On Sat, Jul 2, 2011 at 3:57 PM, Yang <teddyyyy123@gmail.com> wrote:


could you please elaborate more on specifically why they are "not even close"?
 --- I kind of see what you mean (please correct me if I misunderstood): Cassandra failure detector 
is consulted on every write; while HBase failure detector is only used when the tablet server joins or leaves. 

 in order to have the single write entry point approach originally brought up in this thread,
I think you need a strong membership protocol to lock on the key range  leadership, once leadership is acquired, 
failure detectors do not need to be consulted on every write. 

yes by definition of the original requirement brought up in this thread,
Cassandra's write behavior is going to be changed, to be more like Hbase, and mongo in "replica set" mode. but 
it seems that this leader mode can even co-exist with the multi-entry write mode that Cassandra uses now, just as
you can use different CL for each single write request.  in that case you would need to keep both the current lightweight Phi-detector
and add the ZK for leader election for single-entry mode write.


(I should correct my terminology .... it's not a "strong failure detector" that's needed, it's a "strong membership protocol". strongly complete and accurate failure detectors do not exist in 
async distributed systems (Tushar Chandra  " Unreliable Failure Detectors for Reliable Distributed Systems, Journal of the ACM, 43(2):225-267, 1996"  and FLP "Impossibility of  Distributed Consensus with One Faulty Process" )  )

On Sat, Jul 2, 2011 at 10:11 AM, Jonathan Ellis <jbellis@gmail.com> wrote:
The way HBase uses ZK (for master election) is not even close to how
Cassandra uses the failure detector.

Using ZK for each operation would (a) not scale and (b) not work
cross-DC for any reasonable latency requirements.

On Sat, Jul 2, 2011 at 11:55 AM, Yang <teddyyyy123@gmail.com> wrote:
> there is a JIRA completed in 0.7.x that "Prefers" a certain node in snitch,
> so this does roughly what you want MOST of the time
> but the problem is that it does not GUARANTEE that the same node will always
> be read.  I recently read into the HBase vs Cassandra comparison thread that
> started after Facebook dropped Cassandra for their messaging system, and
> understood some of the differences. what you want is essentially what HBase
> does. the fundamental difference there is really due to the gossip protocol:
> it's a probablistic, or eventually consistent failure detector  while
> HBase/Google Bigtable use Zookeeper/Chubby to provide a strong failure
> detector (a distributed lock).  so in HBase, if a tablet server goes down,
> it really goes down, it can not re-grab the tablet from the new tablet
> server without going through a start up protocol (notifying the master,
> which would notify the clients etc),  in other words it is guaranteed that
> one tablet is served by only one tablet server at any given time.  in
> comparison the above JIRA only TRYIES to serve that key from one particular
> replica. HBase can have that guarantee because the group membership is
> maintained by the strong failure detector.
> just for hacking curiosity, a strong failure detector + Cassandra replicas
> is not impossible (actually seems not difficult), although the performance
> is not clear. what would such a strong failure detector bring to Cassandra
> besides this ONE-ONE strong consistency ? that is an interesting question I
> think.
> considering that HBase has been deployed on big clusters, it is probably OK
> with the performance of the strong  Zookeeper failure detector. then a
> further question was: why did Dynamo originally choose to use the
> probablistic failure detector? yes Dynamo's main theme is "eventually
> consistent", so the Phi-detector is **enough**, but if a strong detector
> buys us more with little cost, wouldn't that  be great?
> On Fri, Jul 1, 2011 at 6:53 PM, AJ <aj@dude.podzone.net> wrote:
>> Is this possible?
>> All reads and writes for a given key will always go to the same node from
>> a client.  It seems the only thing needed is to allow the clients to compute
>> which node is the closes replica for the given key using the same algorithm
>> C* uses.  When the first replica receives the write request, it will write
>> to itself which should complete before any of the other replicas and then
>> return.  The loads should still stay balanced if using random partitioner.
>>  If the first replica becomes unavailable (however that is defined), then
>> the clients can send to the next repilca in the ring and switch from ONE
>> write/reads to QUORUM write/reads temporarily until the first replica
>> becomes available again.  QUORUM is required since there could be some
>> replicas that were not updated after the first replica went down.
>> Will this work?  The goal is to have strong consistency with a read/write
>> consistency level as low as possible while secondarily a network performance
>> boost.

Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support

They are not close.

HBase uses zookeeper and its master servers as a single source of truth. When a node responsible for a region is down it can not be read or written to until the master elects a new node to be responsible for the region. Also all the storage is one back namely HDFS.

With Cassandra each node has a failure detector and it's own view of the network. This is why Cassandra is peer to peer. This is why the failure detector is consulted on every write/read because the network is converging. With Cassandra each node has its own storage. This is why you can work at ONE and QUORUM and clients are unaffected by single node failures.

If you want Cassandra to work like hbase you can....
1) Buy a SAN
2) make one LUN for each node
3) Use replication factor=1
4) running linux-ha on each node
5) when a Cassandra fails, linux-ha can detect failure and take over that nodes data and Cassandra instance :)