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Database / ScyllaDB Interview questions

Describe the Seastar framework used by ScyllaDB?

Seastar is the open-source C++ asynchronous programming framework, originally built for ScyllaDB, that underlies its shard-per-core, high-performance design. Seastar provides a future-promise concurrency model, its own userspace network stack and memory allocator, and asynchronous I/O so applications built on it can drive hardware close to its physical limits.

Rather than relying on OS threads and blocking system calls, Seastar programs run one thread per core, each managing its own event loop and non-blocking I/O queue. This eliminates cross-core synchronization overhead and lets ScyllaDB scale near-linearly with core count on a single machine, which is the foundation the shard-per-core architecture is built on top of. Seastar is also used by other performance-sensitive projects beyond ScyllaDB itself.

What concurrency model does Seastar provide?
How many OS threads does Seastar typically run per core?

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What is ScyllaDB? What are the key features of ScyllaDB? What is the shard-per-core architecture in ScyllaDB? Define a partition key in ScyllaDB? What is a clustering key in ScyllaDB? What are the data types supported by ScyllaDB? Describe the Seastar framework used by ScyllaDB? What are the types of consistency levels in ScyllaDB? List the compaction strategies available in ScyllaDB? How do you create a table in ScyllaDB? What is a materialized view in ScyllaDB? Explain the purpose of the commit log in ScyllaDB? What is ScyllaDB Alternator? How do you apply TTL to data in ScyllaDB? What is ScyllaDB Manager? Why doesn't ScyllaDB rely on a JVM? How does ScyllaDB achieve linear scalability per node? What is the difference between ScyllaDB and Apache Cassandra? When should you use a local secondary index versus a global one? What happens when a wide partition forms in ScyllaDB? How is data replicated across nodes in ScyllaDB? Why should you avoid large batch statements in ScyllaDB? What is the difference between ScyllaDB and DynamoDB? How does ScyllaDB handle node failure with hinted handoff? When would you choose LOCAL_QUORUM over QUORUM? How can you optimize write performance in ScyllaDB? What is the difference between memtables and SSTables? Why do we use tombstones in ScyllaDB? How does ScyllaDB's shard-aware driver route requests? What is the difference between STCS and LCS compaction? When should you use lightweight transactions in ScyllaDB? How is repair implemented in ScyllaDB? Why doesn't ScyllaDB support arbitrary ad-hoc joins? What is the difference between ScyllaDB tablets and vnodes? How do you troubleshoot high read latency in ScyllaDB? Explain the internal working of the Seastar future-promise model? Explain the execution flow of a write request in ScyllaDB? Explain the lifecycle of an SSTable in ScyllaDB? How does ScyllaDB guarantee strongly consistent schema changes using Raft? What happens internally when ScyllaDB performs compaction? How can you optimize a multi-datacenter ScyllaDB deployment for latency? Which is better and why: LOCAL_QUORUM or ONE for a globally distributed app? How does token-aware routing improve latency in ScyllaDB? Why is the gossip protocol critical to ScyllaDB's cluster membership? How do you troubleshoot compaction backlog in ScyllaDB? Explain the internal working of ScyllaDB's read path? What happens when a node becomes unavailable in a ScyllaDB cluster? How does ScyllaDB implement Change Data Capture internally? Explain the execution flow of a lightweight transaction in ScyllaDB? How can you optimize schema design to avoid wide partitions at scale?
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