I am assuming
>>>>>>>>> this is a standard sink that Flume provides and not a
custom one.
>>>>>>>>> The reason I ask is because sinks do not introspect the
event, and
>>>>>>>>> hence there is no reason why it will fail during the
event's removal. It is
>>>>>>>>> more likely that there is a problem within the channel
in that it cannot
>>>>>>>>> dereference the event correctly. Looking at the logs
will help us identify
>>>>>>>>> the root cause for what you are experiencing.
>>>>>>>>> Regards,
>>>>>>>>> Arvind Prabhakar
>>>>>>>>> On Wed, Jul 24, 2013 at 3:56 PM, Jeremy Karlson <
>>>>>>>>> jeremykarlson@gmail.com> wrote:
>>>>>>>>>>  Both reasonable suggestions.  What would a custom
sink look
>>>>>>>>>> like in this case, and how would I only eliminate
the problem events since
>>>>>>>>>> I don't know what they are until they are attempted
by the "real" sink?
>>>>>>>>>> My philosophical concern (in general) is that we're
taking the
>>>>>>>>>> approach of exhaustively finding and eliminating
possible failure cases.
>>>>>>>>>>  It's not possible to eliminate every single failure
case, so shouldn't
>>>>>>>>>> there be a method of last resort to eliminate problem
events from the
>>>>>>>>>> channel?
>>>>>>>>>> -- Jeremy
>>>>>>>>>> On Wed, Jul 24, 2013 at 3:45 PM, Hari Shreedharan
>>>>>>>>>> hshreedharan@cloudera.com> wrote:
>>>>>>>>>>> Or you could write a custom sink that removes
this event (more
>>>>>>>>>>> work of course)
>>>>>>>>>>> Thanks,
>>>>>>>>>>> Hari
>>>>>>>>>>>  On Wednesday, July 24, 2013 at 3:36 PM, Roshan
Naik wrote:
>>>>>>>>>>>   if you have a way to identify such events..
you may be able
>>>>>>>>>>> to use the Regex interceptor to toss them out
before they get into the
>>>>>>>>>>> channel.
>>>>>>>>>>> On Wed, Jul 24, 2013 at 2:52 PM, Jeremy Karlson
>>>>>>>>>>> jeremykarlson@gmail.com> wrote:
>>>>>>>>>>>  Hi everyone.  My Flume adventures continue.
>>>>>>>>>>> I'm in a situation now where I have a channel
that's filling
>>>>>>>>>>> because a stubborn message is stuck.  The sink
won't accept it (for
>>>>>>>>>>> whatever reason; I can go into detail but that's
not my point here).  This
>>>>>>>>>>> just blocks up the channel entirely, because
it goes back into the channel
>>>>>>>>>>> when the sink refuses.  Obviously, this isn't
>>>>>>>>>>> I'm wondering what mechanisms, if any, Flume
has to deal with
>>>>>>>>>>> these situations.  Things that come to mind might
>>>>>>>>>>> 1. Ditch the event after n attempts.
>>>>>>>>>>> 2. After n attempts, send the event to a "problem
area" (maybe a
>>>>>>>>>>> different source / sink / channel?)  that someone
can look at later.
>>>>>>>>>>> 3. Some sort of mechanism that allows operators
to manually kill
>>>>>>>>>>> these messages.
>>>>>>>>>>> I'm open to suggestions on alternatives as well.
>>>>>>>>>>> Thanks.
>>>>>>>>>>> -- Jeremy
>>>>> --
>>>>> thanks
>>>>> ashish
>>>>> Blog: http://www.ashishpaliwal.com/blog
>>>>> My Photo Galleries: http://www.pbase.com/ashishpaliwal

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