Details
Major Description
Work on the Phase II of IEP-4 (Baseline topology) [1] has finished. It makes sense to implement some examples 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 All updates history until some point in the past is stored along with the data, so when an outdated node connects to the cluster it will receive all the missing data and apply it locally.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 a 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 a task for changing baseline to queue with configured timeout
- If a new event happens before baseline would be changed task would be removed from the queue and a new task will be added
- When a timeout is 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.
These parameters are cluster-wide and can be changed in real-time because it is based on "Distributed metastore".
Restrictions:
- This mechanism handling events only on active grid
- for in-memory nodes - enabled by default. For persistent nodes - disabled.
- If lost partitions was detected this feature would be disabled
- If baseline was adjusted manually on baselineNodes != gridNodes the exception would be thrown
[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