DataStax Apache Cassandra Developer Associate Certification — Questions and Answers

Question 1: What does setting 'auto_bootstrap: false' in cassandra.yaml do?

Prevents the node from accepting writes until repair completes
Disables automatic MemTable flushing
Disables automatic schema migrations
Prevents the node from streaming data when joining the ring, used when building a new cluster from scratch (Correct answer)

Correct answer: Prevents the node from streaming data when joining the ring, used when building a new cluster from scratch

Setting auto_bootstrap to false tells Cassandra not to stream data when the node joins, which is the correct behavior when forming a brand new cluster where no existing data needs to be replicated.

Question 2: Which authorizer must be set in cassandra.yaml to enable GRANT and REVOKE permission management?

RoleBasedAuthorizer
PermissionAuthorizer
AllowAllAuthorizer
CassandraAuthorizer (Correct answer)

Correct answer: CassandraAuthorizer

CassandraAuthorizer stores and enforces permission grants in the system_auth keyspace, enabling fine-grained access control via GRANT and REVOKE statements.

Question 3: Which NetworkTopologyStrategy parameter controls how many replicas are placed per datacenter?

Per-datacenter replication_factor entries in the strategy options (Correct answer)
snitch_datacenter
replication_factor
consistency_level

Correct answer: Per-datacenter replication_factor entries in the strategy options

NetworkTopologyStrategy accepts a separate replication factor for each datacenter as named options (e.g., 'dc1': 3, 'dc2': 2).

Question 4: How do you enable authentication in Cassandra?

Set authenticator: AllowAllAuthenticator in cassandra.yaml
Set authenticator: PasswordAuthenticator in cassandra.yaml (Correct answer)
Enable TLS in cassandra-env.sh
Create a superuser via cqlsh only

Correct answer: Set authenticator: PasswordAuthenticator in cassandra.yaml

Setting authenticator to PasswordAuthenticator in cassandra.yaml enables username/password authentication using the system_auth keyspace.

Question 5: Which career path is a natural progression for a senior Cassandra DBA with strong distributed systems knowledge?

Transitioning to mainframe COBOL development
Becoming a distributed systems architect or site reliability engineer (Correct answer)
Moving to spreadsheet-based data management
Specializing in single-node SQLite deployments

Correct answer: Becoming a distributed systems architect or site reliability engineer

Deep expertise in distributed databases like Cassandra translates directly into distributed systems architecture and SRE roles.

Question 6: Which of the following data types maintains the order of the supplied elements?

None of these
Map
List (Correct answer)
Set

Correct answer: List

Explanation: <br> In the List data type, the order of the elements entered is maintained. Lists are a collection of ordered elements, where each element is identified by an index or position within the list. Elements can be added, removed, or accessed by their index, and new elements can be added to the end of the list.

Question 7: In Cassandra, what is the 'coordinator node'?

The node that performs compaction
The dedicated master node
The node that holds the schema definition
The node that receives a client request and routes it to the correct replicas (Correct answer)

Correct answer: The node that receives a client request and routes it to the correct replicas

The coordinator is whichever node a client connects to; it routes the request to the appropriate replica nodes based on the partition key.

Question 8: What are the default superuser credentials in a freshly installed Apache Cassandra cluster?

cassandra / cassandra (Correct answer)
user / password
admin / admin
root / cassandra

Correct answer: cassandra / cassandra

Cassandra ships with a default superuser account where both the username and password are 'cassandra', which must be changed immediately in production.

Question 9: What is the purpose of the FROZEN keyword in CQL when used with collections or UDTs?

Serializes the entire collection/UDT as a single blob, preventing partial updates (Correct answer)
Compresses the collection on disk
Marks the column as read-only
Enables secondary indexing on nested fields

Correct answer: Serializes the entire collection/UDT as a single blob, preventing partial updates

FROZEN serializes the entire collection or user-defined type as a single immutable value; updates must replace the entire value, enabling it to be used as a primary key component.

Question 10: Which CQL function generates a new unique time-based UUID suitable for use as a clustering key?

now() (Correct answer)
currentTime()
uuid()
toTimestamp()

Correct answer: now()

The now() function in CQL generates a new TIMEUUID based on the current time, useful for inserting rows with a time-ordered clustering key.

