Steven,
Thank you.
You wrote: "The data of cassandra are partitioned by the row key;
therefore, if you want to put all pairs into the same row, you should
consider the disk size"
Can you please explain why the disk size is / might be a problem?
Thanks,
Shalom.
-----Original Message-----
From: Steven Yen-Liang Su <xpste...@gmail.com>
Reply-to: user@cassandra.apache.org
To: user@cassandra.apache.org
Subject: Re: Cassandra Database Modeling
Date: Wed, 13 Apr 2011 12:16:00 +0800
Is there a limit to the size that can be stored in one
'cell' (by 'cell' I mean the intersection between a key and a
data column)? is there a limit to the size of data of one key?
one data column?
http://wiki.apache.org/cassandra/CassandraLimitations
The data of cassandra are partitioned by the row key; therefore, if you
want to put all pairs into the same row, you should consider the disk
size.
Thanks in advance for any help / guidance.
-----Original Message-----
From: aaron morton <aa...@thelastpickle.com>
Reply-to: user@cassandra.apache.org
To: user@cassandra.apache.org
Subject: Re: Cassandra Database Modeling
Date: Wed, 13 Apr 2011 10:14:21 +1200
Yes for interactive == real time queries. Hadoop based
techniques are non time critical queries, but they do have
greater analytical capabilities.
particle_pairs: 1) Yes and no and sort of. Under the hood the
get_slice api call will be used by your client library to pull
back chunks of (ordered) columns. Most client libraries abstract
away the chunking for you.
2) If you are using a packed structure like JSON then no,
Cassandra will have no idea what you've put in the columns other
than bytes . It really depends on how much data you have per
pair, but generally it's easier to pull back more data than try
to get exactly what you need. Downside is you have to update all
the data.
3) No, you would need to update all the data for the pair. I was
assuming most of the data was written once, and that your
simulation had something like a stop-the-world phase between
time slices where state was dumped and then read to start the
next interval. You could either read it first, or we can come up
with something else.
distance_cf 1) the query would return an list of columns, which
have a name and value (as well as a timestamp and ttl). 2)
depends on the client library, if using python go
for https://github.com/pycassa/pycassa It will return objects
3) returning millions of columns is going to be slow, would also
be slow using a RDBMS. Creating millions objects in python is
going to be slow. You would need to have a better idea of what
queries you will actually want to run to see if it's *too* slow.
If it is one approach is to store the particles at the same
distance in the same column, so you need to read less columns.
Again depends on how your sim works. Time complexity depends
on the number of columns read. Finding a row will not be O(1) as
it it may have to read from several files. Writes are more
constant than reads. But remember, you can have a lot of io and
cpu power in your cluster.
Best advice is to jump in and see if the data model works for
you at a small single node scale, most performance issues can be
solved.
Aaron
On 12 Apr 2011, at 15:34, csharpplusproject wrote:
> Hi Aaron,
>
> Yes, of course it helps, I am starting to get a flavor of
> Cassandra -- thank you very much!
>
> First of all, by 'interactive' queries, are you referring to
> 'real-time' queries? (meaning, where experiments data is
> 'streaming', data needs to be stored and following that, the
> query needs to be run in real time)?
>
> Looking at the design of the particle pairs:
>
> - key: expriement_id.time_interval
> - column name: pair_id
> - column value: distance, angle, other data packed together as
> JSON or some other format
>
> A couple of questions:
>
> (1) Will a query such as pairID[ expriement_id.time_interval ]
> will basically return an array of all paidIDs for the
> experiment, where each item is a 'packed' JSON?
> (2) Would it be possible, rather than returning the whole JSON
> object per every pairID, to get (say) only the distance?
> (3) Would it be possible to easily update certain 'pairIDs'
> with new values (for example, update pairIDs = {2389, 93434}
> with new distance values)?
>
> Looking at the design of the distance CF (for example):
>
> this is VERY INTERESTING. basically you are suggesting a
> design that will save the actual distance between each pair of
> particles, and will allow queries where we can find all
> pairIDs (for an experiment, on time_interval) that meet a
> certain distance criteria. VERY, VERY INTERESTING!
>
> A couple of questions:
>
> (1) Will a query such as
> distanceCF[ expriement_id.time_interval ] will basically
> return an array of all 'zero_padded_distance.pair_id' elements
> for the experiment?
> (2) In such a case, I will get (presumably) a python list
> where every item is a string (and I will need to process it)?
> (3) Given the fact that we're doing a slice on millions of
> columns (?), any idea how fast such an operation would be?
>
>
> Just to make sure I understand, is it true that in both
> situations, the query complexity is basically O(1) since it's
> simply a HASH?
