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https://issues.apache.org/jira/browse/FLINK-7757?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16190804#comment-16190804
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ASF GitHub Bot commented on FLINK-7757:
---------------------------------------
Github user bowenli86 commented on a diff in the pull request:
https://github.com/apache/flink/pull/4764#discussion_r142579521
--- Diff:
flink-contrib/flink-statebackend-rocksdb/src/main/java/org/apache/flink/contrib/streaming/state/RocksDBKeyedStateBackend.java
---
@@ -356,28 +354,26 @@ public int getKeyGroupPrefixBytes() {
final long checkpointTimestamp,
final CheckpointStreamFactory checkpointStreamFactory) throws
Exception {
+ if (db == null) {
+ throw new IOException("RocksDB closed.");
--- End diff --
This exception message looks a bit odd to me. Can you please enrich the
message? E.g. "RocksDB has been closed expected. <suggestion for a solution>"?
> RocksDB lock is too strict and can block snapshots in synchronous phase
> -----------------------------------------------------------------------
>
> Key: FLINK-7757
> URL: https://issues.apache.org/jira/browse/FLINK-7757
> Project: Flink
> Issue Type: Bug
> Components: State Backends, Checkpointing
> Affects Versions: 1.2.2, 1.3.2
> Reporter: Stefan Richter
> Assignee: Stefan Richter
> Priority: Blocker
> Fix For: 1.4.0
>
>
> {{RocksDBKeyedStateBackend}} uses a lock to guard the db instance against
> disposal of the native resources while some parallel threads might still
> access db, which might otherwise lead to segfaults.
> Unfortunately, this locking is a bit to strict and can lead to situations
> where snapshots block the pipeline. This can happen when a snapshot s1 is
> running and somewhere blocking in IO while holding the guarding lock. A
> second snapshot s2 can be triggered in parallel and requires to hold the lock
> in the synchronous part to get a snapshot from db. As s1 is still holding on
> to the lock, s2 can block here and stop the operator from processing further
> elements.
> A simple solution could remove lock acquisition from the synchronous phase,
> because both, synchronous phase and disposing the backend are only allowed to
> be triggered from the thread that also drives element processing.
> A better solution would be to remove long sections under the lock all
> together, because as of now they will always prevent the possibility of
> parallel checkpointing. I think a guard for the rocksdb instance would be
> sufficient that blocks disposal for as long as there are still clients
> potentially accessing the instance in parallel. This could be realized by
> keeping a synchronized counter for active clients and block disposal until
> the client count drops to zero.
> This approach could also be integrated with triggering timers, which have
> always been problematic in the disposal phase are currently unregulated. In
> the new model, they could register as yet another client.
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