[infinispan-dev] Proposal: ISPN-1394 Manual rehashing in 5.2

Sun Feb 5 10:24:44 EST 2012

On Sat, Feb 4, 2012 at 4:49 PM, Manik Surtani <manik at jboss.org> wrote:
>
> On 1 Feb 2012, at 12:23, Dan Berindei wrote:
>
>> Bela, you're right, this is essentially what we talked about in Lisbon:
>> https://community.jboss.org/wiki/AsymmetricCachesAndManualRehashingDesign
>>
>> For joins I actually started working on a policy of coalescing joins
>> that happen one after the other in a short time interval. The current
>> implementation is very primitive, as I shifted focus to stability, but
>> it does coalesce joins 1 second after another join started (or while
>> that join is still running).
>>
>> I don't quite agree with Sanne's assessment that it's fine for
>> getCache() to block for 5 minutes until the administrator allows the
>> new node to join. We should modify startCaches() instead to signal to
>> the coordinator that we are ready to receive data for one or all of
>> the defined caches, and wait with a customizable time limit until the
>> caches have properly joined the cluster.
>>
>> The getCache() timeout should not be increased at all. Instead I would
>> propose that getCache() returns a functional cache immediately, even
>> if the cache didn't receive any data, and it works solely as an L1
>> cache until the administrator allows it to join. I'd even make it
>> possible to designate a cache as an L1-only cache, so it's never an
>> owner for any key.
>
> I presume this would be encoded in the Address?  That would make sense for a node permanently designated as an L1 node.  But then how would this work for a node temporarily acting as L1 only, until it has been allowed to join?  Change the Address instance on the fly?  A delegating Address?  :/
>

Nope, not in the Address. Since we now have virtual cache views, every
node has to explicitly request to join each cache. It would be quite
easy to add an "L1-only" flag to the join request, and nodes with that
flag would never be included in the proper cache view. They would
however get a copy of the cache view on join and on every cache view
installation so they could update their CH and send requests to the
proper owners.

Nodes acting temporarily as L1-only would send a normal join request,
but they would also receive a copy of the cache view and getCache()
would return immediately instead of waiting for the node to receive
state. When the coordinator finally installs a cache view that
includes them, they will perform the initial state transfer as they do
now.

>> For leaves, the main problem is that every node has to compute the
>> same primary owner for a key, at all times. So we need a 2PC cache
>> view installation immediately after any leave to ensure that every
>> node determines the primary owner in the same way - we can't coalesce
>> or postpone leaves.
>
> Yes, manual rehashing would probably just be for joins.  Controlled shutdown in itself is manual, and crashes, well, need to be dealt with immediately IMO.
>

We could extend the policy for craches as well, by adding a
minNumOwners setting and only triggering an automatic rehash when
there is a segment on the hash wheel with <= minNumOwners owners.

We would have a properly installed CH, that guarantees at some point
in the past each key had numOwners owners, and a filter on top of it
that removes any leavers from the result of DM.locate(),
DM.getPrimaryLocation() etc.

It would probably undo our recent optimizations around locate and
getPrimaryLocation, so it's slowing the normal case (without any
leavers) in order to make the exceptional case (organized shutdown or
a part of the cluster) faster. The question is how big the cluster has
to get before the exceptional case becomes common enough that it's
worth optimizing for...

