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the data, so when an outdated node connects to the cluster it will receive all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
