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From Tamar Fraenkel <ta...@tok-media.com>
Subject Re: RE Ordering counters in Cassandra
Date Mon, 21 May 2012 17:51:42 GMT
I also had a similar problem. I have a temporary solution, which is not
best, but may be of help.
I have the coutner cf to count events, but apart from that I hold leaders
CF:

leaders = {
  // key is time bucket
  // values are composites(rank, event) ordered by
  // descending order of the rank
  // set relevant TTL on columns
  time_bucket1 : {
    composite(1000,event1) : ""
    composite(999, event2) : ""
  },
  ...
}

Whenever I increment counter for a specific event, I add a column in the
time bucket row of the leaders CF, with the new value of the counter and
the event name.
There are two ways to go here, either delete the old column(s) for that
event (with lower counters) from leaders CF. Or let them be.
If you choose to delete, there is the complication of not having
getAndSetfor counters, so you may end up not deleting all the old
columns.
If you choose not to  delete old column, and live with duplicate columns
for events (each with different count), it will make your query to retrieve
leaders run longer.
Anyway, when you need to retrieve the leaders, you can do slice query
onleaders CF and ignore
duplicates events using client (I use Java). This will happen less if you
do delete old columns.

Another option is not to use Cassandra for that purpose, http://redis.io/ is
a nice tool

Will be happy to hear you comments.
Thanks,

*Tamar Fraenkel *
Senior Software Engineer, TOK Media

[image: Inline image 1]

tamar@tok-media.com
Tel:   +972 2 6409736
Mob:  +972 54 8356490
Fax:   +972 2 5612956





On Mon, May 21, 2012 at 8:05 PM, Filippo Diotalevi <filippo@ntoklo.com>wrote:

> Hi Romain,
> thanks for your suggestion.
>
> When you say " build every day a ranking in a dedicated CF by iterating
> over events:" do you mean
> - load all the columns for the specified row key
> - iterate over each column, and write a new column in the inversed index
> ?
>
> That's my current approach, but since I have many of these wide rows (1
> per day), the process is extremely slow as it involves moving an entire row
> from Cassandra to client, inverting every column, and sending the data back
> to create the inversed index.
>
> --
> Filippo Diotalevi
>
>
> On Monday, 21 May 2012 at 17:19, Romain HARDOUIN wrote:
>
>
> If I understand you've got a data model which looks like this:
>
> CF Events:
>     "row1": { "event1": 1050, "event2": 1200, "event3": 830, ... }
>
> You can't query on column values but you can build every day a ranking in
> a dedicated CF by iterating over events:
>
> create column family Ranking
>     with comparator = 'LongType(reversed=true)'
>     ...
>
> CF Ranking:
>     "rank": { 1200: "event2", 1050: "event1", 830: "event3", ... }
>
> Then you can make a "top ten" or whatever you want because counter values
> will be sorted.
>
>
> Filippo Diotalevi <filippo@ntoklo.com> a écrit sur 21/05/2012 16:59:43 :
>
> > Hi,
> > I'm trying to understand what's the best design for a simple
> > "ranking" use cases.
> > I have, in a row, a good number (10k - a few 100K) of counters; each
> > one is counting the occurrence of an event. At the end of day, I
> > want to create a ranking of the most occurred event.
> >
> > What's the best approach to perform this task?
> > The brute force approach of retrieving the row and ordering it
> > doesn't work well (the call usually goes timeout, especially is
> > Cassandra is also under load); I also don't know in advance the full
> > set of event names (column names), so it's difficult to slice the get
> call.
> >
> > Is there any trick to solve this problem? Maybe a way to retrieve
> > the row ordering for counter values?
> >
> > Thanks,
> > --
> > Filippo Diotalevi
>
>
>

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