Database / ScyllaDB Interview questions
1. What is ScyllaDB?
ScyllaDB is a distributed, wide-column NoSQL database built in C++ as a drop-in replacement for Apache Cassandra, designed to squeeze far more throughput out of the same hardware. It re-implements Cassandra's data model and wire protocol from scratch on top of the Seastar framework, avoiding the ...
2. What are the key features of ScyllaDB?
ScyllaDB's design goals center on extracting maximum throughput per node while keeping the operational model familiar to Cassandra users. Shard-per-core architecture - each CPU core owns its own data and connections, avoiding lock contention. No JVM / no GC pauses - written in C++ on the Seastar ...
3. What is the shard-per-core architecture in ScyllaDB?
Shard-per-core means each CPU core on a node runs an independent, single-threaded execution engine that owns a dedicated slice of RAM, a dedicated set of data (by token range), and its own network connections. Instead of one process sharing memory across threads with locks, ScyllaDB partitions ev...
4. Define a partition key in ScyllaDB?
A partition key is the portion of a table's primary key that determines which node(s) store a given row, by hashing the key value into a token that maps to a range owned by a specific replica set. Every row with the same partition key lands in the same partition, physically together on disk. CREA...
5. What is a clustering key in ScyllaDB?
A clustering key is the part of the primary key that comes after the partition key and determines how rows within the same partition are sorted on disk. Where the partition key decides which node holds the data, the clustering key decides the physical order of rows inside that partition. PRIMARY ...
6. What are the data types supported by ScyllaDB?
ScyllaDB, following CQL, supports a broad set of scalar and collection types for schema design: text / varchar, ascii - string data. int, bigint, smallint, tinyint, varint - integer types of varying width. float, double, decimal - numeric types, with decimal for exact precision. boolean - true/fa...
7. 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 applicat...
8. What are the types of consistency levels in ScyllaDB?
ScyllaDB, like Cassandra, offers tunable consistency, letting each read or write specify how many replicas must acknowledge before the operation succeeds. Level Meaning ONE / TWO / THREE That exact number of replicas must respond. QUORUM A majority of all replicas across the cluster. LOCAL_QUORUM...
9. List the compaction strategies available in ScyllaDB?
Compaction merges SSTables together to remove obsolete data and reduce read amplification; ScyllaDB supports several strategies suited to different workloads. Size-Tiered Compaction Strategy (STCS) - merges similarly-sized SSTables together; good default for write-heavy workloads. Leveled Compact...
10. How do you create a table in ScyllaDB?
Tables are created with standard CQL CREATE TABLE statements through cqlsh , a driver, or ScyllaDB's REST/CQL tooling, specifying the partition key and any clustering columns as part of the primary key. CREATE TABLE ecommerce.orders ( customer_id uuid, order_id timeuuid, status text, total decima...
11. What is a materialized view in ScyllaDB?
A materialized view is a server-managed table that automatically re-derives its rows from a base table using a different primary key layout, letting the same data be queried efficiently by a column that isn't part of the base table's partition key. CREATE MATERIALIZED VIEW orders_by_status AS SEL...
12. Explain the purpose of the commit log in ScyllaDB?
The commit log is an append-only, on-disk log that ScyllaDB writes to before acknowledging any write, purely for crash recovery. Every mutation is appended sequentially to the commit log at the same time it's applied to the in-memory memtable, so a sequential disk write, which is fast, stands in ...
13. What is ScyllaDB Alternator?
Alternator is ScyllaDB's implementation of the DynamoDB HTTP API, letting applications written against AWS DynamoDB's SDKs point at a ScyllaDB cluster instead, with no application code changes beyond the endpoint URL. aws dynamodb list-tables --endpoint-url http://scylla-node:8000 It supports Dyn...
14. How do you apply TTL to data in ScyllaDB?
Time-to-live (TTL) can be set per-write in seconds, either on an individual INSERT / UPDATE statement or as a table-wide default, after which the data is automatically marked for deletion. INSERT INTO sessions (session_id, user_id, data ) VALUES (uuid(), 'user123' , 'payload' ) USING TTL 3600 ; -...
15. What is ScyllaDB Manager?
ScyllaDB Manager is a centralized operations tool for automating and scheduling cluster-wide maintenance tasks that would otherwise need to be run manually node by node, such as repairs, backups, and rolling restarts. Automated repair scheduling - runs anti-entropy repairs on a rolling schedule a...
16. Why doesn't ScyllaDB rely on a JVM?
Cassandra runs on the JVM, which means its performance is subject to garbage collection pauses: periodically the JVM has to stop application threads to reclaim memory, and under heavy load or large heaps these "stop-the-world" pauses can spike into hundreds of milliseconds, directly hurting tail ...
17. How does ScyllaDB achieve linear scalability per node?
Per-node scalability in ScyllaDB comes from combining the shard-per-core architecture with a design that avoids shared, contended state as core counts grow. Each core runs its own shard with its own memtable, cache, and connections, so adding cores adds independent capacity rather than more conte...
