[IGNITE-18011] Avoid obtaining LogManager to stream RAFT snapshots

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Invalid
• Fix Version/s: 3.0
• Component/s: persistence
• Labels: ignite-3
• Epic Link: Storage support for Rebalancing
• Assignee: Roman Puchkovskiy
• Reporter: Roman Puchkovskiy
• Created: 28/Oct/22 13:59
• Updated: 04/Sep/24 16:19
• Resolved: 16/Nov/22 10:45

Description

When freezing an outgoing snapshot scope (see OutgoingSnapshot#freezeScope()), we need to get snapshot metadata corresponding to the contents of our storages (MV+TX) at that precise moment. The snapshot metadata includes last applied index (which we have), also term and lists of followers and learners which we need to obtain. In JRaft, we can take them from LogManager.

The problem is that JRaft does not provide a way to get the corresponding LogManager from this code. The 'right' way to fix this would be to change JRaft internals so that LogManager instance is made available to snapshot readers. But we should not touch JRaft core when we can avoid this (because, in the future, we might need to merge new version of JRaft into our codebase). So current implementation adds a way to obtain the current JRaft Node and then take LogManager instance from it.

A facility to get peers (followers/learners) is planned in the future, it could be used instead of a LogManager. But it's not clear how a term could be obtained without it.

Issue Links

• is superceded by: IGNITE-17935 Finish implementation of streaming RAFT snapshot sender
• is superceded by: IGNITE-18122 Track last applied term and group config in storages