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From Rishi Bhardwaj <khichri...@yahoo.com>
Subject Re: Atomic Compare and Swap
Date Wed, 23 Jun 2010 18:42:00 GMT
Hmnn... vector clocks seem very interesting, I am not very familiar with the work being done
in 0.7 for the vector clock support but it sounds like a great addition. I was thinking if
we can just use vector clocks to implement CAS in the following way:

Let's assume that a client will only do one CAS on a column at a time using Quorum/ALL consistency
level. Also lets assume that each client will have a unique vector component in the vector
clock associated with the column value to be updated. Now in order to do a CAS on a column,
the client first reads the column value and the associated vector clock. Then it issues a
Quorum/ALL write on the column with the new vector clock obtained by incrementing its own
vector component in the old vector clock value. If the write succeeds, the client issues a
Quorum/ALL read on the column again and checks if the vector clock for the column is logically
greater than the vector clock it wrote. If the column value read has a vector clock greater
than what the client wrote then the CAS succeeded, otherwise it failed.

Now in order for the above CAS scheme to work we would need couple of guarantees from the
server side (i.e. the cassandra backend). Basically when doing a read repair (at compaction
time, or read time, or write time) if we find two writes to the same column with logical clock
values that can't be compared then the "older" column write wins. By "older" I mean whichever
write got to the server first wins. For example if a cassandra replica node is applying a
write for a column and if it already finds write for the same column in the memtable then
it can check the vector clocks for the two writes. If the vector clocks are not comparable
then the write which is in memtable wins as that write got to the cassandra node earlier.
Similarly on a read if two versions of the same column are found with vector clocks that are
not comparable then the write that was in the "older" sstable (sstable written before) wins.

With the above scheme if two CAS writes are racing against each other, we know only one of
them will win and the client who issued that write can be sure about it.

How does the above scheme sound? 

-Rishi



________________________________
From: Mike Malone <mike@simplegeo.com>
To: dev@cassandra.apache.org
Sent: Tue, June 22, 2010 9:27:44 PM
Subject: Re: Atomic Compare and Swap

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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