Home | About | Sematext search-lucene.com search-hadoop.com
NEW: Monitor These Apps!
elasticsearch, apache solr, apache hbase, hadoop, redis, casssandra, amazon cloudwatch, mysql, memcached, apache kafka, apache zookeeper, apache storm, ubuntu, centOS, red hat, debian, puppet labs, java, senseiDB
 Search Hadoop and all its subprojects:

Switch to Threaded View
Zookeeper >> mail # user >> leader election, scheduled tasks, losing leadership


Copy link to this message
-
Re: leader election, scheduled tasks, losing leadership
> My point is that by the time that VM sees SUSPENDED/LOST, another VM may
> have been elected leader and have started running another process.
There's no way around this, right? ZK is not a transactional system so this edge-case is unsolvable.

> The way
> around the problem is to either ensure that no work is done by you once you
> are no longer the leader

You only release leadership when your work is done. If the cluster becomes unstable then you cancel your work. Leadership is denoted by a ZNode. Curator has a top-level watcher that notifies on cluster instability. How does the fence make this better?

-JZ

On Dec 8, 2012, at 9:30 PM, Henry Robinson <[EMAIL PROTECTED]> wrote:

> On 8 December 2012 21:18, Jordan Zimmerman <[EMAIL PROTECTED]>wrote:
>
>> If your ConnectionStateListener gets SUSPENDED or LOST you've lost
>> connection to ZooKeeper. Therefore you cannot use that same ZooKeeper
>> connection to manage a node that denotes the process is running or not.
>> Only 1 VM at a time will be running the process. That process can watch for
>> SUSPENDED/LOST and wind down the task.
>>
>>
> My point is that by the time that VM sees SUSPENDED/LOST, another VM may
> have been elected leader and have started running another process.
>
> It's a classic problem - you need some mechanism to fence a node that
> thinks its the leader, but isn't and hasn't got the memo yet. The way
> around the problem is to either ensure that no work is done by you once you
> are no longer the leader (perhaps by checking every time you want to do
> work), or that the work you do does not affect the system (e.g. by
> idempotent work units).
>
> ZK itself solves this internally by checking with that it has a quorum for
> every operation, which forces an ordering between the disconnection event
> and trying to do something that relies upon being the leader. Other systems
> forcibly terminate old leaders before allowing a new leader to take the
> throne.
>
> Henry
>
>
>>> You can't assume that the notification is received locally before another
>>> leader election finishes elsewhere
>> Which notification? The ConnectionStateListener is an abstraction on
>> ZooKeeper's watcher mechanism. It's only significant for the VM that is the
>> leader. Non-leaders don't need to be concerned.
>
>
>> -JZ
>>
>> On Dec 8, 2012, at 9:12 PM, Henry Robinson <[EMAIL PROTECTED]> wrote:
>>
>>> You can't assume that the notification is received locally before another
>>> leader election finishes elsewhere (particularly if you are running
>> slowly
>>> for some reason!), so it's not sufficient to guarantee that the process
>>> that is running locally has finished before someone else starts another.
>>>
>>> It's usually best - if possible - to restructure the system so that
>>> processes are idempotent to work around these kinds of problem, in
>>> conjunction with using the kind of primitives that Curator provides.
>>>
>>> Henry
>>>
>>> On 8 December 2012 21:04, Jordan Zimmerman <[EMAIL PROTECTED]
>>> wrote:
>>>
>>>> This is why you need a ConnectionStateListener. You'll get a notice that
>>>> the connection has been suspended and you should assume all
>> locks/leaders
>>>> are invalid.
>>>>
>>>> -JZ
>>>>
>>>> On Dec 8, 2012, at 9:02 PM, Henry Robinson <[EMAIL PROTECTED]> wrote:
>>>>
>>>>> What about a network disconnection? Presumably leadership is revoked
>> when
>>>>> the leader appears to have failed, which can be for more reasons than a
>>>> VM
>>>>> crash (VM running slow, network event, GC pause etc).
>>>>>
>>>>> Henry
>>>>>
>>>>> On 8 December 2012 21:00, Jordan Zimmerman <[EMAIL PROTECTED]
>>>>> wrote:
>>>>>
>>>>>> The leader latch lock is the equivalent of task in progress. I assume
>>>> the
>>>>>> task is running in the same VM as the leader lock. The only reason the
>>>> VM
>>>>>> would lose leadership is if it crashes in which case the process would
>>>> die
>>>>>> anyway.
>>>>>>
>>>>>> -JZ
>>>>>>
>>>>>> On Dec 8, 2012, at 8:56 PM, Eric Pederson <[EMAIL PROTECTED]> wrote:
NEW: Monitor These Apps!
elasticsearch, apache solr, apache hbase, hadoop, redis, casssandra, amazon cloudwatch, mysql, memcached, apache kafka, apache zookeeper, apache storm, ubuntu, centOS, red hat, debian, puppet labs, java, senseiDB