Your design should be around how you want to query. If you are only querying by user, then having a user as part of the row key makes sense. To manage row size, you should think of a row as being a bucket of time. Cassandra supports a large (but not without bounds) row size. To manage row size you might say that this row is for user fred for the month of april, or if that's too much perhaps the row is for user fred for the day 4/18/12. To do this you can use composite keys to hold both pieces of information in the key. (user, bucketpos)

The nice thing is that once the time period ha s come and gone, that row is complete, and you can perform background jobs against that row and store summary information for that time period.

----- Original Message -----
From: "Trevor Francis" <>
Sent: Wed, April 18, 2012 15:48
Subject: Re: Column Family per User

Of course, I am new to the Cassandra world, so it is taking some getting used to understand how everything translates into my MYSQL head.
We are building an enterprise application that will ingest log inf ormation and provide metrics and trending based upon the data contained in the logs. The application is transactional in nature such that a record will be written to a log and our system will need to query that record and assign two values to it in addition to using the information to develop trending metrics. 
The logs are being fed into cassandra by Flume.
Each of our users will be assigned their own piece of hardware that generates these log events, some of which can peak at up to 2500 transactions per second for a couple of hours. The log entries are around 150-bytes each and contain around 20 different pieces of information. Neither us, nor our users are interested in generating any queries across the entire database. Users are only concerned with the data that their particular piece of hardware generates. 
Should I just setup a single column family with 20 columns, the first of which bei ng the row key and make the row key the username of that user?
We would also need probably 2 more columns to store Value A and Value B assigned to that particular record.
Our metrics will be be something like this: For this particular user, during this particular timeframe, what is the average of field "X?" And then store that value, which we can generate historical trending over the course a week. We will do this every 15 minutes. 
Any suggestions on where I should head to start my journey into Cassandra for my particular application?

Trevor Francis

On Apr 18, 2012, at 2:14 PM, Janne Jalkanen wrote:

Each CF takes a fair chunk of memory regardless of how much data it has, so this is probably not a good idea, if you have lots of users. Also using a single CF means that compression is likely to work better (more redundant data).
However, Cassandra distributes the load across different nodes based on the row key, and the writes scale roughly linearly according to the number of nodes. So if you can make sure that no single row gets overly burdened by writes (50 million writes/day to a single row would always go to the same nodes - this is in the order of 600 writes/second/node, which shouldn't really pose a problem, IMHO). The main problem is that if a single row gets lots of columns it'll start to slow down at some point, and your row caches become less useful, as they cache the entire row.
Keep your rows suitably sized and you should be fine. To partition the data, you can either distribute it to a few CFs based on use or use some other distribution method (like "user:1234:00" where the "00" is the hour-of-the-day.
(There's a great article by Aaron Morton on how wide rows impact performance at, but as always, running your own tests to determine the optimal setup is recommended.)

On Apr 18, 2012, at 21:20 , Trevor Francis wrote:

Our application has users that can write in upwards of 50 million records per day. However, they all write the same format of records (20 fields.columns). Should I put each user in their own column family, even though the column family schema will be the same per user?
Would this help with dimensioning, if each user is querying their keyspace and only their keyspace?

Trevor Francis