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From Chris Riccomini <criccom...@linkedin.com.INVALID>
Subject Re: Inter-stream time-alignment
Date Sat, 06 Dec 2014 01:12:16 GMT
Hey Ben,

This is a good point. I was thinking more about this as well. Another
example is when a message is sent but not yet replicated to the rest of
the ISR in Kafka. In such a case, the Samza job would think it's at head
(since the last message it received == the high watermark), but there are
still messages in-flight that might be < the timestamp for the stream
you're joining against.

> In your case, it seems like partitioning by shard id should work

Yea, this is an interesting idea. This should work provided that the join
is happening within the same shard (in the example given, that was the
case), and there is only one producer for the shard, which is properly
interleaving the messages from the two strictly ordered files (assuming
the files are one per-table).

Cheers,
Chris

On 12/5/14 4:54 PM, "Ben Kirwin" <ben@kirw.in> wrote:

>> In order to visit each input in the correct order, the
>>event-type-specific
>> streams need to be interleaved/time-aligned for each summarizer Task
>>based
>> on embedded timestamps. However, Samza offers no inter-stream ordering
>> semantics, so this ordering would seem to be the job of a custom
>> MessageChooser. But I don't see how a MessageChooser can do this without
>> one additional bit of context: when we haven't been offered an item from
>> one or more input streams, we need to know whether the missing stream(s)
>> are "at head," to decide whether we must await an upcoming message for
>> timestamp comparison.
>
>Unless I misunderstand your design, I don't think knowing whether the
>stream is 'at head' actually helps you here.
>
>Let's say the upstream task is loading data into two topics:
>'level-changed' and 'purchase-completed'. Suppose a Kafka node goes
>down while it's producing a new batch of messages, and the
>'purchase-completed' events in that batch are written, but the
>'level-changed' events are not. The downstream Samza task will see /
>process those 'purchase-completed' events, but there's no way for it
>to know that it should expect some preceding 'level-changed' events --
>since they never made it into Kafka, Samza think's it's 'caught up'.
>(There are some other race-type problems you can get, but I think
>that's the most obvious.)
>
>Normally, I suggest that whenever you care about the relative ordering
>of some data, you try and put that data in the same partition of the
>same topic. When messages are in the same partition, the ordering's
>obvious -- but as you've noticed, it gets a lot trickier to
>re-establish order when Kafka doesn't enforce it. In your case, it
>seems like partitioning by shard id should work; consumers can just
>filter out the messages they don't care about.
>
>In your case, another option is to add periodic 'marker' messages with
>the current timestamp to each topic/partition. When your samza job
>gets a marker as input, it can be sure that the upstream job will
>never send an event with a smaller timestamp on that partition. When
>your task sees a 'purchase-completed' event, it just needs to buffer
>until it sees either a 'level-changed' event or a marker with a >=
>timestamp -- and then it can be confident it knows the player's level
>at that moment in time. (Still, I suggest the first option if you can
>swing it -- it's a whole lot harder to mess up.)
>
>-- 
>Ben Kirwin
>http://ben.kirw.in/


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