Commonly asked Cassandra famous interview questions and answers.

1. What is the role of a commit log in Cassandra?
   • The commit log in Cassandra is a durable, append-only log that records all write operations to disk before they are applied to the memtable (in-memory table).
   • The commit log is used to ensure durability and to recover data in the event of a crash. When a node is restarted, the commit log is replayed to reconstruct the in-memory data structures, allowing the node to resume serving requests.
2. What is a CQL query in Cassandra?
   • CQL (Cassandra Query Language) is a SQL-like language used to interact with Cassandra.
   • CQL queries are used to retrieve, insert, update, and delete data in Cassandra, and can be executed through the CQL shell or through client drivers.
   • CQL queries are translated into internal Cassandra operations, which are then executed on the cluster.
   • CQL supports a variety of features, including user-defined types and functions, prepared statements, and batch statements.
3. What is the role of a snitch in Cassandra?
   • A snitch in Cassandra is responsible for determining the network topology of the cluster and for assigning IP addresses to nodes.
   • The snitch is used to ensure that each node is aware of the location and status of other nodes in the cluster, and to route requests to the appropriate node.
   • There are several different types of snitches in Cassandra, including SimpleSnitch, PropertyFileSnitch, and GossipingPropertyFileSnitch.
4. What is the purpose of the nodetool in Cassandra?
   • The nodetool in Cassandra is a command-line tool that is used to perform administrative tasks on a Cassandra cluster.
   • The nodetool can be used to view cluster status and metrics, to manage nodes, to compact SSTables, to repair data inconsistencies, and to perform backups and restores.
   • The nodetool is an important tool for managing and monitoring the health and performance of a Cassandra cluster.
5. What is a counter column in Cassandra?
   • A counter column in Cassandra is a column that is used to store a count of a particular event or action.
     • Counter columns can be incremented or decremented atomically, and are designed to support high-write workloads.
     • Counter columns are stored separately from regular columns in Cassandra, and are managed using a separate set of operations.
   • A counter column in Cassandra is a special type of column that is used to store a numeric value that can be incremented or decremented.
     • Counter columns can be used to represent a variety of metrics or statistics, such as page views or likes on a social media platform.
     • Counter columns are implemented using a specialized storage engine that is designed to ensure consistency across replicas when incrementing or decrementing the value.
     • Counter columns cannot be updated using the normal write operation in Cassandra, but must be updated using the special counter update operation.
   • A counter column in Cassandra is a special type of column that is used to store counter values.
     • Counter columns can be incremented or decremented, and the value is guaranteed to be consistent across all replicas of the data.
     • Counter columns are designed to be used in situations where it is important to maintain an accurate count of some event, such as the number of page views or the number of times a user has performed a particular action.

6. What is a quorum in Cassandra?
   • A quorum in Cassandra is the minimum number of replicas that must respond to a read or write request before the request is considered successful.
   • The quorum is used to ensure consistency and availability in a distributed system, and is configurable in Cassandra.
   • By default, the quorum is set to a majority of the replicas (i.e. n/2 + 1, where n is the number of replicas), but it can be adjusted based on the specific requirements of the application.
7. What is the role of the gossip protocol in Cassandra?
   • The gossip protocol in Cassandra is a peer-to-peer protocol that is used to disseminate information about the state and location of nodes in a cluster.
   • The gossip protocol is used to ensure that each node is aware of the status of other nodes in the cluster, and to route requests to the appropriate node.
   • The gossip protocol is designed.
8. What is a CQL statement in Cassandra?
   • A CQL (Cassandra Query Language) statement in Cassandra is a statement that is used to interact with the database.
   • CQL is a SQL-like language that is used to create and modify schema objects, insert and update data, and query data from Cassandra.
   • CQL statements can be executed using the cqlsh command-line tool or using a client library for a programming language such as Java, Python, or Node.js.
9. What is the difference between a partition key and a clustering key in Cassandra?
   • In Cassandra, a partition key is used to determine which node in a cluster a particular row is stored on, while a clustering key is used to determine the order in which rows are stored within a partition.
     • The partition key is the first part of the primary key for a table or column family, while the clustering key is the remaining part of the primary key.
