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From DuyHai Doan <>
Subject Re: Performance migrating from MySQL to C*
Date Wed, 28 May 2014 16:19:51 GMT
Hello Simon

 There is definitely some data modeling issue there.

 Your first data model was quite good, except the usage of
map<text,double>. Collections in C* are not meant to be used to store lots
of values because they are loaded entirely in memory server side every time
you access them. An alternative for the first design would be:

CREATE TABLE sensorDataByIdTypeAndTime (
day timestamp,
sensorId bigint,
        metricType text,
time timestamp,
values map<text, double>,
PRIMARY KEY ((day, sensorId), metricType, time)

 The above table will let you query by (day, sensorId) and then
by metricType and range of time

If you want to query by range of time and then by metricType:

CREATE TABLE sensorDataByIdTimeAndType (
day timestamp,
sensorId bigint,
        metricType text,
time timestamp,
values map<text, double>,
PRIMARY KEY ((day, sensorId), time, metricType)

A quick note on the "IN" clause. Basically behind the scene C* will issue
as many queries as there are distinct values inside the "IN" clause. It's
is as if you create several SELECT requests yourself. This is called
multiGet. There is no magic behind it.

 The advantage of using "IN" is that you issue only 1 query and save some
network bandwith. The drawback is that the C* coordinator blocks until all
the requests are done before sending the response back to the client.

The more elements you put into the  "IN" clause, the more queries will be
done from C* side.


 Duy Hai DOAN

On Wed, May 28, 2014 at 5:25 PM, Simon Chemouil <> wrote:

> Hi,
> First, sorry for the length of this mail. TL;DR: DataModeling timeseries
> with an extra dimension, and C* not handling stress well; MySQL doesn't
> scale as well but handles the queries way better on similar hardware.
> ==============
> Context:
> We've been evaluating Cassandra for a while now (~1 month) as a
> replacement of our current MySQL based solution. While we're very
> interested in the scalability promises of Cassandra, the results we had so
> far are not as good as we expected.
> Our system is providing almost real-time analytics on our (quite large,
> but definitely not 'Big data') dataset, and we are beginning to wonder if
> Cassandra is the right tool or if we're simply doing something wrong. We've
> spent a lot of effort trying to validate our usage of C* internally so I
> would appreciate any pointers or ideas.
> I have read that Cassandra was not so good when it cames to reads, or that
> it was more suited to returning smaller datasets, but I've also noticed it
> is being more and more used and advertised as a Big Data solution (e.g the
> recent partnership between DataBricks and DataStax).
> The problem we try to model is so: we have sensors (millions of them) of
> different types (thousands of them), that each report many pieces of data
> (typed double) every 5 minutes (00:00, 00:05, 00:10, ..., 23:50, 23:55).
> That's about 735K timestamped values per year per data, per sensor.
> We want to be able, for instance, to compute the average value for a given
> piece of data and a given set of sensors over a month as fast as possible.
> ==============
> Setup:
> Cassandra 2.0.7, on a 32-cores Linux 64 machine, using XFS and 4TB SSDs
> with 128 GB of RAM.
> DataStax Java Driver 2.0.2 with -Xmx16G. All queries using
> PreparedStatements.
> ==============
> Data Model:
> We've tried several data models for this:
> CREATE TABLE sensorData (
> day timestamp,
>  sensorId bigint,
> time timestamp,
> values map<text, double>,
>  PRIMARY KEY ((day, sensorId), time)
> );
> In this model, we cram all the data gathered by a single sensor into a
> map, so that we can perform computations on-the-fly when we get the data.
> The problem is that we sometime have up to 10K values stored while we'd
> like to retrieve only 10, and Cassandra is not only unable to let us select
> the map keys we're interested in, it is also unable to partially read that
> cell... and it makes these queries slow.
> Instead we've moved towards storing each value in different tables, with
> this model:
> CREATE TABLE sensorData (
> sensorId bigint,
>  time TIMESTAMP,
> dataName TEXT,
> dataValue DOUBLE,
>  PRIMARY KEY ((dataName, sensorId), time)
> );
> Now, we have to synchronize the time field client-side, which is a bit
> costly but at least we only get the data we need. We removed the day
> component (which was used to further partition the data) and put the
> dataName instead.
> We've also tried changing the compaction strategy (to
> LeveledCompactionStrategy), removing the compression, and generally
> tweaking our tables without any noticeable gain.
> Do these models seem OK for our purpose? They work fine when working with
> a few hundred sensors, but how can we query 300K sensorIds without killing
> Cassandra?
> I tried adding a secondary index on an extra-field (sensorTypeId) to get
> everything and filter client-side, but then we lose the ability to slice on
> the time. I tried introducing an extra info in the table name itself (e.g
> sensorData_<day>) but not only it is ugly, but it also increases the number
> of queries we have to send by the number of days we query, and the amount
> of queries we send already seems too high for Cassandra.
> ==============
> Query volume:
> We want our queries to span from few sensorIds to hundred thousands of
> them. We issue queries such as:
> SELECT * FROM sensorData WHERE dataName = 'yyy' AND sensorID IN (<list>)
> AND time >= <startTime> AND time <= <endTime>;
> We are extremely limited in the size of our list. I read that IN queries
> were not meant for large sets, but if we issue several queries with
> smallers sets we often end-up with the same situation: timeout exceptions
> in the Java driver and quite often dead Cassandra nodes.
> These are the kind of exceptions we often get:
> Exception in thread "Thread-4029" Exception in thread "Thread-3972"
> com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s)
> tried for query failed (tried: [/, /,
> /, /] - use getErrors() for details)
>  at
> com.datastax.driver.core.exceptions.NoHostAvailableException.copy(
> at
> com.datastax.driver.core.DefaultResultSetFuture.extractCauseFromExecutionException(
>  at
> com.datastax.driver.core.DefaultResultSetFuture.getUninterruptibly(
> at com.datastax.driver.core.SessionManager.execute(
>  at com.davfx.cassandra.TestClient$
> at
> Caused by: com.datastax.driver.core.exceptions.NoHostAvailableException:
> All host(s) tried for query failed (tried: /
Timeout during read),
> / (com.datastax.driver.core.exceptions.DriverException:
> Timeout during read))
>  at
> com.datastax.driver.core.RequestHandler.sendRequest(
> at com.datastax.driver.core.RequestHandler$
>  at
> java.util.concurrent.ThreadPoolExecutor.runWorker(
> at
> java.util.concurrent.ThreadPoolExecutor$
>  ... 1 more
> (before starting the query, Cassandra is running fine and showing up as
> healthy in OpsCenter; smaller queries run fine. getErrors() doesn't provide
> so much info).
> I get that these are timeouts, but is changing the timeout settings in
> cassandra.yaml the right solution? It feels hackish. Other times, Cassandra
> nodes just 'die' and we get the sensation that Cassandra is a bit 'fragile'
> (we broke our cluster several times during our tests, by sending too many
> queries concurrently or simply by creating a table just after dropping it).
> Is there any recommended way to avoid stressing too much Cassandra, without
> manually keeping track of ongoing queries client-side?
> I noticed several JIRA issues that could probably help us reduce the
> overhead we have (either by having useless data transfered over the wire,
> or requiring us to flood SELECT queries to Cassandra) by allowing multiple
> IN queries (CASSANDRA-4762), or filtering server-side (CASSANDRA-4914,
> CASSANDRA-6377), but they haven't been updated in a while.
> ==============
> Performance:
> Finally, when we try to avoid stressing our cluster too much, we manage to
> run our queries ('our' queries translate to several Cassandra queries).
> However, where running the same queries on MySQL will take only one CPU
> core to 100%, Cassandra takes our 32 cores to 100% and doesn't reply any
> faster than MySQL. In practice we've found MySQL to be able to concurrently
> run several queries, also suffering a performance loss but not to the
> extent of Cassandra.
> We're looking at Cassandra today because we know that its scaling
> capability is very superior to MySQL's. We know that adding servers will
> help us increase throughput dramatically, but we also must be able to keep
> decent performance on a setup similar to what we're currently running. We
> are thus facing several problems:
> * it is a very hard sell if the performance is too far from MySQL's
> (comparing time performance, and ability to handle the load of several
> concurrent queries). We're not comparing on a single server, because we
> know MySQL has been around longer and is very optimized at what it does,
> but we'd at least expect Cassandra to do with 3 servers as good as MySQL
> does with 2. We've been unable to demonstrate that so far :(.
> * when we stress Cassandra, we get timeouts, very high loads and even make
> the process become unresponsive (doesn't necessarily 'crash')... But since
> we are limited in the queries we can express, we have no choice but to
> split them into many smaller queries (that would be written in a single SQL
> query) which seems to be a significant overhead. This is probably also a
> misuse from our side (even though we're simply using the DataStax Java
> driver and sending a few queries with probably too many elemets in the IN
> relation on the last component of the partition key). Is there any
> recommended (preferrably built-in) way to let Cassandra breathe while
> sending our queries so we're not crashing it?
> * our problem is two-dimensional ... we query on a given time range, but
> also on a bunch of sensorIds (up to 500K). It is a difficult problem
> generally, and we try to pre-compute as much as we can to denormalize (e.g
> give an identifier to a sensor-set frequently used), but our queries are
> very dynamic and we can only do so much. While most NoSQL datastores don't
> seem to have any smarter solution for this, we've found that MySQL does
> pretty good (by using different indices or filtering server-side). How to
> model it best with Cassandra to keep its strengths? Can we expect
> improvements in C* to help us deal with this kind of query?
> Which finally brings us to the more important question: do you feel
> Cassandra is fit for our use-case? I've seen Cassandra being advertised as
> a 'Big data' solution a lot (and we're working with smaller datasets) so
> I'd expect it to be more resilient to stressful usage and more
> feature-complete when it comes to reading large datasets... Maybe I have
> overlooked some pieces of documentation. We would be OK to try to adjust
> Cassandra to fit our needs and contribute to the project, but we have to
> make sure that the project is going in a similar direction (big data, etc).
> Thanks for your help, comments are greatly appreciated.
> Simon

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