Question 11: What is the 'SERIAL' consistency level used for in Cassandra?

The consistency level for the read phase of a lightweight transaction (LWT) using Paxos (Correct answer)
A deprecated consistency level replaced by LOCAL_QUORUM
A serial scan of all SSTables during reads
Linearizable writes to a single partition without read checks

Correct answer: The consistency level for the read phase of a lightweight transaction (LWT) using Paxos

SERIAL consistency is used in LWTs to ensure the read portion of a compare-and-set operation uses Paxos consensus, providing linearizable reads at the cost of higher latency.

Question 12: What is the purpose of a secondary index in Cassandra?

To compress SSTable files
To deduplicate rows with the same clustering key
To allow queries on non-primary-key columns without needing ALLOW FILTERING (Correct answer)
To partition data across more nodes

Correct answer: To allow queries on non-primary-key columns without needing ALLOW FILTERING

A secondary index builds a local index on each node for a non-primary-key column, enabling filtered queries on that column across all partitions.

Question 13: In Cassandra's data model, what is a 'partition key' used for?

Setting the TTL on a row
Sorting rows within a partition
Determining which node stores the data (Correct answer)
Defining secondary indexes

Correct answer: Determining which node stores the data

The partition key is hashed to a token that determines which node(s) are responsible for storing that partition.

Question 14: By "Table in Cassandra," what do you mean?

Column
Row
KeySpace
Column-Family (Correct answer)

Correct answer: Column-Family

Explanation: <br> In Cassandra, a table is a data structure that stores related data in rows and columns. Tables in Cassandra are similar to tables in relational databases, but they are implemented differently and have some unique characteristics.

Question 15: Which configuration file is the primary settings file for a Cassandra node?

logback.xml
cassandra.yaml (Correct answer)
cassandra-env.sh
jvm.options

Correct answer: cassandra.yaml

cassandra.yaml contains the core node configuration including cluster name, data directories, seeds, listen address, and replication settings.

Question 16: What is the 'replication factor' in a Cassandra keyspace?

The number of nodes in the cluster
The consistency level for reads
The number of copies of each partition stored across the cluster (Correct answer)
The compression ratio of SSTables

Correct answer: The number of copies of each partition stored across the cluster

The replication factor defines how many nodes store a replica of each partition, directly affecting fault tolerance.

Question 17: Which CQL collection type stores an ordered list of elements that allows duplicate values?

MAP
FROZEN
LIST (Correct answer)
SET

Correct answer: LIST

The LIST type stores an ordered collection of elements and permits duplicates, unlike SET which is unordered and deduplicated.

Question 18: What is the primary data distribution mechanism in Apache Cassandra?

Master-slave replication
Round-robin assignment
Sharding by row key prefix
Consistent hashing with a token ring (Correct answer)

Correct answer: Consistent hashing with a token ring

Cassandra uses consistent hashing on a token ring to distribute data evenly across nodes without a central coordinator.

Question 19: When writing a resume for a Cassandra DBA position, which metric best demonstrates operational competence?

Cluster size managed, uptime maintained, and throughput handled (Correct answer)
Number of SELECT queries written
Lines of CQL code produced
Number of tables created

Correct answer: Cluster size managed, uptime maintained, and throughput handled

Employers look for scale indicators like nodes managed, SLA uptime, and data throughput to gauge operational experience.

Question 20: What tool can be used to run benchmark load tests against a Cassandra cluster?

cqlsh --bench
cassandra-stress (Correct answer)
sstableloader --test
nodetool benchmark

Correct answer: cassandra-stress

cassandra-stress is a built-in benchmarking tool that generates configurable read/write workloads to measure cluster throughput and latency.

Question 21: How does increasing the 'memtable_flush_writers' setting improve Cassandra write performance?

It reduces the MemTable size, causing more frequent smaller flushes
It increases the number of threads flushing MemTables to SSTables in parallel, improving throughput on multi-disk setups (Correct answer)
It compresses MemTable data before flushing
It enables concurrent writes to the same partition

Correct answer: It increases the number of threads flushing MemTables to SSTables in parallel, improving throughput on multi-disk setups

More flush writer threads allow Cassandra to flush multiple MemTables to disk concurrently, benefiting systems with multiple data directories or fast SSDs.

