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https://issues.apache.org/jira/browse/KAFKA-1955?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16019726#comment-16019726
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Brandon Bradley commented on KAFKA-1955:
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Ok, I am wrong. There are two instances where the current implementation reuses
buffers.
> Explore disk-based buffering in new Kafka Producer
> --------------------------------------------------
>
> Key: KAFKA-1955
> URL: https://issues.apache.org/jira/browse/KAFKA-1955
> Project: Kafka
> Issue Type: Improvement
> Components: producer
> Affects Versions: 0.8.2.0
> Reporter: Jay Kreps
> Assignee: Jay Kreps
> Attachments: KAFKA-1955.patch,
> KAFKA-1955-RABASED-TO-8th-AUG-2015.patch
>
>
> There are two approaches to using Kafka for capturing event data that has no
> other "source of truth store":
> 1. Just write to Kafka and try hard to keep the Kafka cluster up as you would
> a database.
> 2. Write to some kind of local disk store and copy from that to Kafka.
> The cons of the second approach are the following:
> 1. You end up depending on disks on all the producer machines. If you have
> 10000 producers, that is 10k places state is kept. These tend to fail a lot.
> 2. You can get data arbitrarily delayed
> 3. You still don't tolerate hard outages since there is no replication in the
> producer tier
> 4. This tends to make problems with duplicates more common in certain failure
> scenarios.
> There is one big pro, though: you don't have to keep Kafka running all the
> time.
> So far we have done nothing in Kafka to help support approach (2), but people
> have built a lot of buffering things. It's not clear that this is necessarily
> bad.
> However implementing this in the new Kafka producer might actually be quite
> easy. Here is an idea for how to do it. Implementation of this idea is
> probably pretty easy but it would require some pretty thorough testing to see
> if it was a success.
> The new producer maintains a pool of ByteBuffer instances which it attempts
> to recycle and uses to buffer and send messages. When unsent data is queuing
> waiting to be sent to the cluster it is hanging out in this pool.
> One approach to implementing a disk-baked buffer would be to slightly
> generalize this so that the buffer pool has the option to use a mmap'd file
> backend for it's ByteBuffers. When the BufferPool was created with a
> totalMemory setting of 1GB it would preallocate a 1GB sparse file and memory
> map it, then chop the file into batchSize MappedByteBuffer pieces and
> populate it's buffer with those.
> Everything else would work normally except now all the buffered data would be
> disk backed and in cases where there was significant backlog these would
> start to fill up and page out.
> We currently allow messages larger than batchSize and to handle these we do a
> one-off allocation of the necessary size. We would have to disallow this when
> running in mmap mode. However since the disk buffer will be really big this
> should not be a significant limitation as the batch size can be pretty big.
> We would want to ensure that the pooling always gives out the most recently
> used ByteBuffer (I think it does). This way under normal operation where
> requests are processed quickly a given buffer would be reused many times
> before any physical disk write activity occurred.
> Note that although this let's the producer buffer very large amounts of data
> the buffer isn't really fault-tolerant, since the ordering in the file isn't
> known so there is no easy way to recovery the producer's buffer in a failure.
> So the scope of this feature would just be to provide a bigger buffer for
> short outages or latency spikes in the Kafka cluster during which you would
> hope you don't also experience failures in your producer processes.
> To complete the feature we would need to:
> a. Get some unit tests that would cover disk-backed usage
> b. Do some manual performance testing of this usage and understand the impact
> on throughput.
> c. Do some manual testing of failure cases (i.e. if the broker goes down for
> 30 seconds we should be able to keep taking writes) and observe how well the
> producer handles the catch up time when it has a large backlog to get rid of.
> d. Add a new configuration for the producer to enable this, something like
> use.file.buffers=true/false.
> e. Add documentation that covers these new options.
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