18. What is the difference between ScyllaDB and Apache Cassandra?
ScyllaDB Apache Cassandra Written in C++ on the Seastar framework. Written in Java, runs on the JVM. Shard-per-core, no GC pauses. Thread-pool based, subject to GC pauses. Wire-compatible with CQL and Cassandra drivers. Native CQL implementation. Adopted Raft for strongly consistent schema/topolo...
19. When should you use a local secondary index versus a global one?
A local secondary index is stored on the same node as the base data it indexes, so a query using it can only be efficiently satisfied by first knowing (or scanning across) the partition, making it most useful when the query already includes the partition key alongside the indexed column. A global...
20. What happens when a wide partition forms in ScyllaDB?
A wide partition occurs when a single partition key accumulates an unusually large number of rows or a very large amount of data, often from a low-cardinality key or unbounded time-series growth without bucketing. Because a partition is the unit that a single set of replicas must serve, the node(...
21. How is data replicated across nodes in ScyllaDB?
ScyllaDB uses consistent hashing to map each partition key to a token, and that token determines a position on a logical ring of token ranges spread across the cluster's nodes. Each token range is owned by a set of replicas equal to the table's replication factor , with the specific placement str...
22. Why should you avoid large batch statements in ScyllaDB?
CQL's BATCH statement groups multiple writes together, but unless every statement in the batch shares the same partition key, ScyllaDB has to coordinate the batch as a distributed operation across multiple partitions, which adds a lot of overhead compared to sending the same writes individually. ...
23. What is the difference between ScyllaDB and DynamoDB?
ScyllaDB DynamoDB Self-managed or ScyllaDB Cloud; deployable anywhere. Fully managed, AWS-only. CQL as the native query language, plus Alternator for DynamoDB API compatibility. Proprietary DynamoDB API only. Pricing based on provisioned/self-managed infrastructure. Pricing based on provisioned o...
24. How does ScyllaDB handle node failure with hinted handoff?
When a write's coordinator can't reach one of the replicas responsible for storing it, perhaps because that node is temporarily down or unreachable, ScyllaDB can store a hint : a record of the missed write, kept on a live node (often the coordinator) until the target replica comes back. Once the ...
25. When would you choose LOCAL_QUORUM over QUORUM?
QUORUM requires a majority of replicas across all datacenters to respond, which means every read or write pays a cross-datacenter network round trip, even when the application only cares about consistency within its own region. LOCAL_QUORUM requires a majority only among replicas in the coordinat...
26. How can you optimize write performance in ScyllaDB?
Design partition keys to avoid hotspots - uneven key distribution caps throughput on a few nodes regardless of cluster size. Use a shard-aware driver so writes go directly to the owning shard, avoiding an internal network hop. Prefer single-partition batches over multi-partition ones, or skip bat...
27. What is the difference between memtables and SSTables?
Memtable SSTable In-memory, mutable structure holding recent writes. Immutable, on-disk file holding flushed data. Lost on crash unless replayed from the commit log. Durable; survives restarts. One active memtable per table per shard at a time. Many SSTables accumulate over time and get merged vi...
28. Why do we use tombstones in ScyllaDB?
Because SSTables are immutable once written, ScyllaDB can't simply erase a row or column in place the way an update-in-place database would. Instead, a delete (explicit, or implicit via TTL expiration) is recorded as a tombstone , a special marker written like any other mutation, that tells later...
29. How does ScyllaDB's shard-aware driver route requests?
A shard-aware driver understands not just which node owns a given partition key, via the same token-ring logic every driver uses, but which specific CPU shard on that node owns it, since ScyllaDB partitions data by core within a node as well as across nodes. Normal (non-shard-aware) drivers conne...
30. What is the difference between STCS and LCS compaction?
Size-Tiered (STCS) Leveled (LCS) Merges SSTables of similar size together. Organizes SSTables into size-bounded levels. Lower write amplification; simpler. Higher write amplification, but bounded per-read SSTable count. Can temporarily need up to 2x disk space during large compactions. More predi...
31. When should you use lightweight transactions in ScyllaDB?
Lightweight transactions (LWT), expressed with IF clauses like INSERT ... IF NOT EXISTS or UPDATE ... IF column = value , provide linearizable compare-and-swap semantics using a Paxos-based consensus protocol among replicas, rather than the normal fire-and-acknowledge write path. UPDATE inventory...
32. How is repair implemented in ScyllaDB?
Repair is ScyllaDB's anti-entropy process for reconciling data drift between replicas that can build up from missed writes, expired hints, or clock/network issues. It compares data across replicas, typically using Merkle trees, hash-tree structures that let two replicas efficiently find which ran...
33. Why doesn't ScyllaDB support arbitrary ad-hoc joins?
ScyllaDB is architected so that any single query can be efficiently routed and answered by the specific node(s) owning the relevant partition, keeping latency predictable at scale. An arbitrary join across two large tables would require correlating data that could live on completely different, un...
