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From Richard Lee <rd...@tivo.com>
Subject Re: Samza commit guarantees
Date Thu, 19 Feb 2015 23:59:14 GMT


> On Feb 19, 2015, at 3:46 PM, Richard Lee <rdlee@tivo.com> wrote:
>
> I’m looking at the samza-core source code… in particular RunLoop, TaskInstance, TaskInstanceCollector,
and SystemProducers, and I’m having a hard time seeing where this batched sends of output
messages happens.  It seems from RunLoop that:
>
> - one input message envelope is read from the consumer multiplexer
> - it is sent to the task for processing
> - if the task writes a new output message envelope to the collector, the TaskInstanceCollector
immediately looks up the SystemProducer from SystemProducers and immediately sends it onwards.
>
> I don’t see any consultation of either producer.type or batch size.
>
> However, I am also new to scala, so I’m likely not understanding something fairly obvious.

I should add, that the rest of the RunLoop, after processing the one input message envelope,
then calls

- window (which is a no-op if not doing windowing, AFAICT)
- commit, which, if the commit timer has expired
  - tells all the tasks to commit
  - which flushes all their collectors
  - which flushes the SystemProducers
  - which flushes each SystemProducer
  - which guarantees that everything is written out.

So, it naively appears that the core loop is

- read one message
- flush all outputs

I know there must be some magic somewhere that is buffering & batching, but I’m having
trouble finding it.

Richard

>
> Richard
>
>
>> On Feb 19, 2015, at 8:39 AM, Chris Riccomini <criccomini@apache.org> wrote:
>>
>> Hey Tom,
>>
>> It seems that most of your questions are concerned with durability and
>> messaging guarantees. Samza is designed to not lose data, but duplicates
>> can occur. Samza reads messages, and feeds them to your process() method.
>> When you send messages, either via a changelog, or via collector.send,
>> Samza will batch those messages up, and send them at some point BEFORE your
>> input offsets are committed. This looks like:
>>
>> <start>, ... process and send a lot ..., <commit>
>>
>> Samza only guarantees that everything will be flushed to Kafka (or whatever
>> output system you're sending to) *before* committing offsets. Once offsets
>> are committed, you'll never see any prior messages again. If a failure
>> occurs somewhere *before* the offsets are committed, you'll simply fall
>> back to the last checkpointed offsets (<start>) and restart the processing
>> again.
>>
>> In between, for performance reasons, Samza batches output, delays sends,
>> etc. This is safe because we always flush before committing.
>>
>>> a) If using RockDB kv implementation, is there a way to guarantee that a
>> put is committed (at least on that instance disc), I notice that RockDB
>> implementation does nothing for kv.flush().
>>
>> The RocksDB store in Samza is basically used as a durable cache. The only
>> guarantee that Samza really cares about is whether it can get the data
>> after it's been put (whether the data is still in memory, or on disk). The
>> guarantee you, as a user, probably care about is whether your write has
>> been sent to your changelog.
>>
>>> b) When is it guaranteed that the kv put is in the change log (I am using
>> kafka implementation).
>>
>> It will be guaranteed to be written to the changelog when commit() is
>> called, before your offsets are committed. The exact order of commit is:
>> flush storage changelogs, flush producers, commit offsets. You can see this
>> in RunLoop.scala. This guarantees that your changelogs will be fully
>> flushed to Kafka before you commit your offsets. If a failure occurs before
>> the offset commit, you'd see duplicate messages, but you'd never lose
>> messages.
>>
>>> When using messageCollector.send and systems.kafka.producer.producer.type=sync
>> does that guarantee that the message is in kafka log when the send returns.
>>
>> Note quite. Samza batches messages to increase throughput. 'sync' tells
>> Samza to block when a *batch* of messages is being sent. If you wanted to
>> synchronously write each message, and block, you'd have to set the batch
>> size to 1.
>>
>>> If my Samza job fails while processing a message, I fix it and deploy
>> again, will the message offset still point to a value <= the message I
>> failed on.
>>
>> Yes. It should never be higher until the commit() message is called (after
>> process()). The guarantee Samza provides is that you might see duplicates,
>> but you'll not lose data.
>>
>> Cheers,
>> Chris
>>
>> On Thu, Feb 19, 2015 at 8:23 AM, Tom Dearman <tom.dearman@gmail.com> wrote:
>>
>>> Hi,
>>> Can someone help with the following questions please:
>>>
>>> a) If using RockDB kv implementation, is there a way to guarantee that a
>>> put is committed (at least on that instance disc), I notice that RockDB
>>> implementation does nothing for kv.flush().
>>>
>>> b) When is it guaranteed that the kv put is in the change log (I am using
>>> kafka implementation).
>>>
>>> c) When using messageCollector.send and
>>> systems.kafka.producer.producer.type=sync does that guarantee that the
>>> message is in kafka log when the send returns.  I am new to kafka, but it
>>> seems to me that if you have type=sync set, you still need to wait for the
>>> future objects get to return,  is this what Samza does?
>>>
>>> d) If my Samza job fails while processing a message, I fix it and deploy
>>> again, will the message offset still point to a value <= the message I
>>> failed on.  ie I understand it can be earlier, but is it possible the
>>> offset will now point to one higher.
>
>
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