[infinispan-dev] [Cloudtm-discussion] [SPAM] Re: Primary-Backup replication scheme in Infinispan
Paolo Romano
romano at inesc-id.pt
Thu Feb 17 14:51:22 EST 2011
Hi Mark,
let me try to clarify the scope and goals of the work Sebastiano and
Diego are doing, as I think there may have been some misunderstanding here.
First an important premise, which I think was not clear in their
previous message. We are here considering the *full replication mode* of
Infinispan, in which every node maintains all copies of the same data
items. This implies that there is no need to fetch data from remote
nodes during transaction execution. Also, Infinispan was configured NOT
TO use eager locking. In other words, during transaction's execution
Infinispan acquires locks only locally.
What it is being compared is a single-master replication protocol (the
classic primary-backup scheme) vs the 2PC based multi-master replication
protocol (which is the one currently implemented by Infinispan).
The two protocols behave identically during transactions' execution.
What differs between the two protocols is the handling of the commit phase.
In the single master mode, as no remote conflict can occur and local
conflicts are managed by local concurrency control, data updates can be
simply pushed to the backups. More precisely (this should answer also a
question of Mircea), the primary DURING the commit phase, in a
synchronous fashion, propagates the updates to the backups, waits for
their ack, and only then returns from the commit request to the application.
In the multi-master mode, 2PC is used to materialize conflicts among
transactions executed at different nodes, and to ensure agreement on the
transaction's outcome. Thus 2PC serves to enforce both atomicity and
isolation.
The plots attached in the mail from Diego and Sebastiano are comparing
the performance of two replication strategies that provide the same
degree of consistency/failure resiliency...at least assuming perfect
failure detection, I'd prefer to avoid delving into a (actually
interesting) discussion of non-blocking guarantees of the two protocols
in partially or eventually synchronous environments in this mailing
list. Thus I disagree when you say that they are not comparable.
IMO, the only arguable unfairness in the comparison is that the
multi-master scheme allows processing update transactions at every node,
whereas the single-master scheme should rely either on 1) a load
balancing scheme to ensure redirecting all the update transactions to
the master, or 2) rely on a redirection scheme to redirect towards the
master any update transactions "originated" by the backups. The testbed
considered in the current evaluation is "emulating" option 1, which
does favour the PB scheme. On the other hand, we are currently studying
a non-intrusive way to implement a transparent redirection scheme
(option 2) in Infinispan, so suggestions are very welcome! ;-)
I hope this post clarified your perplexities!! :-)
Cheers,
Paolo
On 2/17/11 3:53 PM, Mark Little wrote:
> Hi
>
> On 11 Feb 2011, at 17:56, Sebastiano Peluso wrote:
>
>> Hi all,
>>
>> first let us introduce ourselves given that it's the first time we
>> write in this mailing list.
>>
>> Our names are Sebastiano Peluso and Diego Didona, and we are working
>> at INESC-ID Lisbon in the context of the Cloud-TM project. Our work
>> is framed in the context of self-adaptive replication mechanisms
>> (please refer to previous message by Paolo on this mailing list and
>> to the www.cloudtm.eu <http://www.cloudtm.eu> website for additional
>> details on this project).
>>
>> Up to date we have been working on developing a relatively simple
>> primary-backup (PB) replication mechanism, which we integrated within
>> Infinispan 4.2 and 5.0. In this kind of scheme only one node (called
>> the primary) is allowed to process update transactions, whereas the
>> remaining nodes only process read-only transactions. This allows
>> coping very efficiently with write transactions, as the primary does
>> not have to incur in the cost of remote coordination schemes nor in
>> distributed deadlocks, which can hamper performance at high
>> contention with two phase commit schemes. With PB, in fact, the
>> primary can serialize transactions locally, and simply propagate the
>> updates to the backups for fault-tolerance as well as to allow them
>> to process read-only transactions on fresh data of course. On the
>> other hand, the primary is clearly prone to become the bottleneck
>> especially in large clusters and write intensive workloads.
