Datastax Interview Questions and Answers for 10 years experience

1. What is Cassandra, and why would you choose it over other NoSQL databases?

   • Answer: Cassandra is a highly scalable, distributed, NoSQL database designed to handle large amounts of data across many commodity servers. I'd choose it over other NoSQL databases like MongoDB or Couchbase when I need extremely high availability, fault tolerance, and linear scalability. Its decentralized architecture eliminates single points of failure, making it ideal for mission-critical applications requiring continuous uptime. It excels in handling massive datasets and high write throughput, unlike some other NoSQL options that might struggle under heavy load. The choice also depends on the specific data model and application requirements; Cassandra's column-family model is particularly well-suited to wide-row data.

2. Explain the Cassandra architecture and its key components.

   • Answer: Cassandra's architecture is based on a peer-to-peer distributed system with no single point of failure. Key components include: nodes (servers), which store data and participate in consensus; a gossip protocol for communication and cluster membership; a consistent hashing ring for data distribution; commit logs for data durability; and multiple replicas for data redundancy and high availability. Each node independently manages its data, providing scalability and fault tolerance. The architecture utilizes structured data (column families) for efficient querying and organization.

3. Describe the concept of consistency levels in Cassandra and when you might choose each one.

   • Answer: Cassandra offers various consistency levels to balance read/write performance with data consistency. `ONE` guarantees at least one replica is updated/read; `QUORUM` requires a majority of replicas; `ALL` requires all replicas; `LOCAL_QUORUM` needs a majority within a datacenter; `EACH_QUORUM` needs a quorum in multiple datacenters; and `SERIAL` provides strong linearizability. The choice depends on the application's requirements: `ONE` for high availability/performance, even if data might be slightly stale; `QUORUM` is common for a good balance; `ALL` for applications demanding the strongest consistency; and others for geographically distributed systems.

4. How does Cassandra handle data replication and failure?

   • Answer: Cassandra replicates data across multiple nodes (replicas) to ensure high availability and fault tolerance. If a node fails, the data remains accessible from other replicas. Data replication is configured using replication factors (the number of replicas) and consistency levels. The system automatically manages data distribution and rebalancing across the cluster when nodes are added or removed. When a node fails, the remaining replicas continue to serve read/write requests, ensuring minimal downtime.

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