Question 1

What is the difference between repartition and coalesce in Apache Spark?

Accepted Answer

**Repartition(n)**: Performs a full shuffle to redistribute data across exactly `n` partitions. Can increase or decrease partition count. Uses hash partitioning by default—all rows are exchanged across the network.

**Coalesce(n)**: Merges existing partitions into fewer partitions without a full shuffle. Only decreases partition count....

Question 2

What is the difference between SparkSession and SparkContext in Spark?

Accepted Answer

**SparkContext** (Spark 1.x): Low-level entry point for RDD operations. Manages cluster connections, configuration, and RDD creation. One active SparkContext per JVM. RDD-only.

**SparkSession** (Spark 2.0+): Unified entry point subsuming SparkContext, SQLContext, HiveContext, StreamingContext. Provides DataFrame, Dataset, SQL, and Structured Streaming APIs....

Question 3

What is the difference between partitioning and bucketing in Spark, and when would you use bucketing?

Accepted Answer

**Partitioning**: Physically divides data by column values (e.g., date, region); enables partition pruning; one directory per partition value. **Bucketing**: Divides data within a partition into a fixed number of files via hash of bucketing column(s); co-locates same-key rows. **When to bucket**: Frequent joins or group-bys on a column (e.g., user_id). Same bucket count on both sides enables sort-merge join without shuffle....

Question 4

What strategies can you use to handle skewed data in Spark?

Accepted Answer

**1. Salting**: Add random suffix to skewed keys to spread load; requires two-phase aggregation. **2. Two-phase aggregation**: Aggregate with salted key, then aggregate again without salt. **3. Broadcast**: For small dimension tables, broadcast to avoid shuffle. **4. Custom partitioning**: Pre-partition by known skewed keys. **5. Increase partitions**: Spreads work but doesn't fix root cause. **6. AQE Skew Join (Spark 3.0+)**: Automatically splits skewed partitions....

Question 5

What is the difference between Managed and External tables in Hive/Spark?

Accepted Answer

Managed: Spark/Hive owns metadata and data. DROP TABLE deletes both. External: Metadata only; data lives in specified location. DROP TABLE drops metadata; data remains. Why External: Shared data across tools (Athena, Glue, Spark); production datasets where accidental DROP would be catastrophic; data lifecycle independent of table. Why Managed: Ephemeral tables, temp outputs; simpler—no orphan paths. Cost: Managed DROP can trigger expensive recursive deletes on object store....

Question 6

What is a window function? Explain with an example.

Accepted Answer

**Architectural Logic:** Window functions compute values over a set of rows related to the current row (defined by OVER) without collapsing rows like GROUP BY. **Why windows over self-joins:** Single scan, no explosion; optimal for rankings, running totals, period-over-period. **Scalability:** PARTITION BY and ORDER BY affect sort/spill; large windows with RANGE BETWEEN UNBOUNDED PRECEDING can spill to disk. **Cost:** ROWS vs....

Question 7

Explain the concept of checkpointing in Spark and why it is important.

Accepted Answer

Checkpointing breaks RDD lineage by materializing to storage, trading storage I/O for DAG truncation. **Why it exists**: Long lineage (e.g., 1000+ stages in iterative algorithms like PageRank or ML) causes stack overflow in the driver and makes fault recovery expensive—Spark would replay the entire DAG. **Scalability trade-off**: Checkpoint is a full write; at scale, this is costly (network, disk). For batch, prefer persist/cache when lineage is < ~50 stages....

Question 8

Agile methodologies used?

Accepted Answer

**Scrum**: 2-week sprints, standups, planning, retros, refinement. Jira for epics, stories, bugs. **Kanban**: For support and incidents—continuous flow. **Data**: Sprint on pipeline features, schema changes, quality; incidents through Kanban....

Citi Data Engineer Interview Questions

+14 More Questions with Expert Answers

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Citi Data Engineer Interview Questions

+14 More Questions with Expert Answers

Other Companies