>
> So when you say "serialize transactions locally" does this mean you
> don't do distributed locking? Or is "locally" not referring simply to
> the primary?
>
>>
>> Thus, this scheme does not really represent a replacement for the
>> default 2PC protocol, but rather an alternative approach that results
>> particularly attractive (as we will illustrate in the following with
>> some radargun-based performance results) in small scale clusters, or,
>> in "elastic" cloud scenarios, in periods where the workload is either
>> read dominated or not very intense.
>
> I'm not sure what you mean by a "replacement" for 2PC? 2PC is for
> atomicity (consensus). It's the A in ACID. It's not the C, I or D.
>
>> Being this scheme particularly efficient in these scenarios, in fact,
>> its adoption would allow to minimize the number of resources acquired
>> from the cloud provider in these periods, with direct benefits in
>> terms of cost reductions. In Cloud-TM, in fact, we aim at designing
>> autonomic mechanisms that would dynamically switch among multiple
>> replication mechanisms depending on the current workload characteristics.
>
> Replication and transactions aren't mutually inconsistent things. They
> can be merged quite well :-)
>
> http://www.cs.ncl.ac.uk/publications/articles/papers/845.pdf
> http://www.cs.ncl.ac.uk/publications/inproceedings/papers/687.pdf
> http://www.cs.ncl.ac.uk/publications/inproceedings/papers/586.pdf
> http://www.cs.ncl.ac.uk/publications/inproceedings/papers/596.pdf
> http://www.cs.ncl.ac.uk/publications/trnn/papers/117.pdf
> http://www.cs.ncl.ac.uk/publications/inproceedings/papers/621.pdf
>
> The reason I mention these is because I don't quite understand what
> your goal is/was with regards to transactions and replication.
>
>>
>> Before discussing the results of our preliminary benchmarking study,
>> we would like to briefly overview how we integrated this replication
>> mechanism within Infinispan. Any comment/feedback is clearly highly
>> appreciated. First of all we have defined a new command, namely
>> PassiveReplicationCommand, that is a subclass of PrepareCommand. We
>> had to define a new command because we had to design customized
>> "visiting" methods for the interceptors. Note that our protocol only
>> affects the commit phase of a transaction, specifically, during the
>> prepare phase, in the prepare method of the TransactionXaAdapter
>> class, if the Primary Backup mode is enabled, then a
>> PassiveReplicationCommand is built by the CommandsFactory and it is
>> passed to the invoke method of the invoker. The
>> PassiveReplicationCommand is then visited by the all the interceptors
>> in the chain, by means of the visitPassiveReplicationCommand methods.
>> We describe more in detail the operations performed by the
>> non-trivial interceptors:
>>
>> -TxInterceptor: like in the 2PC protocol, if the context is not
>> originated locally, then for each modification stored in the
>> PassiveReplicationCommand the chain of interceptors is invoked.
>> -LockingInterceptor: first the next interceptor is called, then the
>> cleanupLocks is performed with the second parameter set to true
>> (commit the operations). This operation is always safe: on the
>> primary it is called only after that the acks from all the slaves are
>> received (see the ReplicationInterceptor below); on the slave there
>> are no concurrent conflicting writes since these are already locally
>> serialized by the locking scheme performed at the primary.
>> -ReplicationInterceptor: first it invokes the next iterceptor; then
>> if the predicate shouldInvokeRemoteTxCommand(ctx) is true, then the
>> method rpcManager.broadcastRpcCommand(command, true, false) is
>> performed, that replicates the modifications in a synchronous mode
>> (waiting for an explicit ack from all the backups).
>>
>> As for the commit phase:
>> -Given that in the Primary Backup the prepare phase works as a commit
>> too, the commit method on a TransactionXaAdapter object in this case
>> simply returns.