     • In general, the partition key should be chosen based on the query patterns for the data, while the clustering key should be chosen based on the desired sort order for the data.
   • In Cassandra, a partition key is a column or set of columns that is used to determine which node in the cluster is responsible for storing a particular row of data.
     • The partition key is hashed to generate a token value, which is used to determine which node in the cluster is responsible for the data.
     • A clustering key, on the other hand, is used to determine the physical order of the rows within a partition.
     • When data is written to Cassandra, the rows within a partition are stored in ascending order based on the values of the clustering key.
     • Clustering keys can be used to support range queries on the data within a partition, and can also be used to group related rows together within a partition.
10. What is a memtable in Cassandra?
    • A memtable in Cassandra is an in-memory data structure that is used to store data before it is written to disk.
      • When data is written to Cassandra, it is first written to a memtable, which is stored in memory.
      • When the memtable reaches a certain size, it is written to disk as an SSTable (sorted string table).
      • The memtable is used to improve write performance by reducing the number of disk writes that are required.
    • In Cassandra, a memtable is an in-memory data structure that is used to store write operations before they are flushed to disk as SSTables.
      • Memtables are used to improve write performance by reducing the number of disk writes required to store data.
    • A memtable in Cassandra is an in-memory data structure that is used to store recently written data before it is written to disk as an SSTable.
      • When a write is received by Cassandra, the data is first written to the memtable for the corresponding table.
      • Once the memtable reaches a certain size, it is flushed to disk as an SSTable.
      • Memtables are used to improve write performance by allowing Cassandra to quickly write data to memory without incurring the overhead of writing to disk for every write operation.
11. What is a replica placement strategy in Cassandra?
    • A replica placement strategy in Cassandra is a strategy for determining how and where replicas of data should be stored in the cluster.
      • Cassandra provides several different replica placement strategies, including SimpleStrategy and NetworkTopologyStrategy.
      • SimpleStrategy is used for single datacenter clusters and simply stores replicas on the next node in a clockwise direction around the ring.
      • NetworkTopologyStrategy is used for multi-datacenter clusters and is designed to ensure that replicas are stored in different datacenters for fault tolerance and disaster recovery purposes.

12. What is Cassandra Query Language (CQL)?
    • Cassandra Query Language (CQL) is a SQL-like language that is used to interact with Cassandra databases.
    • CQL is used to create and modify tables, insert and retrieve data, and perform other administrative tasks.
    • CQL provides a simplified syntax and familiar structure for developers who are already familiar with SQL, while also providing additional features and functionality that are specific to Cassandra.
13. What is an SSTable in Cassandra?
    • An SSTable (sorted string table) in Cassandra is a disk-based data structure that is used to store data that has been sorted by partition key and clustering key.
      • SSTables are immutable and append-only, meaning that they can only be written to and not modified or deleted.
      • When a memtable reaches a certain size, it is written to disk as an SSTable.
      • SSTables are used to improve read performance by allowing Cassandra to efficiently retrieve data that is stored on disk.
    • In Cassandra, an SSTable (sorted string table) is a data file that is used to store a sorted collection of key-value pairs.
      • SSTables are immutable, meaning that once they are written to disk, they cannot be modified.
      • This allows for efficient read operations, as SSTables can be read sequentially without needing to perform any expensive seeks.
      • SSTables are also used to provide durability guarantees, as they are the final destination for data before it is written to disk.
14. What is a hinted handoff in Cassandra?
    • A hinted handoff in Cassandra is a process that is used to ensure that write operations are not lost when a node fails or is temporarily unavailable.
      • When a write operation is received by a node that is responsible for a particular partition, and that node is unable to write the data to the replicas because they are not available, the node will create a “hint” for each unavailable replica.
      • The hint is a record of the write operation that is stored on the local disk of the node.
      • When the unavailable replicas come back online, the hints are delivered to the replicas, allowing them to catch up with the missed writes.
      • Hinted handoffs are used to ensure that data is not lost due to temporary node failures.
    • A hinted handoff in Cassandra is a mechanism that is used to ensure that data is not lost in the event of a temporary node failure.
      • When a node goes down, other nodes in the cluster will temporarily hold on to any data that was intended for the failed node, along with a hint indicating that the data should be delivered to the failed node once it comes back online.
