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From Steven A Robenalt <srobe...@stanford.edu>
Subject Re: Read/Write consistency issue
Date Fri, 10 Jan 2014 22:41:58 GMT
My understanding is that it's generally a Cassandra anti-pattern to do
read-before-write in any case, not just because of this issue. I'd agree
with Robert's suggestion earlier in this thread of writing each update
independently and aggregating on read.

Steve



On Fri, Jan 10, 2014 at 2:35 PM, Robert Wille <rwille@fold3.com> wrote:

> Actually, locking won’t fix the problem. He’s getting the problem on a
> single thread.
>
> I’m pretty sure that if updates can occur within the same millisecond (or
> more, if there is clock skew), there is literally nothing you can do to
> make this pattern work.
>
> Robert
>
> From: Todd Carrico <Todd.Carrico@match.com>
> Reply-To: <user@cassandra.apache.org>
> Date: Friday, January 10, 2014 at 3:28 PM
> To: "user@cassandra.apache.org" <user@cassandra.apache.org>
> Subject: RE: Read/Write consistency issue
>
> That, or roll your own locking.  Means multiple updates, but it works
> reliably.
>
>
>
> tc
>
>
>
> *From:* Robert Wille [mailto:rwille@fold3.com <rwille@fold3.com>]
> *Sent:* Friday, January 10, 2014 4:25 PM
> *To:* user@cassandra.apache.org
> *Subject:* Re: Read/Write consistency issue
>
>
>
> Cassandra is a last-write wins kind of a deal. The last write is
> determined by the timestamp. There are two problems with this:
>
>    1. If your clocks are not synchronized, you’re totally screwed. Note
>    that the 2nd and 3rd to last operations occurred just 2 milliseconds apart.
>    A clock skew of 2 milliseconds would definitely manifest itself like that.
>    2. Even if your clocks are perfectly synchronized, timestamps only
>    have millisecond granularity. If multiple writes occur within the same
>    millisecond, its impossible for Cassandra to determine which one occurred
>    last.
>
> Lots of really good information here:
> http://aphyr.com/posts/294-call-me-maybe-cassandra/
>
>
>
> I’d be very interested in hearing what others have to say. In the article
> I just linked to, the author experienced similar problems, even with
> “perfectly synchronized clocks”, whatever that means.
>
>
>
> The conclusion I’ve arrived at after reading and pondering is that if you
> perform multiple updates to a cell, even with synchronous calls from a
> single-threaded app, if those updates occur less than a millisecond apart,
> or approach the sum of the clock drift and network latency, you’re probably
> hosed.
>
>
>
> I think a better approach for Cassandra would be to write new values each
> time, and then sum them up on read, or perhaps have a process that
> periodically aggregates them. It’s a tricky business for sure, not one that
> Cassandra is very well equipped to handle.
>
>
>
> Robert
>
>
>
> *From: *Manoj Khangaonkar <khangaonkar@gmail.com>
> *Reply-To: *<user@cassandra.apache.org>
> *Date: *Friday, January 10, 2014 at 2:50 PM
> *To: *<user@cassandra.apache.org>
> *Subject: *Read/Write consistency issue
>
>
>
> Hi
>
>
>
> Using Cassandra 2.0.0.
>
> 3 node cluster
>
> Replication 2.
>
> Using consistency ALL for both read and writes.
>
>
>
> I have a single thread that reads a value, updates it and writes it back
> to the table. The column type is big int. Updating counts for a timestamp.
>
>
>
> With single thread and consistency ALL , I expect no lost updates. But as
> seem from my application log below,
>
>
>
> 10 07:01:58,507 [Thread-10] BeaconCountersCAS2DAO [INFO] 1389366000  H
>  old=59614 val =252 new =59866
>
> 10 07:01:58,611 [Thread-10] BeaconCountersCAS2DAO [INFO] 1389366000  H
>  old=59866 val =252 new =60118
>
> 10 07:01:59,136 [Thread-10] BeaconCountersCAS2DAO [INFO] 1389366000  H
>  old=60118 val =255 new =60373
>
> 10 07:02:00,242 [Thread-10] BeaconCountersCAS2DAO [INFO] 1389366000  H
>  old=60373 val =243 new =60616
>
> 10 07:02:00,244 [Thread-10] BeaconCountersCAS2DAO [INFO] 1389366000  H
>  old=60616 val =19 new =60635
>
> 10 07:02:00,326 [Thread-10] BeaconCountersCAS2DAO [INFO] 1389366000  H
>  old=60616 val =233 new =60849
>
>
>
> See the last 2 lines of above log.
>
> value 60116 is updated to 60635. but the next operation reads the old
> value 60616 again.
>
>
>
> I am not using counter column type because it does not support TTL and i
> hear there are lot of open issues with counters.
>
>
>
> Is there anything else I can do to further tighten the consistency or is
> this pattern of high volume read - update - write not going to work in C* ?
>
>
>
> regards
>
> MJ
>
>
>
> --
>



-- 
Steve Robenalt
Software Architect
HighWire | Stanford University
425 Broadway St, Redwood City, CA 94063

srobenal@stanford.edu
http://highwire.stanford.edu

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