>>
>> For 5.2 I will try to decouple the cache view installation from the
>> state transfer, so in theory we will be able to coalesce/postpone the
>> state transfer for leaves as well
>> (https://issues.jboss.org/browse/ISPN-1827). I'm kind of need it for
>> non-blocking state transfer, because with the current implementation a
>> leave forces us to cancel any state transfer in progress and restart
>> with the updated cache view - a state transfer rollback will be very
>> expensive with NBST.
>>
>>
>> Erik does raise a valid point - with TACH, if we bring up a node with
>> a different siteId, then it will be an owner for all the keys in the
>> cache. That node probably isn't provisioned to hold all the keys, so
>> it would very likely run out of memory or evict much of the data. I
>> guess that makes it a 5.2 issue?
>
> Yes.
>
>> Shutting down a site should be possible even with what we have now -
>> just insert a DISCARD protocol in the JGroups stack of all the nodes
>> that are shutting down, and when FD finally times out on the nodes in
>> the surviving datacenter they won't have any state transfer to do
>> (although it may cause a few failed state transfer attempts). We could
>> make it simpler though.
>>
>>
>> Cheers
>> Dan
>>
>>
>> On Tue, Jan 31, 2012 at 6:21 PM, Erik Salter <an1310 at hotmail.com> wrote:
>>> ...such as bringing up a backup data center.
>>>
>>> -----Original Message-----
>>> From: infinispan-dev-bounces at lists.jboss.org
>>> [mailto:infinispan-dev-bounces at lists.jboss.org] On Behalf Of Bela Ban
>>> Sent: Tuesday, January 31, 2012 11:18 AM
>>> To: infinispan-dev at lists.jboss.org
>>> Subject: Re: [infinispan-dev] Proposal: ISPN-1394 Manual rehashing in 5.2
>>>
>>> I cannot volunteer either, but I find it important to be done in 5.2 !
>>>
>>> Unless rehashing works flawlessly with a large number of nodes joining
>>> at the same time, I think manual rehashing is crucial...
>>>
>>>
>>>
>>> On 1/31/12 5:13 PM, Sanne Grinovero wrote:
>>>> On 31 January 2012 16:06, Bela Ban<bban at redhat.com>  wrote:
>>>>> This is essentially what I suggested at the Lisbon meeting, right ?
>>>>
>>>> Yes!
>>>>
>>>>> I think Dan had a design wiki on this somewhere...
>>>>
>>>> Just rising it here as it was moved to 6.0, while I think it deserves
>>>> a dedicated thread to better think about it. If it's not hard, I think
>>>> it should be done sooner.
>>>> But while I started the thread to wake up the brilliant minds, I can't
>>>> volunteer for this to make it happen.
>>>>
>>>> Sanne
>>>>
>>>>>
>>>>>
>>>>> On 1/31/12 4:53 PM, Sanne Grinovero wrote:
>>>>>> I think this is an important feature to have soon;
>>>>>>
>>>>>> My understanding of it:
>>>>>>
>>>>>> We default with the feature off, and newly discovered nodes are
>>>>>> added/removed as usual. With a JMX operatable switch, one can disable
>>>>>> this:
>>>>>>
>>>>>> If a remote node is joining the JGroups view, but rehash is off: it
>>>>>> will be added to a to-be-installed view, but this won't be installed
>>>>>> until rehash is enabled again. This gives time to add more changes
>>>>>> before starting the rehash, and would help a lot to start larger
>>>>>> clusters.
>>>>>>
>>>>>> If the [self] node is booting and joining a cluster with manual rehash
>>>>>> off, the start process and any getCache() invocation should block and
>>>>>> wait for it to be enabled. This would need of course to override the
>>>>>> usually low timeouts.
>>>>>>
>>>>>> When a node is suspected it's a bit a different story as we need to
>>>>>> make sure no data is lost. The principle is the same, but maybe we
>>>>>> should have two flags: one which is a "soft request" to avoid rehashes
>>>>>> of less than N members (and refuse N>=numOwners ?), one which is just
>>>>>> disable it and don't care: data might be in a cachestore, data might
>>>>>> not be important. Which reminds me, we should consider as well a JMX
>>>>>> command to flush the container to the CacheLoader.
>>>>>>
>>>>>> --Sanne
>>>>>> _______________________________________________
>>>>>> infinispan-dev mailing list
>>>>>> infinispan-dev at lists.jboss.org
>>>>>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>>>>
>>>>> --
>>>>> Bela Ban
>>>>> Lead JGroups (http://www.jgroups.org)
>>>>> JBoss / Red Hat
>>>>> _______________________________________________
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>>> --
>>> Bela Ban
>>> Lead JGroups (http://www.jgroups.org)
>>> JBoss / Red Hat
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>
> --
> Manik Surtani
> manik at jboss.org
> twitter.com/maniksurtani
>
> Lead, Infinispan
> http://www.infinispan.org
>
>
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