Cassandra Data Modeling Notes

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Modeling principles

  • Know your data
  • Know your query
  • Nest Data
  • Duplicate data

Modeling Good to know

  • Query-driven data modeling. Application queries should be build before creating a data model.
  • Every query should have its own table.
  • Name tables after queries ex (videos_by_tag_added_year).
  • It’s okay to have duplicated data across multiple tables. We don’t care about space, we care about speed.
  • Cassandra will do an upsert if the primary key and clustering key in a cell are not unique.
  • Primary keys, clustering keys and clustering order can’t be change after a table is create. You will need need to create a new table and move old data to new table.
  • You can only do range queries on clustering keys.
  • Make sure data is duplicate at a constant duplication. If data is duplicated 25 x N, limit N to make the duplication factor constant.
  • A partition can only have 2 billion cells. You are likely to hit performance issues before hitting this limit.
  • A partition should only be hundreds of megabytes on disk
  • A cell or column is a key-value pair.
  • Trade-off between efficiency and space. Use client side joins and don’t duplicate data.
  • There are no JOINs in cassandra.
  • Use BATCH statement to update or insert all duplicated data.
  • Cassandra has SQL like sytanx call CQL for creating and manipulating.

Calculate partition size

The formula below can be use to calculate how big a partition will get to overtime. If a partition gets bigger than 2 billion cells, performance will be affected.

  • Nr -> Number of rows
  • Nc -> Number of regular colums
  • Npk -> Number of primary keys
  • Ns -> Number of static columns
  • Nv -> Number of values
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  • Nv = 40000 x (7 - 3 - 0) + 0
  • Nv = 40000 x 4 + 0
  • Nv = 160,000

Estimate table disk space

  • Ck -> Partition key
  • Cs -> Static columns
  • Cr -> Regular column
  • Cc -> Clustering column
  • Nr -> Number of rows
  • Nv -> See formula above
  • St -> Size of a table
  • sizeOf -> Estimate size of column
  • St = 16 + 0 + 40000 x ((55 + (8 + 16)) + (12 + (8 + 16)) + (30 + (8 + 16)) + (2340 + (8 + 16))) + 8 x 160000
  • St = 16 + 0 + 40000 x 2533 + 8 x 160000
  • St = 16 + 0 + 101320000 + 1280000
  • St = 102,600,016 bytes
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