      • This allows for eventual consistency to be maintained, even in the face of temporary failures.

15. What is a key cache in Cassandra?
    • A key cache in Cassandra is an in-memory cache that is used to store recently accessed partition keys and their corresponding row indexes.
    • When a read operation is performed on a partition key that is already in the key cache, Cassandra can quickly retrieve the row indexes from memory rather than needing to perform disk I/O to read the partition index file.
    • Key caches can improve read performance by reducing the amount of disk I/O that is required for frequently accessed partition keys.
16. What is a row cache in Cassandra?
    • A row cache in Cassandra is an in-memory cache that is used to store entire rows of data for frequently accessed partition keys.
    • When a read operation is performed on a partition key that is already in the row cache, Cassandra can quickly retrieve the entire row of data from memory rather than needing to perform disk I/O to read the SSTable.
    • Row caches can improve read performance by reducing the amount of disk I/O that is required for frequently accessed partition keys, but can also increase memory usage and may not be suitable for all workloads.
17. What is the role of the commit log in Cassandra?
    • The commit log in Cassandra is a durable, append-only log that is used to ensure that write operations are not lost in the event of a node failure or system crash.
      • When data is written to Cassandra, it is first written to the commit log before being written to an SSTable.
      • This ensures that the data is stored durably on disk and can be recovered in the event of a failure.
      • During normal operation, the commit log is periodically flushed to disk to free up memory.
    • The commit log in Cassandra is a file that is used to ensure data durability in the event of a node failure.
      • When data is written to Cassandra, it is first written to a memtable in memory.
      • Once the memtable becomes full, it is flushed to disk as an SSTable.
      • At the same time, the write is also appended to the commit log on disk.
      • If a node fails before the memtable can be flushed to disk, the data can be recovered from the commit log.
      • The commit log is also used during the recovery process, to ensure that any data that was not properly written to disk is recovered.
18. What is the role of the memtable in Cassandra?
    • The memtable in Cassandra is an in-memory data structure that is used to buffer write operations before they are written to disk.
      • When data is written to Cassandra, it is first written to the memtable, which is stored in memory.
      • Once the memtable becomes full, it is flushed to disk as an SSTable.
      • The memtable allows Cassandra to write data to disk in an efficient and scalable manner, as it reduces the number of random disk I/O operations that are required for write operations.
    • In Cassandra, a memtable is an in-memory data structure that is used to hold recently updated data before it is written to disk.
      • The memtable is used to provide efficient write performance by allowing Cassandra to quickly record updates to data without incurring the overhead of immediately writing the data to disk.
      • Once the memtable becomes full, it is written to an SSTable on disk.

19. What is the replication factor in Cassandra?
    • The replication factor in Cassandra is the number of nodes that a copy of each piece of data is stored on in the cluster.
    • Replication is used to ensure that data is available even in the event of node failures or network partitions.
    • The replication factor can be set on a per-keyspace basis in Cassandra, and determines how many replicas of each piece of data are stored in the cluster.
    • The replication factor can be increased or decreased dynamically to adjust the durability and availability of data in the cluster.
20. How does Cassandra handle consistency and availability?
    • Cassandra uses a tunable consistency model to balance consistency and availability.
    • The consistency level specifies how many replicas must acknowledge a read or write operation before it is considered successful.
    • Cassandra supports several consistency levels, ranging from ONE (which requires only a single replica to acknowledge the operation) to ALL (which requires all replicas to acknowledge the operation).
    • By tuning the consistency level, applications can balance the need for strong consistency with the need for high availability.
21. What is a repair in Cassandra?
    • A repair in Cassandra is a process that is used to synchronize data between replicas in the cluster.
    • Cassandra uses a gossip protocol to detect inconsistencies between replicas, and repairs are initiated automatically when inconsistencies are detected.
    • During a repair, data is streamed from one replica to another, ensuring that all replicas are consistent with one another.
    • Repairs can be initiated manually using the nodetool utility, or can be scheduled to run automatically on a regular basis.
22. What is the role of the snitch in Cassandra?
    • The snitch in Cassandra is a component that is used to determine the network topology of the cluster.
