Database Reference
In-Depth Information
rows_cached
rows_cached
The number of rows whose entire contents (the complete list of name/value pairs for that
unique row key) will be cached in memory.
comment
comment
This is just a standard comment that helps you remember important things about your
column family definitions.
read_repair_chance
read_repair_chance
This is a value between 0 and 1 that represents the probability that read repair operations
will be performed when a query is performed without a specified quorum, and it returns the
same row from two or more replicas and at least one of the replicas appears to be out of date.
You may want to lower this value if you are performing a much larger number of reads than
writes.
preload_row_cache
preload_row_cache
Specifies whether you want to prepopulate the row cache on server startup.
I have simplified these definitions somewhat, as they are really more about configuration and
server behavior than they are about the data model. They are covered in detail in
Chapter 6
.
Columns
A columnis the most basic unit of data structure in the Cassandra data model. A column is a
triplet of a name, a value, and a clock, which you can think of as a timestamp for now. Again, al-
though we're familiar with the term “columns” from the relational world, it's confusing to think
of them in the same way in Cassandra. First of all, when designing a relational database, you
specify the structure of the tables up front by assigning all of the columns in the table a name;
later, when you write data, you're simply supplying values for the predefined structure.
But in Cassandra, you don't define the columns up front; you just define the column families
you want in the keyspace, and then you can start writing data without defining the columns any-
where. That's because in Cassandra, all of a column's names are supplied by the client. This adds
considerable flexibility to how your application works with data, and can allow it to evolve or-
ganically over time.
