What feature of Cassandra improves its scalability?

The correct answer highlights Cassandra’s ability to add nodes without downtime, which is fundamental to its scalability. This feature allows organizations to seamlessly increase their capacity and performance by incorporating additional nodes into the cluster while the system remains operational. This horizontal scaling capability ensures that as demand grows or as the data workload increases, Cassandra can easily expand to meet those needs without the disruption typically associated with traditional databases.

Adding nodes to a Cassandra cluster enhances its ability to distribute data evenly across the nodes, improving both read and write performance. The decentralized architecture of Cassandra, which avoids single points of failure, means that each node can handle requests independently, leading to a more resilient and scalable system overall.

In contrast, other options focus on aspects that do not inherently facilitate scalability. The use of primary keys is crucial for organizing and retrieving data but does not directly influence the ability to scale the database. Similarly, complex joins and reliance on a relational model are characteristics of traditional relational database management systems that can hinder, rather than enhance, scalability. These approaches often involve significant overhead, making it difficult to achieve the same level of scalability that Cassandra offers.