Tell about the hotspot problem in Cassandra and how you solved it.

The hotspot problem in Cassandra occurs when data is written or read with high concentration on one or several cluster nodes, leading to overload of these nodes and reduced performance.

This is often related to uneven key distribution, for example, when a sequential or too simple key is used, which does not provide good distribution across partitions.

Solutions:

• Using composite keys or adding a random prefix (salting) to keys to evenly distribute data across partitions.
• Designing the data schema considering load balancing.
• Using TimeUUID or other methods for time-based data partitioning.

Example of adding a random prefix to a key:

String originalKey = "user123";
int salt = new Random().nextInt(10); // from 0 to 9
String saltedKey = salt + "_" + originalKey;
// Use saltedKey as the partition key

This helps avoid query concentration on a single node and improves cluster scalability.