>>
>> On the resulting extended Infinispan version a subset of
>> unit/functional tests were executed and successfully passed:
>> - commands.CommandIdUniquenessTest
>> - replication.SyncReplicatedAPITest
>> - replication.SyncReplImplicitLockingTest
>> - replication.SyncReplLockingTest
>> - replication.SyncReplTest
>> - replication.SyncCacheListenerTest
>> - replication.ReplicationExceptionTest
>>
>>
>> We have tested this solution using a customized version of Radargun.
>> Our customizations were first of all aimed at having each thread
>> accessing data within transactions, instead of executing single
>> put/get operations. In addition, now every Stresser thread accesses
>> with uniform probability all of the keys stored by Infinispan, thus
>> generating conflicts with a probability proportional to the number of
>> concurrently active threads and inversely proportional to the total
>> number of keys maintained.
>>
>> As already hinted, our results highlight that, depending on the
>> current workload/number of nodes in the system, it is possible to
>> identify scenarios where the PB scheme significantly outperforms the
>> current 2PC scheme, and vice versa.
>
> I'm obviously missing something here, because it still seems like you
> are attempting to compare incomparable things. It's not like comparing
> apples and oranges (at least they are both fruit), but more like
> apples and broccoli ;-)
>
>> Our experiments were performed in a cluster of homogeneous 8-core
>> (Xeon at 2.16GhZ) nodes interconnected via a Gigabit Ethernet and
>> running a Linux Kernel 64 bit version 2.6.32-21-server. The results
>> were obtained by running for 30 seconds 8 parallel Stresser threads
>> per nodes, and letting the number of node vary from 2 to 10. In 2PC,
>> each thread executes transactions which consist of 10 (get/put)
>> operations, with a 10% of probability of generating a put operation.
>> With PB, the same kind of transactions are executed on the primary,
>> but the backups execute read-only transactions composed of 10 get
>> operations. This allows to compare the maximum throughput of update
>> transactions provided by the two compared schemes, without
>> excessively favoring PB by keeping the backups totally idle.
>>
>> The total write transactions' throughput exhibited by the cluster
>> (i.e. not the throughput per node) is shown in the attached plots,
>> relevant to caches containing 1000, 10000 and 100000 keys. As already
>> discussed, the lower the number of keys, the higher the chance of
>> contention and the probability of aborts due to conflicts. In
>> particular with the 2PC scheme the number of failed transactions
>> steadily increases at high contention up to 6% (to the best of our
>> understanding in particular due to distributed deadlocks). With PB,
>> instead the number of failed txs due to contention is always 0.
>
> Maybe you don't mean 2PC, but pessimistic locking? I'm still not too
> sure. But I am pretty sure it's not 2PC you mean.
>
>>
>> Note that, currently, we are assuming that backup nodes do not
>> generate update transactions. In practice this corresponds to
>> assuming the presence of some load balancing scheme which directs
>> (all and only) update transactions to the primary node, and read
>> transactions to the backup.
>
> Can I say it again? Serialisability?
>
>> In the negative case (a put operation is generated on a backup node),
>> we simply throw a PassiveReplicationException at the CacheDelegate
>> level. This is probably suboptimal/undesirable in real settings, as
>> update transactions may be transparently rerouted (at least in
>> principle!) to the primary node in a RPC-style. Any suggestion on how
>> to implement such a redirection facility in a
>> transparent/non-intrusive manner would be highly appreciated of
>> course! ;-)
>>
>> To conclude, we are currently working on a statistical model that is
>> able to predict the best suited replication scheme given the current
>> workload/number of machines, as well as on a mechanism to dynamically
>> switch from one replication scheme to the other.
>>
>>
>> We'll keep you posted on our progresses!
>>
>> Regards,
>> Sebastiano Peluso, Diego Didona
>> <PCvsPR_WRITE_TX_1000keys.png><PCvsPR_WRITE_TX_10000keys.png><PCvsPR_WRITE_TX_100000keys.png>------------------------------------------------------------------------------
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>
> ---
> Mark Little
> mlittle at redhat.com <mailto:mlittle at redhat.com>
>
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