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From "Brandon Bradley (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-1955) Explore disk-based buffering in new Kafka Producer
Date Wed, 21 Jun 2017 19:47:00 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-1955?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16058087#comment-16058087
] 

Brandon Bradley commented on KAFKA-1955:
----------------------------------------

I just pushed what I had previously completed and closed the PR since the scope of the requirements
changed. But it's there if someone needs to look.

> 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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