Interview Questions and Answers

Apache Cassandra is a highly scalable and distributed NoSQL database system designed to handle large amounts of data across multiple commodity servers without a single point of failure.

A key space in Cassandra is a namespace that defines data replication on nodes. It is the outermost container for data, similar to a database in the relational database world.

A node in Cassandra is an individual server that stores data. Nodes work together to form a distributed and decentralized database system.

In Cassandra, a column family is a container for rows. It is similar to a table in a relational database and consists of columns and rows.

A Cassandra data center is a logical grouping of nodes that are geographically close to each other. Data centers are used for replication and fault tolerance purposes.

Cassandra achieves fault tolerance through data replication. Each piece of data is replicated across multiple nodes in the cluster, ensuring that if a node fails, the data is still available on other nodes.

Nodetool is a command-line utility in Cassandra that provides various administrative tasks, such as monitoring the cluster, managing compaction, and performing other maintenance operations.

CQL is a query language for interacting with Cassandra databases. It is similar to SQL but is specifically designed for NoSQL databases and includes support for creating, updating, and querying data in Cassandra.

The CAP theorem states that a distributed system cannot simultaneously provide more than two out of three guarantees: Consistency, Availability, and Partition Tolerance. Cassandra prioritizes Availability and Partition Tolerance over Consistency, making it an AP system.

A partition key is a primary key assigned to each row in a Cassandra table. It is responsible for distributing data across nodes in the cluster and is crucial for the performance of queries.

The Snitch in Cassandra is responsible for determining the proximity of nodes in a cluster. It helps in optimizing data distribution and ensures that data is stored on nodes that are geographically closer to each other.

The commit log in Cassandra is crucial for durability and fault tolerance. It stores write operations before they are written to the actual data files, ensuring that data is not lost in the event of a node failure.

Compaction in Cassandra is the process of merging and compacting SSTables (sorted string tables) to optimize storage and improve read performance.

Cassandra uses a write-ahead log (WAL) and a memtable for write operations. Data is first written to the commit log for durability and then stored in the memtable, which is periodically flushed to an SSTable on disk.

Eventual consistency in Cassandra means that, given enough time and in the absence of further updates, all replicas of a piece of data will converge to the same value. It allows for high availability and partition tolerance but may result in temporarily inconsistent data.

The Gossip Protocol is used by nodes in a Cassandra cluster to communicate with each other and share information about the state of the cluster. It helps in maintaining a decentralized and dynamic view of the cluster.

Cassandra uses a combination of partition key and clustering key to locate and retrieve data efficiently. Read operations can be served from memory (memtable) or disk (SSTables), depending on the specific scenario.

A wide row in Cassandra contains a large number of columns, while a narrow row has a smaller number of columns. The distinction is important for designing data models based on query requirements and performance considerations.

The coordinator node in Cassandra is responsible for receiving and coordinating client requests. It determines the nodes that need to be involved in the request and communicates with them to fulfill the operation.

A quorum in Cassandra is a majority of replicas that must respond for a read or write operation to be considered successful. The quorum level is configurable and is used to ensure consistency in distributed systems.

A secondary index in Cassandra allows querying on columns other than the primary key. However, the use of secondary indexes should be carefully considered due to potential performance implications.

Cassandra uses a consistent hashing algorithm to distribute data across nodes in the cluster. This ensures a uniform distribution of data and facilitates efficient data retrieval.

Cassandra supports several compaction strategies, including SizeTieredCompactionStrategy, LeveledCompactionStrategy, and TimeWindowCompactionStrategy. Each strategy has its own advantages and is suited for specific use cases.

Tombstones in Cassandra are markers used to represent deleted data. They are necessary for eventual consistency and are used during the process of data cleanup and compaction.

A lightweight transaction in Cassandra is a form of conditional update that ensures atomicity and isolation. It allows for conditional updates based on the current state of the data.