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From aaron morton <aa...@thelastpickle.com>
Subject Re: Atomic Compare and Swap
Date Wed, 23 Jun 2010 11:51:11 GMT
I've been playing with something like CAS, it's not the same but it  
may be of interest.

I write some data into Cassandra with quorum or better consistency,  
that allows me to assert what it should look like when read back. If  
the assertion holds I can then go ahead.

For example, in a CF with Time uuid ordering the client writes a  
column against the key of the thing we want to update. This write does  
not store the value. Then read back the first ordered column, if it's  
name is my uuid then I can proceed. Otherwise delete the column. If  
you know the uuid of the last update you can read back two columns.  
Then assert your the first and the previous is the second.

Perhaps if you were doing a CAS you could then write then actual value  
you want to update and somehow store the uuid from above with it. Say  
as col in another col family with  name as the uuid and value as the  
value. To read get the first colum from both CFs as a multi get, the  
col names must match from both cols for the value to be correct.

(could just use two diff keys in same CF)

Hope that makes sense.
Aaron






On 23/06/2010, at 4:27 PM, Mike Malone <mike@simplegeo.com> wrote:

> I'd be interested in what the folks who want CAS implementations  
> think about
> vector clocks. Can you use them to fulfill your use cases? If not,  
> why not?
>
> I ask because I have found myself wanting CAS in Cassandra too, but  
> I think
> that's only because I'm pretty familiar with HTTP. I think vector  
> clocks
> with client merge give you essentially the same functionality, but  
> in a way
> that fits much more nicely with the rest of the Cassandra  
> architecture. CAS
> really exacerbates Cassandra's weaknesses.
>
> Mike
>
> On Tue, Jun 22, 2010 at 4:52 PM, Rishi Bhardwaj  
> <khichrishi@yahoo.com>wrote:
>
>>
>>
>> S>: An *atomic* CAS is another beast and I see at least two  
>> difficulties:
>>
>> S>: 1) making it atomic locally: Cassandra's implementation is very  
>> much
>>> multi-threaded. On a given node, while you're
>> reading-comparing-and-swapping
>>> on some column c, no other thread should be allowed to write c (even
>> 'normal'
>>> write). You would probably need to have specific column families  
>>> where CAS
>> is
>>> allowed and for which all writes would be slower (since some  
>>> locking would
>> be
>>> involved). Even then, making such locking efficient and right is  
>>> not easy.
>> But
>>> in the end, local atomicity is quite probably the easy part.
>>
>> R: I am curious as to how does Cassandra handle two concurrent  
>> writes to
>> the same column right now? Is there any locking on the write path to
>> serialize two writes to the same column? If there is any locking  
>> then CAS
>> can build on that. If there is no such locking then we could  
>> exclude normal
>> writes from the synchronization/locking required for CAS. So the  
>> normal
>> write path remains the same, and we let the client know that atomic  
>> CAS
>> wouldn't work if normal writes are also happening on the same  
>> column values.
>> In short a client should not mix normal writes with Atomic CAS for  
>> writing
>> some column value. This will hopefully make things simpler.
>>
>> S:>2) making it atomic cluster-wide: data is replicated and an  
>> atomic CAS
>> would
>>> need to apply on the exact same column version in every node.  
>>> Which, with
>>> eventual consistency especially, is pretty hard to accomplish unless
>> you're
>>> locking the cluster (but that's what Cages/ZK do).
>>
>> R: For starters it would be great if atomic CAS could work for  
>> consistency
>> level Quorum and ALL and not be supported for other consistency  
>> levels. Even
>> for other consistency levels what would stop CAS to work? Why would  
>> one
>> require cluster wide locking? I might be mistaken here but the  
>> atomic CAS
>> operation would happen individually at all the replica nodes (either
>> directly or through hinted writes) and would succeed or fail  
>> depending on
>> the timestamp/version of the column at the replica. If we do Quorum  
>> reads
>> and CAS writes then we can also be sure about consistency.
>>
>> S:>That being said, if you have a neat solution for efficient and
>> distributed
>>> atomic CAS that doesn't require rewriting 80% of Cassandra, I'm  
>>> sure there
>>> will be interest in that.
>>
>>
>> R: That sounds great. I am definitely going to look into this and  
>> report
>> back if I have a good solution.
>>
>>
>> Thanks,
>> Rishi
>>
>>
>>
>>
>> ________________________________
>> From: Sylvain Lebresne <sylvain@yakaz.com>
>> To: dev@cassandra.apache.org
>> Sent: Tue, June 22, 2010 1:21:51 AM
>> Subject: Re: Atomic Compare and Swap
>>
>> On Mon, Jun 21, 2010 at 11:19 PM, Rishi Bhardwaj <khichrishi@yahoo.com 
>> >
>> wrote:
>>> I have read the post on cages and it is definitely very  
>>> interesting. But
>>> cages seems to be too coarse grained compared to an Atomic Compare  
>>> and
>> Swap
>>> on Cassandra column value. Cages would makes sense when one wants  
>>> to do
>>> multiple atomic row, column updates. Also, I am not so sure about  
>>> the
>>> scalability when it comes to using zookeeper for keeping locks on
