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From Gyula Fóra <gyf...@apache.org>
Subject Re: About stateful transformations
Date Mon, 24 Oct 2016 07:29:49 GMT
Hi Juan,

Let me try to answer some of your questions :)

We have been running Flink Streaming at King for quite some time now with
multiple jobs having several hundred gigabytes of KV state stored in
RocksDB. I would say RocksDB state backend is definitely the best choice at
the moment for large deployments as you can also keep the heap relatively
small to save some time on GC. But you have to play around with the rocks
configuration to get the most out of it depending on your hardware.

I am not aware of any caching/TTL functionality exposed in the Flink APIs
currently. But if you are willing to dig a llittle deeper you could
implement a lower lever operator that uses timers like the windowing
mechanisms to clear state after some time.

When you are selecting a checkpoint directory (URL) you need to make sure
that it is accessible from all the task managers. HDFS is convenient but is
not strictly necessary. We for instance use CEPH that is mounted as a
regular disk from the OS's perspective so we can use file:// and still save
to the distributed storage. As far as I know using yarn doesnt give much
benefit, I am not sure if Flink exploits any data locality at this moment.

When you are running rocks db state backend there are two concepts you need
to think about for checkpointing. Your local rocks db directory, and the
checkpoint directory. The local directory is where the rocks instances are
created and they live on the taskmanagers local disk/memory. When Flink
takes a checkpoint a backup of all K-V pairs is copied as one blob to HDFS
or to the selected checkpoint directory. This means there is no data
fragmentation in the checkpoints. Similar applies to the FsStateBackend but
that keeps the local state strictly in memory.

I think you should definitely give RocksDB + HDFS a try. It works extremely
well for very large state sizes given some tuning, but should also perform
out-of-the-box :)


Juan Rodríguez Hortalá <juan.rodriguez.hortala@gmail.com> ezt írta
(időpont: 2016. okt. 23., V, 22:29):

> Hi all,
> I don't have much experience with Flink, so please forget me if I ask some
> obvious questions. I was taking a look to the documentation on stateful
> transformations in Flink at
> https://ci.apache.org/projects/flink/flink-docs-release-1.2/dev/state.html.
> I'm mostly interested in Flink for stream processing, and I would like to
> know:
> - What is the biggest state that has been used in production deployments?
> I'm interested in how many GB of state, among all key-value pairs, have
> been used before in long running streaming jobs deployed in production.
> Maybe some company has shared this in some conference or blog post. I guess
> for that RocksDB backend is the best option for big states, to avoid being
> limited by the available memory.
> - Is there any pre built functionality for state eviction? I'm thinking of
> LRU cache-like behavior, with eviction based on time or size, similar to
> Guava cache (https://github.com/google/guava/wiki/CachesExplained). This
> is probably easy to implement anyway, by using the clear() primitive, but I
> wouldn't like to reinvent the wheel if this is already implemented
> somewhere.
> - When using file:// for the checkpointing URL, is the data replicated in
> the workers, or a failure in a worker leads to losing the state stored in
> that worker? I guess with hdfs:// we get the replication of HDFS, and we
> don't have that problem. I also guess S3 can be used for checkpointing
> state too, is there any remarkable performance impact of using s3 instead
> of HDFS for checkpointing? I guess some performance is lost compared to a
> setup running in YARN with collocated DataNodes and NodeManagers, but I
> wanted to know if the impact is negible, as checkpointing is performed at a
> relatively slow frequency. Also I'm interested on Flink running on EMR,
> where the impact of this should be even smaller because the access to S3 is
> faster from EMR than from an in-house YARN cluster outside the AWS cloud.
> - Is there any problem with the RocksDB backend on top of HDFS related to
> defragmentation? How is clear handled for long running jobs? I'm thinking
> on a streaming job that has a state with a size of several hundred GBs,
> where each key-pair is stored for a week and then deleted. How does clear()
> work, and how do you deal with the "small files problem" of HDFS (
> http://inquidia.com/news-and-info/working-small-files-hadoop-part-1) for
> the FsState and RocksDB backend on top of HDFS? I guess this wouldn' t be a
> problem for S3, as it is an object store that has no problem with small
> files.
> Thanks a lot for your help!
> Greetings,
> Juan Rodriguez Hortala

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