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From Sergey Kalashnikov <zkilling...@gmail.com>
Subject cache size() calculation for MVCC
Date Tue, 24 Apr 2018 13:28:51 GMT
Hi Igniters,

I need your advice on a task at hand.

Currently cache API size() is a constant time operation, since the
number of entries is maintained as a separate counter.
However, for MVCC-enabled cache there can be multiple versions of the
same entry.
In order to calculate the size we need to obtain a MVCC snapshot and
then iterate over data pages filtering invisible versions.
So, it is impossible to keep the same complexity guarantees.

My current implementation internally switches to "full-scan" approach
if cache in question is a MVCC-enabled cache.
It happens unbeknown to users, which may expect lightning-fast
response as before.
Perhaps, we might add a new constant to CachePeekMode enumeration that
is passed to cache size() to make it explicit?

The second concern is that cache size calculation is also included
into Cache Metrics API and Visor functionality.
Will it be OK for metrics and things alike to keep returning raw
unfiltered number of entries?
Is there any sense in showing raw unfiltered number of entries which
may vary greatly from invokation to invokation with just simple
updates running in background?

Please share your thoughts.

Thanks in advance.
--
Sergey

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