34. What is the difference between ScyllaDB tablets and vnodes?
Vnodes (legacy) Tablets (newer architecture) Fixed number of token ranges assigned per node at join time. Dynamically sized, independently balanced units of data per table. Rebalancing after adding/removing a node can be slow and coarse-grained. Rebalancing is finer-grained and much faster. Manua...
35. How do you troubleshoot high read latency in ScyllaDB?
Check for wide partitions - a large partition takes longer to scan and can dominate latency for that key. Check tombstone counts in query tracing - excessive tombstones force reads to skip past dead data. Review the consistency level - QUORUM or ALL reads pay more coordination latency than ONE or...
36. Explain the internal working of the Seastar future-promise model?
Seastar's concurrency model is built around futures (a placeholder for a value that will eventually be ready) and promises (the producer side that eventually fulfills that value), composed together instead of using blocking calls or OS-level thread synchronization. sequenceDiagram participant App...
37. 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 pa...
38. Explain the lifecycle of an SSTable in ScyllaDB?
An SSTable's life begins in memory and ends when its data is either merged into a newer SSTable or fully expired and removed. flowchart LR A[Writes land in memtable] --> B[Memtable fills / flush triggered] B --> C[Memtable flushed to disk as new immutable SSTable] C --> D[SSTable participates in ...
39. How does ScyllaDB guarantee strongly consistent schema changes using Raft?
Older versions of ScyllaDB (and Cassandra) propagated schema changes via gossip, an eventually-consistent protocol, which could momentarily leave different nodes with slightly different views of the schema during a rollout, an acceptable risk for schema but not ideal. ScyllaDB has since adopted R...
40. What happens internally when ScyllaDB performs compaction?
Compaction reads several existing SSTables, merges their contents row by row, and writes the result as new, consolidated SSTables, all while the affected data remains readable and writable through the process. flowchart TD A[Compaction strategy selects candidate SSTables] --> B[Open selected SSTa...
41. How can you optimize a multi-datacenter ScyllaDB deployment for latency?
Use LOCAL_QUORUM (or ONE/LOCAL_ONE) for regular application traffic so requests don't pay cross-datacenter round trips. Set NetworkTopologyStrategy replication per-datacenter so each region has enough local replicas to satisfy local consistency without depending on a remote datacenter. Route appl...
42. Which is better and why: LOCAL_QUORUM or ONE for a globally distributed app?
Neither is universally better; the choice trades off consistency strength against latency and availability, and the right answer depends on what the read or write is for. ONE LOCAL_QUORUM Fastest possible latency; only one replica must respond. Slightly higher latency; needs a majority in the loc...
43. How does token-aware routing improve latency in ScyllaDB?
Without token awareness, a driver sends a request to an arbitrary node, which then has to act as a coordinator and forward the request to whichever node(s) actually own the relevant data, adding an extra network hop before the "real" work even starts. // Token-aware driver (conceptual) token = mu...
44. Why is the gossip protocol critical to ScyllaDB's cluster membership?
In a large peer-to-peer cluster with no central coordinator for membership, every node needs a way to learn which other nodes exist, whether they're alive, and their basic state (load, schema version, token ownership) without a single point of failure or a central registry becoming a bottleneck. ...
45. How do you troubleshoot compaction backlog in ScyllaDB?
Check pending compaction metrics in the Monitoring Stack - a steadily growing backlog means compaction can't keep pace with incoming writes. Review the compaction strategy fit - STCS under heavy, uneven write patterns can lag; LCS or ICS may handle the shape of the workload better. Check disk I/O...
46. Explain the internal working of ScyllaDB's read path?
A read has to reconstruct the current value of a row from potentially several places at once, since data for one partition can be spread across the memtable and multiple SSTables. flowchart TD A[Coordinator receives read] --> B[Identify replicas for the token] B --> C[Send read request to require...
47. What happens when a node becomes unavailable in a ScyllaDB cluster?
Because data is replicated across multiple nodes per the table's replication factor, a single node going down doesn't make its data unavailable - the remaining replicas for its token ranges continue serving reads and writes. Other nodes' failure detectors (built on gossip-based heartbeat exchange...
48. How does ScyllaDB implement Change Data Capture internally?
When CDC is enabled on a table, ScyllaDB automatically creates a companion log table alongside it, and every insert, update, or delete on the base table also writes a corresponding row into that log table describing the change, as part of the same write path. flowchart TD A[Write to base table] -...
49. Explain the execution flow of a lightweight transaction in ScyllaDB?
A lightweight transaction (LWT) needs every replica involved to agree on both the current value and the outcome of the conditional check before anything is applied, so it runs a Paxos round instead of the normal single-phase write path. sequenceDiagram participant C as Client participant Coord as...
50. How can you optimize schema design to avoid wide partitions at scale?
Bucket time-series data by a natural window (hour/day) appended to the partition key, so one logical entity's data spreads across many bounded partitions instead of one unbounded one. Add a synthetic bucket suffix (e.g. a hash or modulo of an ID) when a naturally low-cardinality key would otherwi...
