ID | IEP-31 |
Author | |
Sponsor | |
Created | 4 March 2019 |
Status | DRAFT |
Apache Ignite provides a consistency guarantee, each backup should contain the same value for the same key, at least eventually.
But this guarantee can be violated because of ... bugs, cosmic rays, hackers, modern SSD, uborshitca with shwabra, but ... most likely because of bugs.
And yes, we had some and fixed them.
But, is there any chances we fixed all and will not write new? :)
So, we have to have additional failover layer to check and fix such violations.
The main idea is to provide special read mode which will read a value from primary and all backups and will check that values are the same.
In case values differ they should be fixed according to the appropriate strategy.
But what if we have 3+ different values for the same key at topology?
It's not true.
Bugs able to make every node outdated.
Not a 100% guarantee, but Simple!
Seems to be suitable because of each value related to the GridCacheVersion which is comparable.
Best case.
Can be implemented as an addon.
Also, we have to have special process checks we have no inconsistency.
Values should be compared and fixed periodically, but should not cause cooldown (hot pages to be replaced with pages from persistence).
1) LWW and any other strategy do not guarantee that the correct value will be chosen.
We have to record the event contains all values and the chosen one.
The event will allow to
- got we have an inconsistent state situation
- investigate which value is correct manually and refix if necessary
1.1) Seems, it's not possible to fix any cases.
For any transactional cache we able to perform pessimistic serializable transaction per key.
But, atomic caches cannot be fixed this way.
2) Consistency guard able to produce the following problems
- replace hot data with cold data
We have to have special configuration property allows or restricts to check (read) data available only at the persistence layer
- decrease throughput/latency metrics
Some throttle feature should be implemented
// Links to discussions on the devlist, if applicable.
// Links to various reference documents, if applicable.