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Database / ScyllaDB Interview questions

Explain the execution flow of a write request in ScyllaDB?

A write in ScyllaDB moves from client to coordinator to replicas, with durability and consistency handled at distinct points along the way.

flowchart TD A[Client sends write] --> B[Coordinator node receives request] B --> C[Coordinator computes token, identifies replicas] C --> D[Write sent in parallel to all replicas] D --> E[Each replica appends to commit log] E --> F[Each replica applies write to memtable] F --> G{Enough replicas acked per consistency level?} G -- Yes --> H[Coordinator returns success to client] G -- No, some unreachable --> I[Hint stored for missed replica] I --> H

The client sends the write to any node, which acts as coordinator for that request and computes which replicas own the relevant token range. It forwards the write to all of them in parallel; each replica durably appends it to its commit log and applies it to its in-memory memtable before acknowledging. The coordinator waits only for the number of acknowledgments required by the requested consistency level (e.g. a quorum) before telling the client the write succeeded - it doesn't need every replica to respond immediately, and any replica it couldn't reach gets a hint stored for later replay. This is why writes in ScyllaDB are fast even at high consistency levels: the expensive part (durable commit log append) is spread in parallel across replicas rather than serialized through a single leader.

What must a replica do before acknowledging a write?
Does the coordinator need every replica to acknowledge before returning success?

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