>> Cassandra
>>> columns... there would also be performance hit with an added RPC for
>> every
>>> write. I feel Cages maybe fine for systems when one has few locks  
>>> but I
>> feel
>>> an atomic CAS in Cassandra would help us avoid distributed locking
>> systems
>>> and zookeeper in many other simpler scenarios. For more complicated
>>> (transaction like) things, using Cages may be fine. Then again  
>>> doing a
>> read
>>> before write for CAS in cassandra will make CAS at least as slow  
>>> as a
>> read,
>>> which I believe will still be better than taking a single column  
>>> lock
>> from
>>> zookeeper.
>>>
>>> What do other folks think in this regard? From whatever I have  
>>> read, I
>>> believe CAS is feasible in Cassandra without hurting the normal  
>>> write
>> path
>>> performance. Only for CAS writes would we have to pay for the read  
>>> before
>>> write penalty. I am going to do feasibility study for this and  
>>> would love
>>> any pointers from others about this.
>>
>> Making a (non atomic) CAS is easy (doing it client side is fine,  
>> and there
>> has been some discussion about 'callbacks' that may or may not  
>> someday
>> allow
>> to do that server-side).
>>
>> An *atomic* CAS is another beast and I see at least two difficulties:
>>
>> 1) making it atomic locally: Cassandra's implementation is very much
>> multi-threaded. On a given node, while you're
>> reading-comparing-and-swapping
>> on some column c, no other thread should be allowed to write c (even
>> 'normal'
>> write). You would probably need to have specific column families  
>> where CAS
>> is
>> allowed and for which all writes would be slower (since some  
>> locking would
>> be
>> involved). Even then, making such locking efficient and right is  
>> not easy.
>> But
>> in the end, local atomicity is quite probably the easy part.
>>
>> 2) making it atomic cluster-wide: data is replicated and an atomic  
>> CAS
>> would
>> need to apply on the exact same column version in every node.  
>> Which, with
>> eventual consistency especially, is pretty hard to accomplish  
>> unless you're
>> locking the cluster (but that's what Cages/ZK do).
>>
>> That being said, if you have a neat solution for efficient and  
>> distributed
>> atomic CAS that doesn't require rewriting 80% of Cassandra, I'm  
>> sure there
>> will be interest in that.
>>
>> --
>> Sylvain
>>
>>>
>>> Thanks,
>>> Rishi
>>>
>>>
>>>
>>> ________________________________
>>> From: Rauan Maemirov <rauan@maemirov.com>
>>> To: dev@cassandra.apache.org
>>> Sent: Mon, June 21, 2010 11:27:02 AM
>>> Subject: Re: Atomic Compare and Swap
>>>
>>> Have you read this post?
>>>
>> http://ria101.wordpress.com/2010/05/12/locking-and-transactions-over-cassandra-using-cages/
>>> I guess, you will like it.
>>>
>>> 2010/6/22 Rishi Bhardwaj <khichrishi@yahoo.com>
>>>
>>>> I am definitely interested in taking this work up. I believe the  
>>>> CAS
>>>> functionality would help in a lot of different scenarios and  
>>>> could help
>>>> avoid use of other external services (like zookeeper) to provide  
>>>> similar
>>>> functionality. I am new at Cassandra development and would really
>> appreciate
>>>> pointers from the dev. community about how to approach/start on  
>>>> this
>>>> project. Also how feasible is the approach mentioned below to  
>>>> implement
>> the
>>>> CAS functionality? It would be great if we could have a  
>>>> discussion on
>> the
>>>> pros and cons.
>>>>
>>>> Thanks,
>>>> Rishi
>>>>
>>>>
>>>>
>>>> ________________________________
>>>> From: Sriram Srinivasan <sriram@malhar.net>
>>>> To: dev@cassandra.apache.org
>>>> Sent: Sun, June 20, 2010 9:47:37 PM
>>>> Subject: Re: Atomic Compare and Swap
>>>>
>>>>
>>>> I too am interested in a CAS facility.
>>>>
>>>> I like Rishi's proposal. One could simply use a version number as  
>>>> the
>>>> logical timestamp. If we promote CAS to a consistency level, it  
>>>> would
>> rate
>>>> higher than a quorum. One pays the price for a more complex write  
>>>> path
>> to
>>>> obtain the requisite guarantee.
>>>>
>>>>
>>>> On Jun 21, 2010, at 4:03 AM, Rishi Bhardwaj wrote:
>>>>
>>>>>
>>>>> Heres another thought I had, if say the user always wrote with  
>>>>> quorum
>> (or
>>>> to all) nodes then can't we implement CAS (compare and swap)  
>>>> assuming
>> that
>>>> user employs logical timestamp and Cassandra doesn't allow writes  
>>>> to a
>>>> column with same or older timestamp. Here's the scenario I am  
>>>> thinking
>>>> about:
>>>>> Say we use logical timestamp for a column value and lets assume  
>>>>> the
>>>> current timestamp is t. Now say two clients read this column and
>> generate
>>>> concurrent CAS (compare and swap) operations on timestamp t and  
>>>> for both
>> the
>>>> writes the resulting new timestamp would become (t+1). Now if we  
>>>> don't
>> allow
>>>> writes to a column with same timestamp then only one of these  
>>>> writes
>> would
>>>> succeed. Of course another assumption is that if a third CAS  
>>>> write with
>>>> compare on logical timestamp (t - 1) came in, that would be  
>>>> denied as I
>>>> believe Cassandra doesn't allow "older" writes to win over "newer"
>> writes.
>>>> Do you think such a thing can be accomplished?
>>>>
>>>>
>>>>
>>>>
>>
>>
>>
>>
>>

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