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From Milind Parikh <milindpar...@gmail.com>
Subject RE: Cassandra Counters
Date Mon, 24 Sep 2012 18:02:51 GMT
IMO

You would use Cassandra Counters (or other variation of distributed
counting) in case of having determined that a centralized version of
counting is not going to work.

You'd determine the non_feasibility of centralized counting by figuring the
speed at which you need to sustain writes and reads and reconcile that with
your hard disk seek times (essentially).

Once you have "proved" that you can't do centralized counting, the second
layer of arsenal comes into play; which is distributed counting.

In distributed counting , the CAP theorem comes into life. & in Cassandra,
Availability and Network Partitioning trumps over Consistency.

So yes, you sacrifice strong consistency for availability and partion
tolerance; for eventual consistency.
On Sep 24, 2012 10:28 AM, "Roshni Rajagopal" <roshni_rajagopal@hotmail.com>
wrote:

>  Hi folks,
>
>    I looked at my mail below, and Im rambling a bit, so Ill try to
> re-state my queries pointwise.
>
> a) what are the performance tradeoffs on reads & writes between creating a
> standard column family and manually doing the counts by a lookup on a key,
> versus using counters.
>
> b) whats the current state of counters limitations in the latest version
> of apache cassandra?
>
> c) with there being a possibilty of counter values getting out of sync,
> would counters not be recommended where strong consistency is desired. The
> normal benefits of cassandra's tunable consistency would not be applicable,
> as re-tries may cause overstating. So the normal use case is high
> performance, and where consistency is not paramount.
>
> Regards,
> roshni
>
>
>
> ------------------------------
> From: roshni_rajagopal@hotmail.com
> To: user@cassandra.apache.org
> Subject: Cassandra Counters
> Date: Mon, 24 Sep 2012 16:21:55 +0530
>
>  Hi ,
>
> I'm trying to understand if counters are a good fit for my use case.
> Ive watched http://blip.tv/datastax/counters-in-cassandra-5497678 many
> times over now...
> and still need help!
>
> Suppose I have a list of items- to which I can add or delete a set of
> items at a time,  and I want a count of the items, without considering
> changing the database  or additional components like zookeeper,
> I have 2 options_ the first is a counter col family, and the second is a
> standard one
>   1. List_Counter_CF    TotalItems    ListId 50        2.List_Std_CF
>
>     TimeUUID1 TimeUUID2 TimeUUID3 TimeUUID4 TimeUUID5  ListId 3 70 -20 3
> -6
>
> And in the second I can add a new col with every set of items added or
> deleted. Over time this row may grow wide.
> To display the final count, Id need to read the row, slice through all
> columns and add them.
>
> In both cases the writes should be fast, in fact standard col family
> should be faster as there's no read, before write. And for CL ONE write the
> latency should be same.
> For reads, the first option is very good, just read one column for a key
>
> For the second, the read involves reading the row, and adding each column
> value via application code. I dont think there's a way to do math via CQL
> yet.
> There should be not hot spotting, if the key is sharded well. I could even
> maintain the count derived from the List_Std_CF in a separate column family
> which is a standard col family with the final number, but I could do that
> as a separate process  immediately after the write to List_Std_CF
> completes, so that its not blocking.  I understand cassandra is faster for
> writes than reads, but how slow would Reading by row key be...? Is there
> any number around after how many columns the performance starts
> deteriorating, or how much worse in performance it would be?
>
> The advantage I see is that I can use the same consistency rules as for
> the rest of column families. If quorum for reads & writes, then you get
> strongly consistent values.
> In case of counters I see that in case of timeout exceptions because the
> first replica is down or not responding, there's a chance of the values
> getting messed up, and re-trying can mess it up further. Its not idempotent
> like a standard col family design can be.
>
> If it gets messed up, it would need administrator's help (is there a a
> document on how we could resolve counter values going wrong?)
>
> I believe the rest of the limitations still hold good- has anything
> changed in recent versions? In my opinion, they are not as major as the
> consistency question.
> -removing a counter & then modifying value - behaviour is undetermined
> -special process for counter col family sstable loss( need to remove all
> files)
> -no TTL support
> -no secondary indexes
>
>
> In short, I can recommend counters can be used for analytics or while
> dealing with data where the exact numbers are not important, or
> when its ok to take some time to fix the mismatch, and the performance
> requirements are most important.
> However where the numbers should match , its better to use a std column
> family and a manual implementation.
>
> Please share your thoughts on this.
>
> Regards,
> roshni
>
>

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