CODEWITHSUNDEEP
cassandradata-modeling
Rajesh Gaur
Rajesh Gaur

Reputation: 287

Cassandra - Distributing data and Multiple tables (Data modeling)

I am trying to learn cassandra. One thing I am not clear is how to ask Cassandra to distribute various tables. ie say I have time series data coming into table t1,t2,t3

T1 is heavily loaded ( ratio is 2000: 2:4 for num of rows). I want the data of T1 for a given day to be not on the same machine as T2 or T3; so my queries are equally distributed ie not put too much load on one machine.

Also as the data gets older, its queried less, how can I take into account this factor.

regards

Upvotes: 0

Views: 114

Answers (1)

Alexis Wilke
Alexis Wilke

Reputation: 20731

Cassandra is automatically distributed, you do not have a direct control on how the data gets distributed. In most cases, by default it makes use of an md5 on the row key and depending on that selects which nodes (computers) will be used to save the data.

What you are talking about would be more of a planning for a standard SQL database. However, if you generate extremely large amount of statistical data that is only to be used by some backend processes and users, you could have a separate cluster of 2 or 3 nodes. That way your other tables would not be affected by those statistics.

However, the true power of Cassandra is to be used with one large cluster. If it slows down, add nodes to it and do the necessary repair to spread the data properly. That's it... pretty much.

As for the way a table is used, you can use all the parameters defined on a table to tweak its setup. If you mainly do writes to a table, then you can tweak the parameters to get faster writes and slower reads. The other way around is also available: one write, many reads. And also many writes and many reads. To tweak those settings, in most cases you will need to have your software running and gather various stats and make changes as time passes.

Update:

There is actually a solution, thinking about it, just... I never use that mode so I did not think about it.

When you use a cluster which supports sorted rows, you can use a specific row name and the data will then go to a specific node. Again, you do not directly have control over what goes where, but if you really really want to do it that way, that's probably the solution you are looking for.

In this case, the row name would start with a number such as 0x0001 for T1 data, and 0x0100 and 0x0200 for T2 and T3. Since you do not know where the data really goes and how Cassandra decides to use it, it is rather complicated to obtain the right results here. And if you change your cluster (i.e. add nodes) then all your assumptions of where the data goes may very well go to the toilet! (and that's not speaking of upgrading to a new version of Cassandra...)

Upvotes: 1

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