>
>
> Thank you for all of your help!
>
> Shalom.
>
> -----Original Message-----
> From: aaron morton <aa...@thelastpickle.com>
> Reply-to: user@cassandra.apache.org
> To: user@cassandra.apache.org
> Subject: Re: Cassandra Database Modeling
> Date: Tue, 12 Apr 2011 10:43:42 +1200
>
> The tricky part here is the level of flexibility you want for
> the querying. In general you will want to denormalise to
> support the read queries.
>
> If your queries are not interactive you may be able to use
> Hadoop / Pig / Hive
> e.g. http://www.datastax.com/products/brisk In which case you
> can probably have a simpler data model where you spend less
> effort supporting the queries. But it sounds like you need
> interactive queries as part of the experiment.
>
> You could store the data per pair in a standard CF (lets call
> it the pair cf) as follows:
>
> - key: expriement_id.time_interval - column name: pair_id -
> column value: distance, angle, other data packed together as
> JSON or some other format
>
> This would support a basic record of what happened, for each
> time interval you can get the list of all pairs and read their
> data.
>
> To support your spatial queries you could use two standard
> standard CFs as follows:
>
> distance CF: - key: experiment_id.time_interval - colunm name:
> zero_padded_distance.pair_id - column value: empty or the
> angle
>
> angle CF : - key: experiment_id.time_interval - colunm name:
> zero_padded_angle.pair_id - column value: empty or the
> distance
>
> (two pairs can have the same distance and/or angle in same
> time slice)
>
> Here we are using the column name as a compound value, and am
> assuming they can be byte ordered. So for distance the column
> name looks something like 000500.123456789. You would then use
> the Byte comparator (or similar) for the columns.
>
> To find all of the particles for experiment 2 at t5 where
> distance is < 100 you would use a get_slice
> (see http://wiki.apache.org/cassandra/API or your higher level
> client docs) against the key "2.5" with a SliceRange start at
> "000000.000000000" and finish at "000100.999999999". Once you
> have this list of columns you can either filter client side
> for the angle or issue another query for the particles inside
> the angle range. Then join the two results client side using
> the pair_id returned in the column names.
>
> By using the same key for all 3 CF's all the data for a time
> slice will be stored on the same nodes. You can potentially
> spread this around by using slightly different keys so they
> may hash to different areas of the cluster. e.g.
> expriement_id.time_interval."distance"
>
> Data volume is not a concern, and it's not possible to talk
> about performance until you have an idea of the workload and
> required throughput. But writes are fast and I think your
> reads would be fast as well as the row data for distance and
> angle will not change so caches will be be useful.
>
> Hope that helps. Aaron
>
> On 12 Apr 2011, at 03:01, Shalom wrote:
>
> > I would like to save statistics on 10,000,000 (ten millions)
> > pairs of
> > particles, how they relate to one another in any given space
> > in time.
> >
> > So suppose that within a total experiment time of T1..T1000
> > (assume that T1
> > is when the experiment starts, and T1000 is the time when
> > the experiment
> > ends) I would like, per each pair of particles, to measure
> > the relationship
> > between every Tn -- T(n+1) interval:
> >
> > T1..T2 (this is the first interval)
> >
> > T2..T3
> >
> > T3..T4
> >
> > ......
> >
> > ......
> >
> > T9,999,999..T10,000,000 (this is the last interval)
> >
> > For each such a particle pair (there are 10,000,000 pairs) I
> > would like to
> > save some figures (such as distance, angel etc) on each
> > interval of [
> > Tn..T(n+1) ]
> >
> > Once saved, the query I will be using to retrieve this data
> > is as follows:
> > "give me all particle pairs on time interval [ Tn..T(n+1) ]
> > where the
> > distance between the two particles is smaller than X and the
> > angle between
> > the two particles is greater than Y". Meaning, the query
> > will always take
> > place for all particle pairs on a certain interval of time.
> >
> > How would you model this in Cassandra, so that the
> > writes/reads are
> > optimized? given the database size involved, can you
> > recommend on a suitable
> > solution? (I have been recommended to both MongoDB /
> > Cassandra).
> >
> > I should mention that the data does change often -- we run
> > many such
> > experiments (different particle sets / thousands of
> > experiments) and would
> > need a very decent performance of reads/writes.
> >
> > Is Cassandra suitable for this time of work?
> >
> >
> > --
> > View this message in context:
> >
http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/Cassandra-Database-Modeling-tp6261778p6261778.html
> > Sent from the cassandra-u...@incubator.apache.org mailing
> > list archive at Nabble.com.
> >
>
>
>
>
>
