Hi igniters,
I need your advice with the following issue. When in-memory cache
reaches it's memory limit, some data may be purged to avoid OOM error.
This process is described in [1]. For MVCC caches this eviction may
break repeatable read semantics. For example, if transaction reads key
before eviction, this key is visible for it. But if key is evicted some
time later, this key become invisible to anyone, including our
transaction, which means broken repeatable read semantics.
Now we see the following solutions of this problem:
1. Ignore broken repeatable read semantics. If cache is set to allow
data eviction, it may lose it's data. This means that there is no
valuable information stored in cache and occasional repeatable read
violations can be tolerated.
2. Prohibit eviction of MVCC caches at all. For example, stop writing to
caches and throw an appropriate exception in the case when there is no
free space in page memory. Before this exception Ignite should do it's
best to avoid this situation, for example, evict all non-mvcc caches and
run full vacuum to free as much space as possible.
First approach is bad because it leads to cache consistency violation.
Second approach is bad because it's behavior may be unexpected to user
if he has set an eviction policy for cache, but instead of eviction
Ignite trying to avoid it as much as possible.
IMO first approach looks better - it is much simpler to implement and
met user expectations in all points except possible repeatable read
violations.
What do you think?
[1] https://apacheignite.readme.io/docs/evictions
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Kind Regards
Roman Kondakov
