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From "Josh Elser (JIRA)" <>
Subject [jira] [Resolved] (ACCUMULO-3320) Replication reference prematurely closed and removed.
Date Mon, 10 Nov 2014 21:55:34 GMT


Josh Elser resolved ACCUMULO-3320.
    Resolution: Fixed

Changed the GC from only inspecting the WALs in use from the metadata table (the tablet definitions)
to also reaching out to each TServer to ask for its active logs.

Including the extra logs is the correct thing to make sure replication entries are not prematurely
deleted; however, the manner in which it's done is RPC heavy and influenced by process/network

Should address the issue described here; however there is likely a better long term solution
which involves persistent storage of WALs in some other storage (ZK or metadata table).

> Replication reference prematurely closed and removed.
> -----------------------------------------------------
>                 Key: ACCUMULO-3320
>                 URL:
>             Project: Accumulo
>          Issue Type: Bug
>          Components: replication
>            Reporter: Josh Elser
>            Assignee: Josh Elser
>             Fix For: 1.7.0
>          Time Spent: 50m
>  Remaining Estimate: 0h
> Saw the following situation in MultiInstanceReplicationIT across GC, tserver and master.
03b6dad5-56c9-4f35-8daf-a444f3252038 appeared to never get cleaned up:
> {noformat}
> 2014-11-08 12:05:34,898 [tserver.Tablet] DEBUG: Logs to be destroyed: 2<< tserver:37732/file:////.../accumulo/test/target/mini-tests/org.apache.accumulo.test.replication.MultiInstanceReplicationIT_dataWasReplicatedToThePeer/accumulo/wal/tserver+37732/03b6dad5-56c9-4f35-8daf-a444f3252038
> 2014-11-08 12:05:34,904 [tserver.TabletServer] DEBUG: ScanSess tid
!0 0 entries in 0.03 secs, nbTimes = [25 25 25.00 1] 
> 2014-11-08 12:05:34,914 [tserver.Tablet] DEBUG: Recording that data has been ingested
into 2<< using [file:////.../accumulo/test/target/mini-tests/org.apache.accumulo.test.replication.MultiInstanceReplicationIT_dataWasReplicatedToThePeer/accumulo/wal/tserver+37732/03b6dad5-56c9-4f35-8daf-a444f3252038,
> 2014-11-08 12:05:34,914 [util.ReplicationTableUtil] DEBUG: Updating replication status
for 2<< with [file:////.../accumulo/test/target/mini-tests/org.apache.accumulo.test.replication.MultiInstanceReplicationIT_dataWasReplicatedToThePeer/accumulo/wal/tserver+37732/03b6dad5-56c9-4f35-8daf-a444f3252038,
using [begin: 0 end: 0 infiniteEnd: true closed: false]
> 2014-11-08 12:05:35,008 [replication.CloseWriteAheadLogReferences] INFO : Found 1 WALs
referenced in metadata in 100.3 ms
> 2014-11-08 12:05:35,127 [replication.CloseWriteAheadLogReferences] DEBUG: Closing unreferenced
WAL (~replfile:/.../accumulo/test/target/mini-tests/org.apache.accumulo.test.replication.MultiInstanceReplicationIT_dataWasReplicatedToThePeer/accumulo/wal/tserver+37732/03b6dad5-56c9-4f35-8daf-a444f3252038
stat:2 [] 5 false) in metadata table
> 2014-11-08 12:05:35,221 [replication.CloseWriteAheadLogReferences] INFO : Closed 1 WAL
replication references in replication table in 206.2 ms
> 2014-11-08 12:05:36,505 [tserver.Tablet] DEBUG: Logs to be destroyed: !0<;~ ip-172-31-47-246:37732/file:////.../accumulo/test/target/mini-tests/org.apache.accumulo.test.replication.MultiInstanceReplicationIT_dataWasReplicatedToThePeer/accumulo/wal/tserver+37732/03b6dad5-56c9-4f35-8daf-a444f3252038
> 2014-11-08 12:05:36,517 [replication.StatusMaker] DEBUG: Creating replication status
record for file:/.../accumulo/test/target/mini-tests/org.apache.accumulo.test.replication.MultiInstanceReplicationIT_dataWasReplicatedToThePeer/accumulo/wal/tserver+37732/03b6dad5-56c9-4f35-8daf-a444f3252038
on table 2 with [begin: 0 end: 0 infiniteEnd: true closed: true createdTime: 1415448333352].
> 2014-11-08 12:06:27,097 [util.ReplicationTableUtil] DEBUG: Updating replication status
for 2<< with [file:////.../accumulo/test/target/mini-tests/org.apache.accumulo.test.replication.MultiInstanceReplicationIT_dataWasReplicatedToThePeer/accumulo/wal/tserver+37732/03b6dad5-56c9-4f35-8daf-a444f3252038]
using [begin: 0 end: 0 infiniteEnd: true closed: false]
> {noformat}
> This is problematic due to cross-process interaction. The GC happened to run just after
the tserver performed a compaction and removed the log reference from the tablet. Thus, at
a very small point in time, the WAL was not referenced by any tablets in the metadata table,
and, as such, the GC "closed" the WAL replication reference.
> The master saw that it was closed, cleaned up the reference and started replication.
The tserver continued to use the WAL (as it does) and placed some more updates into metadata.
> The other problem is that when the master saw that replication of the file was completed,
it removed the references from the replication table (as expected). However, when it went
to the next round of StatusMaker, it made a new record which had lost the fact that the old
file had been fully replicated.

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