Hi Fabian,
thanks for quick reply, what you suggest seems to work at first sight, I
will try it. Is there any reason not to implement a RocksDBListState
this way in general? Is there any increased overhead of this approach?
Thanks,
Jan
On 12/12/2017 11:17 AM, Fabian Hueske wrote:
Hi Jan,
I cannot comment on the internal design, but you could put the data into a
RocksDBStateBackend MapState<Integer, X> where the value X is your data
type and the key is the list index. You would need another ValueState for
the current number of elements that you put into the MapState.
A MapState allows to fetch and traverse the key, value, or entry set of the
Map without loading it completely into memory.
The sets are traversed in sort order of the key, so should be in insertion
order (given that you properly increment the list index).
Best, Fabian
2017-12-12 10:23 GMT+01:00 Jan Lukavský <je...@seznam.cz>:
Hi all,
I have a question that appears as a user@ question, but brought me into
the dev@ mailing list while I was browsing through the Flink's source
codes. First I'll try to briefly describe my use case. I'm trying to do a
group-by-key operation with a limited number of distinct keys (which I
cannot control), but a non trivial count of values. The operation in the
GBK is non-combining, so that all values per key (many) have to be stored
in a state. Running this on testing data led to a surprise (for me), that
even when using RocksDBStateBackend, the whole list of data is serialized
into single binary blob and then deserialized into List, and therefore has
to fit in memory (multiple times, in fact).
I tried to create an alternative RocksDBStateBackend, that would store
each element of list in ListState to a separate key in RocksDB, so that the
whole blob would not have to be loaded by a single get, but a scan over
multiple keys could be made. Digging into the source code I found there was
a hierarchy of classes mirroring the public API in 'internal' package -
InternalKvState, InternalMergingState, InternalListState, and so on. These
classes however have different hierarchy than the public API classes that
they mirror, most notably InternalKvState is superinterface of all others.
This fact seems to be used on multiple places throughout the source code.
My question is - is this intentional? Would it be possible to store each
element of a ListState in a separate key in RocksDB (probably by adding
some suffix to the actual key of the state for each element)? What are the
pitfalls? And is it necessary for the InternalListState to be actually
subinterface of InternalKvState? I find this to be a related problem.
Many thanks for any comments or thoughts,
Jan
