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https://issues.apache.org/jira/browse/ZOOKEEPER-4643?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kezhu Wang resolved ZOOKEEPER-4643.
-----------------------------------
Release Note: Superceded by ZOOKEEPER-4785.
Resolution: Duplicate
> Committed txns may be improperly truncated if follower crashes right after
> updating currentEpoch but before persisting txns to disk
> -----------------------------------------------------------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-4643
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-4643
> Project: ZooKeeper
> Issue Type: Bug
> Components: quorum, server
> Affects Versions: 3.6.3, 3.7.0, 3.8.0, 3.7.1, 3.9.0, 3.8.1, 3.9.1
> Reporter: Sirius
> Priority: Critical
> Labels: pull-request-available
> Attachments: Trace-ZK-4643.pdf
>
> Time Spent: 4h 50m
> Remaining Estimate: 0h
>
> When a follower is processing the NEWLEADER message in SYNC phase, it will
> update its {{_currentEpoch_}} to the file *before* writing the txns (from the
> PROPOSALs sent by leader in SYNC) to the log file. Such execution order may
> lead to improper truncation of *committed* txns on other servers in later
> rounds.
> The critical step to trigger this problem is to make a follower node crash
> right after it updates its {{_currentEpoch_}} to the file but before writing
> the txns to the log file. The potential risk is that, this node with
> incomplete committed txns might be later elected as the leader with its
> larger {{{}_currentEpoch_{}}}, and then improperly uses TRUNC to ask other
> nodes to truncate their committed txns!
>
> h2. Trace
> [^Trace-ZK-4643.pdf]
> Here is an example to trigger the bug. (Focus on {{_currentEpoch_}} and
> {{{}_lastLoggedZxid_{}}})
> {*}Round 1 (Running nodes with their acceptedEpoch & currentEpoch set to
> 1{*}{*}):{*}
> - Start the ensemble with three nodes: S{+}0{+}, +S1+ & {+}S2{+}.
> - +S2+ is elected leader.
> - For all of them, _{{currentEpoch}}_ = 1, {{_lastLoggedZxid_}} (the last
> zxid in the log)= <1, 3>, {{_lastProcessedZxid_}} = <1, 3>.
> - +S0+ crashes.
> - A new txn <1, 4> is logged and committed by +S1+ & {+}S2{+}. Then, +S1+ &
> +S2+ have {{_lastLoggedZxid_}} = <1, 4>, {{_lastProcessedZxid_}} = <1, 4> .
> - Verify clients can read the datatree with latest zxid <1, 4>.
> *Round 2* {*}(Running nodes with their acceptedEpoch & currentEpoch set to
> 2{*}{*}){*}{*}:{*}
> * +S0+ & +S2+ restart, and +S1+ crashes.
> * Again, +S2+ is elected leader.
> * Then, during the SYNC phase, the leader +S2+ ({{{}_maxCommittedLog_{}}} =
> <1, 4>) uses DIFF to sync with the follower +S0+ ({{{}_lastLoggedZxid_{}}} =
> <1, 3>), and their {{_currentEpoch_}} will be set to 2 (and written to disk).
> * ( Note that the follower +S0+ updates its currentEpoch file before writing
> the txns to the log file when receiving NEWLEADER message. )
> * *Unfortunately, right after the follower +S0+ finishes updating its
> currentEpoch file, it crashes.*
> *Round 3* {*}(Running nodes with their acceptedEpoch & currentEpoch set to
> 3{*}{*}){*}{*}:{*}
> * +S0+ & +S1+ restart, and +S2+ crashes.
> * Since +S0+ has {{_currentEpoch_}} = 2, +S1+ has {{_currentEpoch_}} = 1,
> +S0+ will be elected leader.
> * During the SYNC phase, the leader +S0+ ({{{}_maxCommittedLog_{}}} = <1,
> 3>) will use TRUNC to sync with +S1+ ({{{}_lastLoggedZxid_{}}} = <1, 4>).
> Then, +S1+ removes txn <1, 4>.
> * ( However, <1, 4> was committed and visible by clients before, and is not
> supposed to be truncated! )
> * Verify clients of +S0+ & +S1+ do NOT have the view of txn <1, 4>, a
> violation of ZAB.
>
> Extra note: The trace can be constructed with quorum nodes alive at any
> moment with careful time tuning of node crash & restart, e.g., let +S1+
> restart before +S0+ crashes at the end of Round 2.
