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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-2354) Recover running jobs on JobManager failure
Date Mon, 05 Oct 2015 15:34:27 GMT

    [ https://issues.apache.org/jira/browse/FLINK-2354?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14943531#comment-14943531
] 

ASF GitHub Bot commented on FLINK-2354:
---------------------------------------

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

    https://github.com/apache/flink/pull/1153#discussion_r41158214
  
    --- Diff: flink-runtime/src/main/java/org/apache/flink/runtime/checkpoint/ZooKeeperCheckpointIDCounter.java
---
    @@ -0,0 +1,130 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one
    + * or more contributor license agreements.  See the NOTICE file
    + * distributed with this work for additional information
    + * regarding copyright ownership.  The ASF licenses this file
    + * to you under the Apache License, Version 2.0 (the
    + * "License"); you may not use this file except in compliance
    + * with the License.  You may obtain a copy of the License at
    + *
    + *     http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.flink.runtime.checkpoint;
    +
    +import org.apache.curator.framework.CuratorFramework;
    +import org.apache.curator.framework.recipes.shared.SharedCount;
    +import org.apache.curator.framework.recipes.shared.VersionedValue;
    +import org.apache.curator.framework.state.ConnectionState;
    +import org.apache.curator.framework.state.ConnectionStateListener;
    +import org.apache.flink.runtime.jobmanager.RecoveryMode;
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +import static com.google.common.base.Preconditions.checkNotNull;
    +
    +/**
    + * {@link CheckpointIDCounter} instances for JobManagers running in {@link RecoveryMode#ZOOKEEPER}.
    + *
    + * <p>Each counter creates a ZNode:
    + * <pre>
    + * +----O /flink/checkpoint-counter/&lt;job-id&gt; 1 [persistent]
    + * .
    + * .
    + * .
    + * +----O /flink/checkpoint-counter/&lt;job-id&gt; N [persistent]
    + * </pre>
    + *
    + * <p>The checkpoints IDs are required to be ascending (per job). In order to guarantee
this in case
    + * of job manager failures we use ZooKeeper to have a shared counter across job manager
instances.
    + */
    +public class ZooKeeperCheckpointIDCounter implements CheckpointIDCounter {
    --- End diff --
    
    As talked offline: It's seems to be ok that concurrently running jobs have overlapping
checkpoint IDs. Thus, using ZooKeeper's provided sequential ID to distinguish checkpoint nodes
unambiguously and using a checkpoint ID counter on a JobManager basis should be ok.


> Recover running jobs on JobManager failure
> ------------------------------------------
>
>                 Key: FLINK-2354
>                 URL: https://issues.apache.org/jira/browse/FLINK-2354
>             Project: Flink
>          Issue Type: Sub-task
>          Components: JobManager
>    Affects Versions: master
>            Reporter: Ufuk Celebi
>            Assignee: Ufuk Celebi
>             Fix For: 0.10
>
>
> tl;dr Persist JobGraphs in state backend and coordinate reference to state handle via
ZooKeeper.
> Problem: When running multiple JobManagers in high availability mode, the leading job
manager looses all running jobs when it fails. After a new leading job manager is elected,
it is not possible to recover any previously running jobs.
> Solution: The leading job manager, which receives the job graph writes 1) the job graph
to a state backend, and 2) a reference to the respective state handle to ZooKeeper. In general,
job graphs can become large (multiple MBs, because they include closures etc.). ZooKeeper
is not designed for data of this size. The level of indirection via the reference to the state
backend keeps the data in ZooKeeper small.
> Proposed ZooKeeper layout:
> /flink (default)
>   +- currentJobs
>        +- job id i
>             +- state handle reference of job graph i
> The 'currentJobs' node needs to be persistent to allow recovery of jobs between job managers.
The currentJobs node needs to satisfy the following invariant: There is a reference to a job
graph with id i IFF the respective job graph needs to be recovered by a newly elected job
manager leader.
> With this in place, jobs will be recovered from their initial state (as if resubmitted).
The next step is to backup the runtime state handles of checkpoints in a similar manner.
> ---
> This work will be based on [~trohrmann@apache.org]'s implementation of FLINK-2291. The
leader election service notifies the job manager about granted/revoked leadership. This notification
happens via Akka and thus serially *per* job manager, but results in eventually consistent
state between job managers. For some snapshots of time it is possible to have a new leader
granted leadership, before the old one has been revoked its leadership.
> [~trohrmann@apache.org], can you confirm that leadership does not guarantee mutually
exclusive access to the shared 'currentJobs' state?
> For example, the following can happen:
> - JM 1 is leader, JM 2 is standby
> - JOB i is running (and hence /flink/currentJobs/i exists)
> - ZK notifies leader election service (LES) of JM 1 and JM 2
> - LES 2 immediately notifies JM 2 about granted leadership, but LES 1 notification revoking
leadership takes longer
> - JOB i finishes (TMs don't notice leadership change yet) and JM 1 receives final JobStatusChange
> - JM 2 resubmits the job /flink/currentJobs/i
> - JM 1 removes /flink/currentJobs/i, because it is now finished
> => inconsistent state (wrt the specified invariant above)
> If it is indeed a problem, we can circumvent this with a Curator recipe for [shared locks|http://curator.apache.org/curator-recipes/shared-lock.html]
to coordinate the access to currentJobs. The lock needs to be acquired on leadership.
> ---
> Minimum required tests:
> - Unit tests for job graph serialization and writing to state backend and ZooKeeper with
expected nodes
> - Unit tests for job submission to job manager in leader/non-leader state
> - Unit tests for leadership granting/revoking and job submission/restarting interleavings
> - Process failure integration tests with single and multiple running jobs



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