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From Kostas Kloudas <k.klou...@data-artisans.com>
Subject Re: maximum size of window
Date Tue, 28 Jun 2016 08:35:53 GMT
Hi Vishnu,

RocksDB allows for storing the window contents on disk when the state of a window becomes
too big.
BUT when you have to trigger and apply the computation of your window function on that big
then all of its state is loaded in memory.

So although during the window formation phase, RocksDB allows you to not  worry about storage
when it is time to fire your computation, then you have to consider how much RAM you have
and if the 
window fits in it.

> On Jun 28, 2016, at 1:25 AM, Vishnu Viswanath <vishnu.viswanath25@gmail.com> wrote:
> Hi Kostas,
> Thank you. 
> Yes 2) was exactly what I wanted to know.
> - So if I am using RocksDB as state backend, does that mean that I don't have to worry
much about the memory available per node since RocksDB will use RAM and Disk to store the
window state? 
> Regards,
> Vishnu
> On Mon, Jun 27, 2016 at 6:19 PM, Kostas Kloudas <k.kloudas@data-artisans.com <mailto:k.kloudas@data-artisans.com>>
> Hi Vishnu,
> I hope the following will help answer your question:
> 1) Elements are first split by key (apart from global windows) and then are put into
windows. In other words, windows are keyed.
> 2) A window belonging to a certain key is handled by a single node. In other words, no
matter how big the window is, its
>         state (the elements it contains) will never be split between two or more nodes.
> 3) Where the state is stored, depends on your state backend. Currently Flink supports
an in-memory one, a filesystem one, and
>         a rocksDB one which is in the middle (first in-memory and then disk when needed).
Of course you can implement your own.
> From the above, you can see that if you use the memory-backed state backend, then your
window size is limited by the memory
> available at each of your nodes. If you use the fs state backend, then your state is
stored on disk. Finally, rocksDB will initially
> use RAM and then spill on disk when no more memory is available.
> Here I have to add that the window documentation is currently being re-written to explain
new features introduced in Flink 1.1,
> which include more flexible handling of late events and more explicit state garbage collection.
> So please stay tuned!
> I hope this helps at answering your question,
> Kostas
> > On Jun 27, 2016, at 8:22 PM, Vishnu Viswanath <vishnu.viswanath25@gmail.com <mailto:vishnu.viswanath25@gmail.com>>
> >
> > Hi All,
> >
> > - Is there any restriction on the size of a window in Flink with respect to the
memory of the nodes?
> > - What happens if a window size grows more than size of a node, will it be split
into multiple nodes?
> >
> > if I am going to have a huge window, should I have fewer nodes with more memory.
> > Is there any documentation on how memory is managed/handled in the case of windows
and also in the case of joins.
> >
> > Regards,
> > Vishnu

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