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Data Modeling In Cassandra

Charmy Garg walks us through some of the basics of modeling tables in Cassandra:

Two basic goals in Cassandra which we should keep in mind:

  • Spread data evenly around the cluster – You want every node in the cluster to have roughly the same amount of data. Rows are spread around the cluster based on a hash of the partition key, which is the first element of the PRIMARY KEY. So, the key to spreading data evenly is this: pick a good primary key.

  • Minimize the number of partitions read – Partitions are groups of rows that share the same partition key. When you issue a read query, you want to read rows from as few partitions as possible. Why is this important? [Each partition may reside on a different node. The coordinator will generally need to issue separate commands to separate nodes for each partition you request. This adds a lot of overhead and increases the variation in latency. Furthermore, even on a single node, it’s more expensive to read from multiple partitions than from a single one due to the way rows are stored.]

Charmy also has a couple of pitfalls that people used to the relational database model may hit.