Cassandra Primary Key = ((Partitioning Key), Clustering Key): A simple explanation

Cassandra primary key (a unique identifier for a row) is made up of two parts - 1) one or more partitioning columns and 2) zero or more clustering columns. To search a table without any indexes, all partitioning columns must be provided to avoid error message: Cannot execute this query as it might involve data filtering … Continue reading Cassandra Primary Key = ((Partitioning Key), Clustering Key): A simple explanation

How and when to index data in Cassandra for fast and efficient retrieval? – A simple explanation

Cassandra is a multi-node, peer-to-peer cluster/distributed system that distributes/stores data across all nodes in the cluster. Every table in Cassandra is physically stored in multiple SSTable files spread across one or multiple nodes. Rows are spread around the cluster based on a hash of the partition key, which is the first part of the primary key. … Continue reading How and when to index data in Cassandra for fast and efficient retrieval? – A simple explanation

How does Cassandra store data? – A simple explanation

Every time a record is inserted into Cassandra - it follows the write-path as per the diagram above. First, the record is written to a commit log (on disk). All records irrespective of schema tables  are written to the commit log. The commit log enables recovery of memtable in case of hardware failure. Second,  the … Continue reading How does Cassandra store data? – A simple explanation

Cassandra Migration Tips

#1. Cassandra allows creating a composite primary key (primary key consisting of more than one columns - first part being partition key and rest being clustering keys) For any queries using a composite pk column without including all of its partitioning key columns will require an index for lookup. For example: CREATE TABLE test.groupmechanism ( … Continue reading Cassandra Migration Tips

Migrating SQL applications to Cassandra – Strategy

Today's typical Java application consists of SQL statements defined either using Data Access Object pattern or Java Persistence Query Language (JPQL). These SQLs can be categorized into three categories: Simple SQL: These are one table SQLs like select * from Employee where employeeID=1 Simple multi-table SQL: These are simple SQLs but involve more than one … Continue reading Migrating SQL applications to Cassandra – Strategy

Migrating SQL applications to Cassandra – Pattern #3

Pattern #3: Get rid of all NOT operators from sql where clauses. For example, consider a typical SQL query to find all employees who have NOT completed a mandatory information security training: select ee1.* from employee ee1 where NOT exists (     select ‘true’ from employee_training et1     where et1.employeeID = ee1.employeeID     and … Continue reading Migrating SQL applications to Cassandra – Pattern #3

Migrating SQL applications to Cassandra – Pattern #2

Pattern #2: Get rid of all EXISTS and IN from sql where clauses. For example, consider a typical SQL query to find all employees who have completed a mandatory information security training: select ee1.* from employee ee1 where exists (     select 'true' from employee_training et1     where et1.employeeID = ee1.employeeID     and … Continue reading Migrating SQL applications to Cassandra – Pattern #2