Question

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

/.../cassandra/commitlog/CommitLog-<id>.log
/.../cassandra/data/Keyspace1/Standard1-1-Data.db
/.../cassandra/data/Keyspace1/Standard1-1-Filter.db
/.../cassandra/data/Keyspace1/Standard1-1-Index.db
/.../cassandra/data/system/LocationInfo-1-Data.db
/.../cassandra/data/system/LocationInfo-1-Filter.db
/.../cassandra/data/system/LocationInfo-1-Index.db
/.../cassandra/data/system/LocationInfo-2-Data.db
/.../cassandra/data/system/LocationInfo-2-Filter.db
/.../cassandra/data/system/LocationInfo-2-Index.db
/.../cassandra/data/system/LocationInfo-3-Data.db
/.../cassandra/data/system/LocationInfo-3-Filter.db
/.../cassandra/data/system/LocationInfo-3-Index.db
/.../cassandra/system.log

The general structure seems to be:

/.../cassandra/commitlog/CommitLog-ID.log
/.../cassandra/data/KEYSPACE/COLUMN_FAMILY-N-Data.db
/.../cassandra/data/KEYSPACE/COLUMN_FAMILY-N-Filter.db
/.../cassandra/data/KEYSPACE/COLUMN_FAMILY-N-Index.db
/.../cassandra/system.log

What is the Cassandra file structure? More specifically, how are the data, commitlog directories used, and what is the structure of the files in the data directory (Data/Filter/Index)?

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