Question 22: Which compaction strategy is best suited for time-series data that is mostly appended and rarely updated?

LeveledCompactionStrategy (LCS)
UnifiedCompactionStrategy (UCS)
TimeWindowCompactionStrategy (TWCS) (Correct answer)
SizeTieredCompactionStrategy (STCS)

Correct answer: TimeWindowCompactionStrategy (TWCS)

TWCS groups SSTables into time windows and compacts within each window, making it ideal for time-series data with little or no overwrites.

Question 23: Which directory parameter in cassandra.yaml specifies where SSTables are stored on disk?

commitlog_directory
saved_caches_directory
data_file_directories (Correct answer)
hints_directory

Correct answer: data_file_directories

data_file_directories defines the on-disk path(s) where Cassandra writes SSTable files for all keyspaces and tables.

Question 24: What does the TTL (Time To Live) option do when inserting data in Cassandra?

Limits the number of reads before expiry
Reserves space in the MemTable
Automatically deletes the data after the specified number of seconds (Correct answer)
Sets a timeout for the write operation

Correct answer: Automatically deletes the data after the specified number of seconds

TTL marks data with an expiration time; Cassandra automatically creates a tombstone for that data once the TTL expires.

Question 25: What is the default native transport port used by Cassandra for CQL client connections?

7000
7199
9042 (Correct answer)
7001

Correct answer: 9042

Port 9042 is the default CQL native transport port that drivers and tools like cqlsh use to connect to Cassandra.

Question 26: Cassandra utilizes _________ by default for cluster communication.

3000
9000
7000 (Correct answer)
5000

Correct answer: 7000

Explanation: <br> Cassandra uses port numbers 9042 for native protocol clients, 7000 for cluster communication (or 7001 if SSL is enabled), and 7199 for JMX by default.

Question 27: What does the 'nodetool cfstats' (or 'tablestats') command report?

Column family (table) level statistics including read/write latency, SSTable count, partition size, and tombstone metrics (Correct answer)
Network bandwidth consumed per table
CommitLog segment usage per table
Cross-node cluster topology statistics

Correct answer: Column family (table) level statistics including read/write latency, SSTable count, partition size, and tombstone metrics

nodetool tablestats provides per-table metrics such as mean partition size, SSTable count, read/write counts, and tombstone warnings, critical for diagnosing table-level issues.

Question 28: In Cassandra, what is 'upsert' behavior?

Cassandra always treats INSERT and UPDATE as an upsert — existing rows are overwritten at the cell level (Correct answer)
Cassandra requires a prior SELECT to check existence before writing
Cassandra merges new and existing rows using a conflict resolution algorithm
Cassandra throws an error if a row already exists during INSERT

Correct answer: Cassandra always treats INSERT and UPDATE as an upsert — existing rows are overwritten at the cell level

Cassandra's INSERT and UPDATE both perform cell-level upserts: they write the new value regardless of whether the row exists, with no existence check by default.

Question 29: Which compaction strategy is best suited for time-series data where old data is rarely updated?

SizeTieredCompactionStrategy (STCS)
LeveledCompactionStrategy (LCS)
UnifiedCompactionStrategy (UCS)
TimeWindowCompactionStrategy (TWCS) (Correct answer)

Correct answer: TimeWindowCompactionStrategy (TWCS)

TWCS groups SSTables into time windows and compacts within each window, minimizing read amplification for time-series workloads.

Question 30: What is the purpose of 'incremental_backups' in Cassandra?

Automatically compacts SSTables on a schedule
Replicates CommitLog segments to S3
Copies each new SSTable to a backups/ subdirectory after it is flushed from MemTable (Correct answer)
Streams data to a remote backup cluster

Correct answer: Copies each new SSTable to a backups/ subdirectory after it is flushed from MemTable

When incremental_backups is enabled, every newly flushed SSTable is hard-linked into a backups/ folder, enabling point-in-time restore between snapshots.

Question 31: A write with consistency level LOCAL_ONE means Cassandra will wait for acknowledgment from:

One replica plus the coordinator node
The closest replica determined by dynamic snitch
One replica only in the local datacenter (Correct answer)
One replica across all datacenters

Correct answer: One replica only in the local datacenter

LOCAL_ONE requires acknowledgment from exactly one replica in the same datacenter as the coordinator, reducing cross-DC latency.

