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.dear...@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.
