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From Anton Kalashnikov <kaa....@yandex.ru>
Subject Baseline auto-adjust`s discuss
Date Thu, 24 Jan 2019 15:33:04 GMT

Hello, Igniters!

Work on the Phase II of IEP-4 (Baseline topology) [1] has started. I want to start to discuss
of implementation of "Baseline auto-adjust" [2].

"Baseline auto-adjust" feature implements mechanism of auto-adjust baseline corresponding
to current topology after event join/left was appeared. It is required because when a node
left the grid and nobody would change baseline manually it can lead to lost data(when some
more nodes left the grid on depends in backup factor) but permanent tracking of grid is not
always possible/desirible. Looks like in many cases auto-adjust baseline after some timeout
is very helpfull.

Distributed metastore[3](it is already done):

First of all it is required the ability to store configuration data consistently and cluster-wide.
Ignite doesn't have any specific API for such configurations and we don't want to have many
similar implementations of the same feature in our code. After some thoughts is was proposed
to implement it as some kind of distributed metastorage that gives the ability to store any
data in it. 
First implementation is based on existing local metastorage API for persistent clusters (in-memory
clusters will store data in memory). Write/remove operation use Discovery SPI to send updates
to the cluster, it guarantees updates order and the fact that all existing (alive) nodes have
handled the update message. As a way to find out which node has the latest data there is a
"version" value of distributed metastorage, which is basically <number of all updates,
hash of updates>. All updates history until some point in the past is stored along with
the data, so when an outdated node connects to the cluster it will receive all the missing
data and apply it locally. If there's not enough history stored or joining node is clear then
it'll receive shapshot of distributed metastorage so there won't be inconsistencies.

Baseline auto-adjust:

Main scenario:
	- There is grid with the baseline is equal to the current topology
	- New node joins to grid or some node left(failed) the grid
	- New mechanism detects this event and it add task for changing baseline to queue with configured
timeout
	- If new event are happened before baseline would be changed task would be removed from queue
and new task will be added
	- When timeout are expired the task would try to set new baseline corresponded to current
topology

First of all we need to add two parameters[4]:
	- baselineAutoAdjustEnabled - enable/disable "Baseline auto-adjust" feature.
	- baselineAutoAdjustTimeout - timeout after which baseline should be changed.

This parameters are cluster wide and can be changed in real time because it is based on "Distributed
metastore". On first time this parameters would be initiated by corresponded parameters(initBaselineAutoAdjustEnabled,
initBaselineAutoAdjustTimeout) from "Ignite Configuration". Init value valid only before first
changing of it after  value would be changed it is stored in "Distributed metastore".

Restrictions:
	- This mechanism handling events only on active grid
	- If baselineNodes != gridNodes on activate this feature would be disabled
	- If lost partitions was detected this feature would be disabled
	- If baseline was adjusted manually on baselineNodes != gridNodes this feature would be disabled

Draft implementation you can find here[5]. Feel free to ask more details and make suggestions.

[1] https://cwiki.apache.org/confluence/display/IGNITE/IEP-4+Baseline+topology+for+caches
[2] https://issues.apache.org/jira/browse/IGNITE-8571
[3] https://issues.apache.org/jira/browse/IGNITE-10640
[4] https://issues.apache.org/jira/browse/IGNITE-8573
[5] https://github.com/apache/ignite/pull/5907

-- 
Best regards,
Anton Kalashnikov


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