    • The snitch is responsible for assigning IP addresses to nodes and determining which nodes are in the same data center and which nodes are in different data centers.
    • The snitch is important in Cassandra because it determines which nodes are used as replicas for a given piece of data.
    • Different snitches can be used to support different network topologies, such as multi-data center or multi-region deployments.
23. What is the role of the memtable_flush_writers parameter in Cassandra?
    • The memtable_flush_writers parameter in Cassandra is used to control the number of threads that are used to flush memtables to disk.
    • When data is written to Cassandra, it is first written to a memtable in memory.
    • Once the memtable becomes full, it is flushed to disk as an SSTable.
    • The memtable_flush_writers parameter controls the number of threads that are used to flush memtables to disk, and can be adjusted to optimize performance based on the available hardware resources.
24. What is a nodetool in Cassandra?
    • Nodetool is a command-line utility in Cassandra that is used to perform administrative tasks and manage the cluster.
    • Nodetool can be used to perform tasks such as starting and stopping nodes, querying cluster status, running repairs, and changing configuration settings.
    • Nodetool is an important tool for managing and monitoring Cassandra clusters, and is used extensively by administrators and developers.
25. What is a partition key in Cassandra?
    • A partition key in Cassandra is a value that is used to determine the partition to which a row belongs.
      • In Cassandra, rows are organized into partitions based on the partition key value.
      • The partition key is used to distribute data across the nodes in the cluster, and is an important component of Cassandra’s distributed architecture.
      • The partition key is specified when a table is created, and must be included in any queries that involve that table.
    • In Cassandra, a partition key is the part of a primary key that is used to determine which partition a particular row belongs to.
      • The partition key is hashed to determine the token that corresponds to the partition, and the node responsible for that token is responsible for storing the data for that partition.
      • Partition keys are used to distribute data across the cluster and to enable efficient data retrieval based on the partition key.
    • In Cassandra, a partition key is the part of a primary key that is used to determine the location of a piece of data within the cluster.
      • The partition key is hashed and used to determine which node in the cluster is responsible for storing and serving the data.
      • Partition keys are typically chosen to evenly distribute data across the cluster and to support efficient querying of data based on specific attributes.

26. What is the difference between a primary key and a partition key in Cassandra?
    • In Cassandra, the primary key is composed of the partition key and zero or more clustering columns.
    • The partition key is used to determine the partition to which a row belongs, while the clustering columns are used to determine the order of the rows within a partition.
    • The partition key is required in any query that involves the table, while the clustering columns are optional.
    • In summary, the partition key is a subset of the primary key and is used to distribute data across the nodes in the cluster, while the clustering columns are used to control the ordering of data within a partition.
27. What is a data center in Cassandra?
    • In Cassandra, a data center is a collection of nodes that are physically located in the same geographic location and connected by a high-speed network.
    • Data centers are an important concept in Cassandra because they are used to control the replication and availability of data.
    • Cassandra supports multi-data center deployments, which allow data to be replicated across multiple geographic regions for improved performance and disaster recovery.
28. What is a token ring in Cassandra?
    • A token ring in Cassandra is a mechanism that is used to distribute data across the nodes in the cluster.
    • Each node in the cluster is assigned a range of tokens, and partitions are assigned to nodes based on the range of tokens that they fall within.
    • The token ring is used to ensure that data is distributed evenly across the nodes in the cluster, and to facilitate efficient routing of queries.
29. What is the role of the hinted handoff in Cassandra?
    • The hinted handoff in Cassandra is a mechanism that is used to ensure data consistency in the event of a node failure.
    • When a node fails, any writes that were in progress on that node are lost.
    • However, if the failed node comes back online before a certain time period has elapsed (known as the hinted handoff window), the other nodes in the cluster will send the missed writes to the failed node.
    • This helps to ensure that all nodes have a consistent view of the data, even in the presence of node failures.
30. What is the read repair mechanism in Cassandra?
    • The read repair mechanism in Cassandra is a mechanism that is used to ensure data consistency when reading data from multiple replicas.
    • When a read is performed in Cassandra, the client contacts multiple replicas of the data and compares the results.
    • If the results are not consistent, the replicas will exchange data to bring them into agreement.