Question 32: What does 'nodetool decommission' do?

Deletes all data on a node immediately
Marks a node as down in gossip
Safely removes a node by streaming its data to other nodes before leaving the ring (Correct answer)
Forces a node to restart

Correct answer: Safely removes a node by streaming its data to other nodes before leaving the ring

nodetool decommission moves the node's token ranges to other nodes and then cleanly removes it from the ring.

Question 33: What does the 'NetworkTopologyStrategy' replication strategy allow you to configure?

Different replication factors per data center (Correct answer)
Token range assignment per vnode
Node rack assignment
Per-table replication factors

Correct answer: Different replication factors per data center

NetworkTopologyStrategy lets you specify a separate replication factor for each data center, enabling flexible multi-DC deployments.

Question 34: What is a 'materialized view' in Cassandra?

A view stored in memory only, not persisted to disk
A cross-keyspace join of two tables
A writable denormalized copy of a table managed by the user
A read-only cached copy of a base table with a different primary key for alternate access patterns (Correct answer)

Correct answer: A read-only cached copy of a base table with a different primary key for alternate access patterns

A materialized view is automatically maintained by Cassandra as a separate table built from a base table, allowing efficient queries on non-primary-key columns.

Question 35: What does the 'nodetool assassinate' command do?

Kills the Cassandra process immediately without draining
Deletes all tombstones on a target node
Permanently removes a node's data from all replicas
Forcefully removes a node that is stuck in a leaving/joining state from the ring (Correct answer)

Correct answer: Forcefully removes a node that is stuck in a leaving/joining state from the ring

nodetool assassinate is used as a last resort to forcefully remove a node that is stuck in the ring (e.g., in LEAVING state) when normal decommission has failed.

Question 36: What does the 'listen_address' parameter in cassandra.yaml define?

The address of the seed node
The address Cassandra listens on for CQL client connections
The IP address the node uses to communicate with other nodes in the cluster (Correct answer)
The Thrift RPC address

Correct answer: The IP address the node uses to communicate with other nodes in the cluster

listen_address is the IP that other Cassandra nodes use to reach this node for inter-node communication (gossip, streaming, repairs).

Question 37: Which CQL data type should be used to store a universally unique identifier in Cassandra?

TEXT
BIGINT
BLOB
UUID or TIMEUUID (Correct answer)

Correct answer: UUID or TIMEUUID

UUID stores a random version-4 UUID while TIMEUUID stores a version-1 time-based UUID, both natively supported as primary key types in Cassandra.

Question 38: Which CQL statement correctly grants SELECT permission on table 'orders' in keyspace 'sales' to role 'analyst'?

PERMIT SELECT ON TABLE sales.orders TO analyst
GRANT READ ON TABLE sales.orders TO analyst
GRANT SELECT ON TABLE sales.orders TO analyst (Correct answer)
ALLOW SELECT ON sales.orders FOR analyst

Correct answer: GRANT SELECT ON TABLE sales.orders TO analyst

The correct syntax is GRANT <permission> ON <resource> TO <role>, using standard SQL-style permission names like SELECT, MODIFY, and ALTER.

Question 39: What availability risk is introduced by leaving the system_auth keyspace at its default replication factor of 1?

Users will be unable to change their passwords
A single node failure can make authentication unavailable cluster-wide (Correct answer)
Superuser credentials become visible to other nodes
Performance degrades significantly under concurrent logins

Correct answer: A single node failure can make authentication unavailable cluster-wide

With RF=1 for system_auth, if the single node holding a credential replica goes down, authentication requests routed to that token range will fail, potentially locking users out.

Question 40: What is 'tombstone' in Cassandra?

A marker written at node startup
A log entry for schema changes
A backup snapshot file
A deletion marker stored in SSTables to represent deleted data (Correct answer)

Correct answer: A deletion marker stored in SSTables to represent deleted data

A tombstone is a deletion marker written to an SSTable that tells Cassandra to ignore older versions of that data during reads.

Question 41: What is the recommended strategy for the system_auth keyspace replication in a multi-datacenter production cluster?

