Cassandra Practice Test

When does a client acknowledge a write in Cassandra?

1. After it is logged in the commit log and MemTable

2. Immediately after sending the request

3. After receiving a response from all nodes

4. Once it is stored in the keyspace

The correct answer is: After it is logged in the commit log and MemTable

In Cassandra, a write is acknowledged by the client after it is logged in the commit log and MemTable. This design emphasizes durability and efficiency, ensuring that the data is not only temporarily in memory but has also been safely recorded on disk. When a write operation is performed, it first gets written to the commit log, which is a crucial process for recovery purposes. This logging guarantees that data can be reconstructed in case of a failure. Immediately following this, the data is placed into the MemTable, which is an in-memory structure that allows for fast read and write operations. This approach allows Cassandra to balance performance with data durability, as clients can receive an acknowledgment more quickly than if they waited for data to be fully replicated to all nodes. The acknowledgment process is designed this way to avoid bottlenecks and to ensure that applications can perform writes at a high throughput level while still maintaining high availability and fault tolerance. The other options do not accurately reflect the acknowledgment process in Cassandra. For instance, immediate acknowledgment after sending the request would not provide sufficient guarantees about data persistence. Waiting for responses from all nodes would lead to increased latency, and acknowledging only once stored in the keyspace does not account for the critical step of writing to the commit log and