    • This process is known as read repair, and helps to ensure that all replicas have a consistent view of the data.
31. What is a hinted handoff window in Cassandra?
    • The hinted handoff window in Cassandra is a time period during which nodes in the cluster will store hints about writes that could not be completed due to node failures.
    • The hinted handoff window is configured on a per-replica basis, and specifies the length of time that hints should be stored before they are discarded.
    • During this time period, if a failed node comes back online, the other nodes in the cluster will send the missed writes to the failed node, to help ensure that all nodes have a consistent view of the data.
32. What is the difference between a partition key and a clustering column in Cassandra?
    • In Cassandra, the partition key is used to determine the partition to which a row belongs, while the clustering columns are used to determine the order of the rows within a partition.
    • The partition key is required in any query that involves the table, while the clustering columns are optional.
    • The partition key is used to distribute data across the nodes in the cluster, while the clustering columns are used to control the ordering of data within a partition.
33. What is the difference between a token and a key in Cassandra?
    • In Cassandra, a token is a value that is used to determine the node to which a particular partition belongs.
    • Each node in a Cassandra cluster is assigned a range of tokens, and partitions are assigned to nodes based on the range of tokens that they fall within.
    • A key, on the other hand, is a value that is used to identify a particular row within a table. Keys are unique within a table, while tokens are unique within a cluster.
34. What is the difference between a wide and a narrow row in Cassandra?
    • In Cassandra, a wide row is a row that contains many columns, while a narrow row is a row that contains relatively few columns.
    • Wide rows are useful for storing denormalized data or for implementing time series data models, while narrow rows are useful for storing data that is more normalized.
    • Wide rows can help to improve query performance by reducing the number of round-trips to the database, while narrow rows can help to improve storage efficiency.
35. What is the purpose of a tombstone in Cassandra?
    • In Cassandra, a tombstone is a special marker that is used to indicate that a column has been deleted.
    • Tombstones are used to ensure that deleted data is not resurrected in the event of a node failure or during the compaction process.
    • Tombstones are stored in SSTables and are deleted during the compaction process.
36. What is the difference between a single-node cluster and a multi-node cluster in Cassandra?
    • In Cassandra, a single-node cluster consists of a single node, while a multi-node cluster consists of multiple nodes.
      • Single-node clusters are useful for development and testing, while multi-node clusters are used in production environments to provide fault tolerance, scalability, and high availability.
      • Multi-node clusters require careful planning and configuration to ensure that data is distributed evenly across the cluster and that all nodes are working efficiently.
    • In Cassandra, a single-node cluster is a deployment in which a single instance of Cassandra is running on a single machine.
      • In contrast, a multi-node cluster is a deployment in which multiple instances of Cassandra are running on multiple machines that are connected by a network.
      • Multi-node clusters are used to provide fault tolerance, high availability, and scalability, as data can be replicated across multiple nodes and queries can be distributed across the cluster.

37. What is the difference between a consistency level and a replication factor in Cassandra?
    • In Cassandra, a consistency level is a setting that determines how many replicas of a piece of data must respond before a read or write operation is considered successful.
    • The replication factor, on the other hand, is the number of replicas of a piece of data that are stored in the cluster.
    • Consistency level and replication factor are closely related, since a higher replication factor allows for greater consistency guarantees.
    • However, a higher replication factor can also have performance implications.
38. What is a virtual node in Cassandra?
    • A virtual node in Cassandra is a technique for partitioning data in a cluster that allows for more granular control over data placement and distribution.
    • Virtual nodes work by dividing each node’s range of tokens into a configurable number of smaller ranges, allowing for finer-grained control over data distribution.
    • This allows for better load balancing across nodes and can improve the efficiency of certain operations, such as range scans.
39. What is the purpose of a bloom filter in Cassandra?
    • In Cassandra, a bloom filter is a probabilistic data structure that is used to quickly determine whether a given key is present in a particular SSTable.
    • Bloom filters are used to optimize read performance by allowing Cassandra to quickly skip over SSTables that do not contain the requested data, without having to read the entire file.
    • Bloom filters are not 100% accurate, but they provide a very high probability of detecting whether a key is present in an SSTable.