Cassandra file structure - how are the files used?

Question

When experimenting with Cassandra I\'ve observed that Cassandra writes to the following files:

/.../cassandra/commitlog/CommitLog-.log
/.../cassandra/d         


        

Answer 1

A write to a Cassandra node first hits the CommitLog (sequential). (Then Cassandra stores values to column-family specific, in-memory data structures called Memtables. The Memtables are flushed to disk whenever one of the configurable thresholds is exceeded. (1, datasize in memtable. 2, # of objects reach certain limit, 3, lifetime of a memtable expires.))

The data folder contains a subfolder for each keyspace. Each subfolder contains three kind of files:

• Data files: An SSTable (nomenclature borrowed from Google) stands for Sorted Strings Table and is a file of key-value string pairs (sorted by keys).
• Index file: (Key, offset) pairs (points into data file)
• Bloom filter: all keys in data file