NetworkTopologyStrategy with RF=1 per DC
SimpleStrategy with RF=1 (default)
SimpleStrategy with RF=3 globally
NetworkTopologyStrategy with RF matching the number of nodes in each DC (Correct answer)

Correct answer: NetworkTopologyStrategy with RF matching the number of nodes in each DC

Using NetworkTopologyStrategy with a replication factor matching each datacenter's node count ensures authentication data is available even during node failures across all DCs.

Question 42: How can you monitor the number of pending compaction tasks on a Cassandra node in real time?

SELECT * FROM system.compactions
nodetool cfstats | grep pending
nodetool compactionstats (Correct answer)
cqlsh -e 'SHOW COMPACTIONS'

Correct answer: nodetool compactionstats

`nodetool compactionstats` displays currently running and pending compaction tasks along with their progress and estimated completion.

Question 43: What does the 'endpoint_snitch' configuration control in Cassandra?

The CQL protocol version
How Cassandra determines the topology (rack/DC) of nodes for replica placement (Correct answer)
The gossip heartbeat interval
Network traffic encryption

Correct answer: How Cassandra determines the topology (rack/DC) of nodes for replica placement

The endpoint_snitch tells Cassandra how nodes are organized into racks and data centers so it can place replicas intelligently to maximize fault tolerance.

Question 44: What does the 'LOCAL_QUORUM' consistency level ensure in a multi-datacenter Cassandra deployment?

A quorum of nodes within the local data center must respond (Correct answer)
Only one node in the local data center must respond
A quorum of nodes across all data centers must respond
All nodes in the local data center must respond

Correct answer: A quorum of nodes within the local data center must respond

LOCAL_QUORUM satisfies quorum using only replicas in the same data center as the coordinator, avoiding cross-DC latency while still providing strong consistency locally.

Question 45: Cassandra was created by who?

Microsoft
Google
Apple
Facebook (Correct answer)

Correct answer: Facebook

Explanation: <br> Avinash Lakshman and Prashant Malik created Apache Cassandra while both were engineers at Facebook. Thanks to the database, which powers Facebook's inbox search feature, users can quickly discover the conversations and other items they're looking for.

Question 46: What is a tombstone in Apache Cassandra?

A stale Bloom filter entry pointing to a missing key
A deleted SSTable that has been compacted away
A marker written to represent a deletion, which is purged after the gc_grace_seconds period (Correct answer)
A committed-log entry that references a rolled-back transaction

Correct answer: A marker written to represent a deletion, which is purged after the gc_grace_seconds period

Cassandra uses tombstones to flag deleted data; they are retained for gc_grace_seconds to prevent deleted data from reappearing via hinted handoff, then removed during compaction.

Question 47: What is the default port used for SSL-encrypted internode (storage) communication in Apache Cassandra?

7000
9142
7001 (Correct answer)
9042

Correct answer: 7001

Port 7001 (ssl_storage_port) is used for encrypted internode communication, while 7000 is the unencrypted storage port, 9042 is native CQL, and 9142 is native CQL over TLS.

Question 48: What is the impact of too many tombstones in a Cassandra partition?

Increased write throughput
Faster compaction cycles
Reduced replication overhead
Degraded read performance because Cassandra must scan and filter tombstones during reads (Correct answer)

Correct answer: Degraded read performance because Cassandra must scan and filter tombstones during reads

Reads must traverse tombstones to determine what data is live; a partition with millions of tombstones can cause read timeouts and TombstoneOverwhelmingException.

Question 49: What is 'read repair' in Apache Cassandra?

Synchronizing divergent replicas by comparing and updating them during a read (Correct answer)
Rewriting outdated partition keys
Compacting SSTables after a read-heavy period
Rebuilding corrupt SSTables

Correct answer: Synchronizing divergent replicas by comparing and updating them during a read

Read repair detects inconsistencies between replicas during a read operation and writes the most recent data back to out-of-date replicas.

Question 50: What are virtual nodes (vnodes) in Apache Cassandra?

Separate JVM instances on a single machine
Dedicated nodes that handle only read traffic
Multiple smaller token ranges assigned to each node instead of one large range (Correct answer)
Logical containers for materialised views

Correct answer: Multiple smaller token ranges assigned to each node instead of one large range

Vnodes allow each physical node to own many small, non-contiguous token ranges, improving load distribution and simplifying cluster scaling.

Question 51: What mechanism does Cassandra use to detect and recover from failed nodes automatically?

