I have heard on cassandra database engine few days ago and searching for a good documentation on it. after studying on cassandra I got cassandra is more scalable than other data engine. I also read on Amazon SimpleDB but as SimpleDB has a limitation 10GB/table and Google Datastore is slower than Amazon SimpleDB, I prefer not to use them (Google Datastore, Amazon SimpleDB). So for making our site scaled specially high write rates with massive data, I like to use Cassandra as our Data Engine.

But before starting using cassandra I am confused on "How to handle complex data using casssandra". I am giving you the MySQL database structure below, Please read this and give me a good suggestion.

Users Table
hasColum ID Primary
hasColum email Unique
hasColum FirstName
hasColum LastName

Category Table
hasColum ID Primary
hasColum Parent
hasColum Category

Posts Table
hasColum ID Primary
hasColum UID Index foreign key linked to users->ID
hasColum CID Index foreign key linked to Category->ID
hasColum Title
hasColum Post Index
hasColum PunDate

Comments
hasColum ID primary
hasColum UID Index foreign key linked to users->ID
hasColum PID Index foreign key linked to Posts->ID
hasColum Comment

User Group
hasColum ID primary
hasColum Name

UserToGroup Table (for many to many relation only)
hasColum UID foreign key linked to Users->ID
hasColum GID foreign key linked to Group->ID

Finally for your information, I like to use SimpleCassie PHP Class http://code.google.com/p/simpletools-php/ So, it will be very helpful if you can give me example using SimpleCassie