>
> h2. Analysis
> *Root Cause:*
> When a follower updates its current epoch, it should guarantee that it has
> already synced the uncommitted txns to the disk (or, taken snapshot).
> Otherwise, after the current epoch is updated to the file but the history
> (transaction log) of the follower is not updated yet, environment failures
> might prevent the latter from going on smoothly. It is dangerous for a node
> with updated current epoch but stale history to be elected leader. It might
> truncate committed txns on other nodes.
>
> *Property Violation:*
> * From the server side, the ensemble deletes a committed txn, which is not
> allowed; the committed log of the ensemble does not append monotonically;
> different nodes have inconsistent committed logs.
> * From the client side, clients connected to different nodes may have
> inconsistent views. A client may read stale data after a newer version is
> obtained. That newer version can only be obtained from certain nodes of the
> ensemble rather than all nodes. What's worse, that newer version may also be
> removed later.
> ZOOKEEPER-3911 shows similar symptoms, but its fix only mitigates the
> occurrance of the problem without solving it at the root. We also raise an
> issue related to that fix in ZOOKEEPER-4646 .
> Although ZOOKEEPER-4646 has similar symptoms and property violations, it
> should be regarded as a distinct problem because it has different root cause
> and risk pattern compared to this one. More specifically,
> * ZOOKEEPER-4643 : the risk lies in the order of updating currentEpoch
> before logging txns to disk. The bug can be triggered by interrupting the
> action of logging txns after currentEpoch is updated.
> * ZOOKEEPER-4646 : the risk lies in the order of replying ACK-LD before
> logging txns to disk. The bug can be triggered by interrupting the action of
> logging txns after ACK-LD is replied.
>
> *Gap between Theory & Reality:*
> The critical step here is to interrupt two updates that should be done
> together. However, there lie some gaps between the theory and the reality. In
> the Zab paper, a follower updates its current epoch and history during the
> SYNC phase in an *atomic* action, and the correctness of the Zab protocol is
> based on that. Unfortunately, in actual environment, a node crash (or other
> environment failures) may occur at any time, breaking the procedures that
> will not be interrupted at the protocol level. By digging into the code, the
> *synchronized* section in the method {{syncWithLeader(long)}} in
> {{Learner.java}} can only prevent the procedure from interrupted by other
> threads, but environment failures can still interrupt the process. We think
> it important to keep ZooKeeper still in a correct condition even under such
> type of environment circumstances.
>
> *Affected Versions:*
> The above trace has been generated in multiple versions such as 3.7.1 & 3.8.1
> (the latest stable & current version till now) by our testing tools. The
> affected versions might be more, because the critical update order between
> the follower's current epoch and its history during SYNC stay unchanged
> across multiple versions.
>
> h2. Possible Fix
> In theory, this issue can be avoided by exchanging the order of writing
> {{_currentEpoch_}} to the currentEpoch file and writing the txns (in the
> queue {_}packetsNotCommitted{_}) to the log file when the follower is
> processing NEWLEADER message. Although it is a more {_}c{_}onservative
> strategy to persist txns to disk before updating {{_currentEpoch_}} to disk,
> it prevents the problem that a node with the incomplete committed txns can
> win an election with its higher _{{{}currentEpoch{}}}._
> When it comes to the implementation of multi-threading, things become a
> little more complicated. In the procedure of processing NEWLEADER message in
> the method {{syncWithLeader(long)}} in {{{}Learner.java{}}}, simply
> exchanging the order of updating {{_currentEpoch_}} and writing txns to the
> log file cannot fix the problem, because these two actions are conducted
> asynchronously by different threads. When the QuorumPeer thread calls
> {{logRequest(..)}} to submit log persistence task to the SyncThread, the
> latter thread will write txns to disk asynchronously, without promising to
> finish that before the node's {{_currentEpoch_}} updated.
> Considering this issue and ZOOKEEPER-4646, one possible fix is to guarantee
> the following partial orders to be satisfied:
> * A follower updates its {{_currentEpoch_}} only after it has persisted the
> txns that might be applied to the leader's datatree before the leader gets
> into the BROADCAST phase (so as to avoid this issue).
> * A follower replies ACK-LD only after it has persisted the txns that might
> be applied to the leader's datatree before the leader gets into the BROADCAST
> phase (so as to avoid the issue of ZOOKEEPER-4646 ).
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