ASF GitHub Bot commented on ZOOKEEPER-2845:

Github user revans2 commented on a diff in the pull request:

    --- Diff: src/java/main/org/apache/zookeeper/server/ZKDatabase.java ---
    @@ -233,14 +233,32 @@ public long getDataTreeLastProcessedZxid() {
          * @throws IOException
         public long loadDataBase() throws IOException {
    -        PlayBackListener listener=new PlayBackListener(){
    +        PlayBackListener listener = new PlayBackListener(){
                 public void onTxnLoaded(TxnHeader hdr,Record txn){
                     Request r = new Request(0, hdr.getCxid(),hdr.getType(), 
hdr, txn, hdr.getZxid());
    -        long zxid = 
    +        long zxid = snapLog.restore(dataTree, sessionsWithTimeouts, 
    +        initialized = true;
    +        return zxid;
    +    }
    +    /**
    +     * Fast forward the database adding transactions from the committed 
log into memory.
    +     * @return the last valid zxid.
    +     * @throws IOException
    +     */
    +    public long fastForwardDataBase() throws IOException {
    +        PlayBackListener listener = new PlayBackListener(){
    --- End diff --
    Will do

> Data inconsistency issue due to retain database in leader election
> ------------------------------------------------------------------
>                 Key: ZOOKEEPER-2845
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2845
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: quorum
>    Affects Versions: 3.4.10, 3.5.3, 3.6.0
>            Reporter: Fangmin Lv
>            Assignee: Robert Joseph Evans
>            Priority: Critical
> In ZOOKEEPER-2678, the ZKDatabase is retained to reduce the unavailable time 
> during leader election. In ZooKeeper ensemble, it's possible that the 
> snapshot is ahead of txn file (due to slow disk on the server, etc), or the 
> txn file is ahead of snapshot due to no commit message being received yet. 
> If snapshot is ahead of txn file, since the SyncRequestProcessor queue will 
> be drained during shutdown, the snapshot and txn file will keep consistent 
> before leader election happening, so this is not an issue.
> But if txn is ahead of snapshot, it's possible that the ensemble will have 
> data inconsistent issue, here is the simplified scenario to show the issue:
> Let's say we have a 3 servers in the ensemble, server A and B are followers, 
> and C is leader, and all the snapshot and txn are up to T0:
> 1. A new request reached to leader C to create Node N, and it's converted to 
> txn T1 
> 2. Txn T1 was synced to disk in C, but just before the proposal reaching out 
> to the followers, A and B restarted, so the T1 didn't exist in A and B
> 3. A and B formed a new quorum after restart, let's say B is the leader
> 4. C changed to looking state due to no enough followers, it will sync with 
> leader B with last Zxid T0, which will have an empty diff sync
> 5. Before C take snapshot it restarted, it replayed the txns on disk which 
> includes T1, now it will have Node N, but A and B doesn't have it.
> Also I included the a test case to reproduce this issue consistently. 
> We have a totally different RetainDB version which will avoid this issue by 
> doing consensus between snapshot and txn files before leader election, will 
> submit for review.

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