Repartition and Coalesce in Apache Spark

Repartition and coalesce are two key functions in Apache Spark that help control the number of partitions in a DataFrame or RDD. Efficient partitioning can significantly impact the performance of your Spark jobs, as it determines how data is distributed across the cluster and how tasks are executed in parallel.

Repartition

The repartition function allows you to increase or decrease the number of partitions in your DataFrame or RDD. It performs a full shuffle of the data, which means that data is redistributed across the new set of partitions. This operation can be expensive due to the shuffling process but is useful in scenarios where you need to evenly distribute data.

Use Cases

• Increasing Partitions: When you have fewer partitions and want to increase the level of parallelism to improve performance.
• Balancing Data: When the data distribution across partitions is uneven and you want to balance it out.
• After Join Operations: When performing joins that result in a large DataFrame, repartitioning can help distribute the data more evenly for subsequent operations.

val df = spark.read.csv("path/to/file.csv")
val repartitionedDF = df.repartition(10)        

or

df.repartition(10, 'age')
df.repartition(10,'age','height')
df.repartition('age','height')
df.repartition('age')        

• You can create uniform partitions using repartition(n) for your dataframe.
• You can also use one or more columns to repartition your dataframe.
• Repartition causes shuffle/sort, number of partitions depends on shuffle partitions configuration.
• You can change shuffle partition configuration using numPartitions argument.
• Repartitioning on column name doesnt guarantee uniform partitions.

Coalesce

The coalesce function is used to decrease the number of partitions in a DataFrame or RDD. Unlike repartition, coalesce avoids a full shuffle of the data, making it a more efficient operation when reducing the number of partitions. It works by moving data from multiple partitions into fewer partitions without redistributing all of the data.

Note: It can cause skewed partitions. It merges local partitions only and avoids shuffle/sort.

Use Cases

• Decreasing Partitions: When you have too many partitions, resulting in overhead, and you want to reduce the number of partitions.
• Optimization: Before writing output to disk, coalescing can reduce the number of output files.
• Post-Filter Operations: After filtering operations that result in smaller datasets, coalescing can consolidate partitions for better performance.

val df = spark.read.csv("path/to/file.csv")
val coalescedDF = df.coalesce(2)        

Key Differences

Shuffle:

• repartition: Performs a full shuffle of the data.
• coalesce: Avoids a full shuffle and simply merges partitions.

Performance:

• repartition: More computationally expensive due to the shuffle.
• coalesce: More efficient for reducing partitions as it avoids shuffling.

Use Case:

• repartition: Useful for both increasing and balancing the number of partitions.
• coalesce: Ideal for reducing the number of partitions without a shuffle.

Best Practices

• Use repartition for Increasing Partitions: When you need to increase parallelism or balance partition sizes, use repartition.
• Use coalesce for Reducing Partitions: When you need to reduce the number of partitions without the cost of a full shuffle, use coalesce.
• Combine Both: In some cases, you may start with repartition to balance data and then use coalesce to fine-tune the number of partitions for output.