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https://issues.apache.org/jira/browse/RATIS-2274?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Tsz-wo Sze resolved RATIS-2274.
-------------------------------
Fix Version/s: 3.2.0
Resolution: Fixed
The pull request is now merged. Thanks, [~gewu12207] !
> Newly added peer may retain outdated configuration after membership change,
> causing election failure
> ----------------------------------------------------------------------------------------------------
>
> Key: RATIS-2274
> URL: https://issues.apache.org/jira/browse/RATIS-2274
> Project: Ratis
> Issue Type: Bug
> Components: Leader, raft-group
> Reporter: Xinhao GU
> Assignee: Xinhao GU
> Priority: Major
> Fix For: 3.2.0
>
> Attachments: image-2025-04-04-16-53-49-441.png
>
> Time Spent: 0.5h
> Remaining Estimate: 0h
>
> h1. *Problem Description*
> When dynamically adding a new peer (G) through membership change, the peer
> might retain an obsolete configuration (e.g., ancient {{{}[A,B,C]{}}})
> instead of adopting the latest configuration {{{}[D,E,F]{}}}. If the original
> leader (D) and another peer (E) fail simultaneously, the surviving peer (F)
> will trigger an election. The newly added peer G (retaining old config) will
> reject F's vote request because F doesn't exist in its configuration
> ({{{}[A,B,C]{}}}), leading to election deadlock and cluster unavailability.
> h2. *Steps to Reproduce*
> # Initial Cluster: 3 peers {{[D(Leader), E, F]}} with all confirmed at
> config {{[D,E,F]}}
> # Client sends {{setConfiguration(D,E,F,G)}}
> # Leader D creates {{GrpcLogAppender}} thread for peer G
> # New peer G joins and starts log catch-up
> # Leader marks G as "caught up" based on current criteria, triggers joint
> consensus mechanism to commit new config {{[D,E,F,G]}}
> # Client receives success response, cluster considers config {{[D,E,F,G]}}
> active
> # Simulate failures: Shut down Leader D and peer E
> # Peer F initiates election, sends {{RequestVote}} RPC to G
> h2. *Expected Behavior*
> After new config {{[D,E,F,G]}} is committed, all peers including G should use
> the latest config for elections and log replication.
> h2. *Actual Behavior*
> Peer G retains outdated config {{[A,B,C]}} (not containing F), rejecting F's
> vote request and preventing leader election.
> h2. *Log Evidence*
> {code:java}
> 2025-01-18 00:29:35,164 [grpc-default-executor-1] INFO
> o.a.r.s.i.RaftServerImpl:1410 - G@group-00020000000F: receive
> requestVote(PRE_VOTE, F, group-00020000000F, 149, (t:149, i:606))
> 2025-01-18 00:29:35,164 [grpc-default-executor-1] INFO
> o.a.r.s.impl.VoteContext:49 - G@group-00020000000F-FOLLOWER: reject PRE_VOTE
> from F: F is not in current conf [A|192.168.130.3:10750,
> B|192.168.130.5:10750, C|192.168.130.4:10750]
> 2025-01-18 00:29:35,164 [grpc-default-executor-1] INFO
> o.a.r.s.i.RaftServerImpl:1444 - G@group-00020000000F replies to PRE_VOTE vote
> request: F<-G#0:FAIL-t149. Peer's state: G@group-00020000000F:t149, leader=D,
> voted=null,
> raftlog=Memoized:G@group-00020000000F-SegmentedRaftLog:OPENED:c333:last(t:78,
> i:357), conf=conf: {index: 447, cur=peers:[A|192.168.130.3:10750,
> B|192.168.130.5:10750, C|192.168.130.4:10750]|listeners:[], old=null} {code}
> h1. *Root Cause Analysis*
> The current catch-up validation in {{checkProgress()}} relies on:
> * {{stagingCatchupGap}} (default 1000 entries)
> * {{LastRpcResponseTime}}
> * {{hasAttemptedToInstallSnapshot}}
> !image-2025-04-04-16-53-49-441.png|width=804,height=307!
> {{{*}Critical Flaw{*}: When the leader has no snapshot (forcing log-based
> catch-up), the new peer might *lack timely configuration change logs* due to
> }}{{{}stagingCatchupGap{}}}{{{}. This leaves the new peer with outdated
> configurations (e.g., {}}}{{{}[A,B,C]{}}}{{{}), violating Raft's membership
> change safety guarantees.{}}}
> {{To solve this problem, the Leader is essentially required to ensure that
> the configuration of the Follower is the latest when the Follower completes
> the catch-up.}}
> h1. *Proposed Solutions*
> {{Observe the checkProgress() function to determine the CAUGHTUP logic. We
> can modify the logic in the following ways.}}
> h2. First Idea
> *More Strict Catch-Up* *Validation*
> Add one more condition to the CAUGHTUP judgment logic. That is, CAUGHTUP is
> complete only after a follower receives the latest conf logs from the leader.
> {code:java}
> follower.getMatchIndex() >= server.getRaftConf().getLogEntryIndex() {code}
> h2. Second Idea
> *Snapshot Enforcement*
> {{If the follower is in the Bootstrapping state, the snapshot is forcibly
> transmitted first. If the leader does not have the snapshot, a new snapshot
> is generated and then sent. (Because the snapshot carries the latest conf)}}
> {code:java}
> if (follower.isBootstrapping() && !leader.hasSnapshot()) {
> leader.triggerSnapshotCreation();
> }{code}
> h2. Third Idea
> *Disable* *Staging* ** *Gap* *for Config Changes*
> {{Set stagingCatchupGap to 0. CAUGHTUP is complete only when the matIndex of
> followers is completely equal to the commitIndex of the current leader.}}
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