Hi Igniters, Currently I’m working on possible approaches how to implement historical rebalance (delta rebalance using WAL iterator) over MVCC caches.
The main difficulty is that MVCC writes changes on tx active phase while partition update version, aka update counter, is being applied on tx finish. This means we cannot start iteration over WAL right from the pointer where the update counter updated, but should include updates, which the transaction that updated the counter did. These updates may be much earlier than the point where the update counter was updated, so we have to be able to identify the point where the first update happened. The proposed approach includes: 1) preserve list of active txs, sorted by the time of their first update (using WAL ptr of first WAL record in tx) 2) persist this list on each checkpoint (together with TxLog for example) 4) send whole active tx list (transactions which were in active state at the time the node was crushed, empty list in case of graceful node stop) as a part of partition demand message. 4) find a checkpoint where the earliest tx exists in persisted txs and use saved WAL ptr as a start point or apply current approach in case the active tx list (sent on previous step) is empty 5) start iteration. Your thoughts? Regards, Igor
