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From Andrey Kuznetsov <stku...@gmail.com>
Subject IgniteSet implementation: changes required
Date Mon, 30 Oct 2017 10:07:52 GMT
Hi, Igniters!

Current implementation of IgniteSet is fragile with respect to cluster
recovery from a checkpoint. We have an issue (IGNITE-5553) that addresses
set's size() behavior, but the problem is slightly broader. The text below
is my comment from Jira issue. I encourage you to discuss it.

We can put current set size into set header cache entry. This will fix
size(), but we have broken iterator() implementation as well.

Currently, set implementation maintains plain Java sets on every node, see
CacheDataStructuresManager.setDataMap. These sets duplicate backing-cache
entries, both primary and backup. size() and iterator() calls issue
distributed queries to collect/filter data from all setDataMap's. And
setDataMaps remain empty after cluster is recovered from checkpoint.

Now I see the following options to fix the issue.

#1 - Naive. Iterate over all datastructure-backing caches entries during
recover from checkpoint procedure, filter set-related entries and refill
setDataMap's.
Pros: easy to implement
Cons: inpredictable time/memory overhead.

#2 - More realistic. Avoid node-local copies of cache data. Maintain linked
list in datastructure-backing cache: key is set item, value is next set
item. List head is stored in set header cache entry (this set item is
youngest one). Iterators build on top of this structure are fail-fast.
Pros: less memory overhead, no need to maintain node-local mirrors of cache
data
Cons: iterators are not fail-safe.

#3 - Option #2 modified. We can store reference counter and 'removed' flag
along with next item reference. This allows to make iterators fail safe.
Pros: iterators are fail-safe
Cons: slightly more complicated implementation, may affect performance,
also I see no way to handle active iterators on remote nodes failures.


Best regards,

Andrey.

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