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From Hari Shreedharan <hshreedha...@cloudera.com>
Subject Re: Optional Channels
Date Mon, 02 Dec 2013 21:48:16 GMT
What version of Flume are you using? If the channel does not accept the events, the transaction
does get rolled back (so that the channel drops the references to the events), but the source
would not retry the events again - since we do not throw a ChannelException to the source.
You will see the rolled back log message, but the events are dropped and not tried again -
the next set would get tried. 


Thanks,
Hari


On Monday, December 2, 2013 at 9:21 AM, James Estes wrote:

> Hoping someone can point me in the right direction. We're indexing our logs into elastic
search just for added real time convenience and want to make that step optional. Essentially,
if we fall behind writing to ES, we would prefer to just skip ES (since we have a more durable
channel for higher latency querying of the same data). Optional Channels seemed to fit, but
we haven't had much success. 
> 
> First, we set our config to have a Memory Channel and made it optional. If the ES sink
fell behind, the channel would fill and reject new events. However, the channel throws an
exception and the Channel Processor rolls back the transaction, causing the events to be put
back on the queue to be attempted again. The doc for getOptionalChannels says "A failure in
writing the event to these channels must be ignored." Should the transaction just always commit
when optional channels fail (basically a best-effort commit-what-you-could since it was optional
anyway)?
> 
> Second, we tried the PseudoTxMemoryChannel, but found it to also continue to bottleneck
on ES. Turns out that it uses queue.put instead of queue.offer, which means it will block
until there is room in the queue to add the event. MemoryChannel uses offer. Should PseudoTxMemoryChannel
switch to using offer always, or at least have an optional 'failFast' to enable that behavior?
> 
> Is there another way I can accomplish truly optional channels? I do find it encouraging
it takes this much effort to make Flume drop events :)
> 
> Thanks,
> 
> James 


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