DataStax Cassandra Developer Certification Certification Questions and Answer (Dumps and Practice Questions)
Question : What is linearizable consistency
1. Order of record updates does not matter, all the statement should be successful at the end of transaction.
2. Order of record updates matter, and should be executed in reverse order of that defined.
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4. None of this
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Explanation: : In ACID terms, linearizable consistency (or serial consistency) is a serial (immediate) isolation
level for lightweight transactions. Cassandra does not use employ traditional mechanisms like locking or transactional
dependencies when concurrently updating multiple rows or tables.
However, some operations must be performed in sequence and not interrupted by other operations. For example, duplications or
overwrites in user account creation can have serious consequences. Situations like race conditions (two clients updating the
same record) can introduce inconsistency across replicas. Writing with high consistency does nothing to reduce this. You can
apply linearizable consistency to a unique identifier, like the userID or email address, although is not required for all
aspects of the user's account. Serial operations for these elements can be implemented in Cassandra with the Paxos consensus
protocol, which uses a quorum-based algorithm. Lightweight transactions can be implemented without the need for a master
database or two-phase commit process.
Lightweight transaction write operations use the serial consistency level for Paxos consensus and the regular consistency
level for the write to the table.
Question : You have following consistency level configured, assume replication factor is
read operations using 2 out of 3 replicas to verify the value
write operations using 2 out of 3 replicas to verify the value
What is the overall consistency in this case?
1. Eventual
2. Light
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4. Initial
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Explanation: Reliability of read and write operations depends on the consistency used to verify the operation.
Strong consistency can be guaranteed when the following condition is true:
R + W > N
where
R is the consistency level of read operations
W is the consistency level of write operations
N is the number of replicas
If the replication factor is 3, then the consistency level of the reads and writes combined must be at least 4. For example,
read operations using 2 out of 3 replicas to verify the value, and write operations using 2 out of 3 replicas to verify the
value will result in strong consistency. If fast write operations are required, but strong consistency is still desired, the
write consistency level is lowered to 1, but now read operations have to verify a matched value on all 3 replicas. Writes will
be fast, but reads will be slower.
Eventual consistency occurs if the following condition is true:
R + W =< N
where
R is the consistency level of read operations
W is the consistency level of write operations
N is the number of replicas
If the replication factor is 3, then the consistency level of the reads and writes combined are 3 or less. For example, read
operations using QUORUM (2 out of 3 replicas) to verify the value, and write operations using ONE (1 out of 3 replicas) to do
fast writes will result in eventual consistency. All replicas will receive the data, but read operations are more vulnerable
to selecting data before all replicas write the data.
Question : Cassandra offers atomic, isolated, and durable transactions with eventual/tunable consistency
1. True
2. False
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Explanation: : Cassandra does not use RDBMS ACID transactions with rollback or locking mechanisms, but instead
offers atomic, isolated, and durable transactions with eventual/tunable consistency that lets the user decide how strong or
eventual they want each transactionas consistency to be.
Related Questions
Question : Let's say we want to implement the following schema:
Enity: Staff Member (id , name)
Enity: Permanent (specialises Staff Member) (next_appraisal_date )
Enity: Temporary (specialises Staff Member) (contract_end_date)
a Staff Member is either a Permanent employee or a Temporary contractor. How do you define disjoint in parent child
relationship?
1. An entity can only participate in only one subtype role
2. An entity cannot participate in the parent type role
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4. An entity is not complete
Question : The complete flow of a video website is below
1. login to site
2. It will show all recent video channels updated
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This flow is known as
1. Logical Diagram
2. Application Workflow
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4. Login flow
Question : What is an application workflow?
1. All the tasks in an application formed by casual dependencies
2. It is an alternate to ER diagram
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4. It is an alternate to DFD
Question : In Application Workflow Model, how do you define access patterns?
1. Using access pattern, you determine how data is accessed and you will know, what queries you will be
running.
2. Using access pattern, you determine how data will be arranged in table, so that we can have least
normalization.
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4. 1 and 3
5. 2 and 3
Question : What all are true, with regards to logical model
1. To design logical model use top down approach
2. You can define logical model algorithmically
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4. 1 and 3
5. 1,2 and 3
Question : Conceptual data model is specific to NoSQL databases?
1. True
2. False