Paxos consensus
Gossip protocol with hinted handoff (Correct answer)
ZooKeeper coordination
Raft leader election

Correct answer: Gossip protocol with hinted handoff

Cassandra uses gossip for failure detection and hinted handoff to store missed writes temporarily on behalf of a down replica.

Question 52: After changing the replication factor of a keyspace in Cassandra, what must you do to apply the change?

Drop and recreate the keyspace
Run nodetool repair on each node (Correct answer)
Run nodetool flush
Restart all nodes

Correct answer: Run nodetool repair on each node

Running nodetool repair streams the correct replicas to nodes that should now own additional copies according to the new replication factor.

Question 53: Which Cassandra feature lets you define a secondary sort order within a partition using the PRIMARY KEY definition?

Clustering columns (Correct answer)
Static columns
Secondary index
Partition index

Correct answer: Clustering columns

Clustering columns are the non-partition components of the primary key and determine the sort order of rows within a partition.

Question 54: What does the CQL statement 'UPDATE ... IF EXISTS' provide?

A batch update across multiple partitions
A conditional update using Paxos consensus
A lightweight transaction that only applies the update if the row already exists (Correct answer)
An upsert that creates the row if missing

Correct answer: A lightweight transaction that only applies the update if the row already exists

IF EXISTS uses Cassandra's lightweight transaction (LWT) mechanism backed by Paxos to perform a compare-and-swap, only updating if the row already exists.

Question 55: What Cassandra version do you use?

3.2
2.4
3
3.9 (Correct answer)

Correct answer: 3.9

The version in use is Cassandra 3.9, a specific release in the 3.x line. The other options (2.4, 3.2, and 3) are different or incomplete version numbers and don't match the deployed release.

Question 56: What is 'key cache' in Cassandra and how does it improve performance?

Caches Bloom filter results to avoid recalculation
Caches partition key to SSTable offset mappings, reducing disk seeks for frequently accessed partitions (Correct answer)
Caches the most recently executed CQL queries
Caches the entire row for hot partitions

Correct answer: Caches partition key to SSTable offset mappings, reducing disk seeks for frequently accessed partitions

The key cache stores the position of a partition key within an SSTable, so Cassandra can skip the index lookup and seek directly to the data on subsequent accesses.

Question 57: A Cassandra professional is asked to estimate cluster sizing for a new project. Which factor is most important to establish first?

Preferred programming language
Number of developers on the team
Budget for licensing fees
Expected read/write throughput and data volume (Correct answer)

Correct answer: Expected read/write throughput and data volume

Throughput requirements and data volume directly determine the number of nodes, replication factor, and hardware specs needed.

Question 58: What does the 'nodetool tpstats' command display?

Table-level partition size statistics
Time-series performance metrics
Token placement statistics per node
Thread pool statistics showing active, pending, and blocked tasks (Correct answer)

Correct answer: Thread pool statistics showing active, pending, and blocked tasks

nodetool tpstats shows the state of all Cassandra internal thread pools, helping identify bottlenecks when pending or blocked counts are high.

Question 59: What does 'nodetool compact' do?

Rebalances token ranges across nodes
Deletes expired tombstones immediately
Flushes all MemTables to disk
Triggers a major compaction of all SSTables for a keyspace/table into a single SSTable (Correct answer)

Correct answer: Triggers a major compaction of all SSTables for a keyspace/table into a single SSTable

nodetool compact merges all SSTables for the specified table into one, removing tombstones and reducing read amplification at the cost of high temporary disk usage.

Question 60: What does the 'off-heap' memory setting for Cassandra caches mean?

Caches are stored on a separate server
Caches are stored in native OS memory outside the JVM heap, reducing GC pressure (Correct answer)
Caches are disabled and data is read from disk each time
Caches are stored in compressed SSTable files

Correct answer: Caches are stored in native OS memory outside the JVM heap, reducing GC pressure

Off-heap memory is allocated outside the JVM heap using direct memory, so GC does not scan or collect it, greatly reducing GC pause frequency and duration.

DataStax Apache Cassandra Developer Associate Certification

The DataStax Apache Cassandra Developer Associate certification validates knowledge of CQL, data modeling, Cassandra architecture, and cluster operations. It targets developers and administrators working with Apache Cassandra in production environments.

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