Apache Spark - Reusing data across multiple stage using Caching
Published: 2023-05-05 19:46:58

## What is Spark?

Apache Spark is a fast and powerful open-source distributed computing system designed for processing large-scale data sets. It was developed at the University of California, Berkeley, and is now maintained by the Apache Software Foundation.

Spark is built on the concept of Resilient Distributed Datasets (RDDs), which are fault-tolerant and can be distributed across a cluster of computers for parallel processing. Spark provides a set of high-level APIs in multiple programming languages, including Java, Scala, Python, and R, making it accessible to a wide range of developers and data scientists.

Spark's core engine allows for in-memory computation, which means that data can be stored and processed in memory, providing faster processing times than traditional disk-based systems. Spark also provides a wide range of libraries and tools for data processing, including SQL, machine learning, graph processing, and streaming.


## What are “Stages” in Spark?

In Apache Spark, a stage is a collection of tasks that can be executed in parallel on a cluster of computers. Tasks within a stage are typically dependent on one another, meaning that they need to be executed in a specific order to produce the correct result.

A Spark job is divided into stages based on the dependencies between RDDs (Resilient Distributed Datasets), which are the primary data abstraction in Spark. When an action is called on an RDD, Spark creates a series of stages that are executed in sequence to produce the final result. Each stage contains one or more tasks that can be executed in parallel across multiple nodes in the cluster.

There are two types of stages in Spark: narrow and wide stages. A narrow stage has dependencies on a single parent RDD, meaning that each partition of the parent RDD is used by at most one partition of the child RDD. A wide stage has dependencies on multiple parent RDDs, meaning that each partition of the parent RDDs may be used by multiple partitions of the child RDD.

Spark uses a technique called pipelining to optimize narrow stages. In pipelining, multiple narrow stages are combined into a single stage, reducing the number of shuffles (data transfers between nodes) and improving performance.

Understanding the concept of stages is important for optimizing the performance of Spark jobs. By minimizing the number of wide stages and reducing the amount of data shuffled between stages, you can improve the overall efficiency and speed of your Spark applications.


## How to reuse data across multiple stage?

Let's say you have a large dataset that needs to be processed in multiple stages. Each stage may involve a different operation or transformation on the dataset, but some of the stages may use the same subset of the data repeatedly. Caching the data can help avoid recomputing the same subset of the data multiple times, and speed up the overall processing time.

For example, let's say you have a dataset of customer transactions and you want to analyze the customer behavior over time. You have the following stages:


1. Filter the transactions to only include those from the last year.
2. Group the transactions by customer ID.
3. Calculate the total transaction amount for each customer.
4. Calculate the average transaction amount for each customer.

The first stage involves filtering the transactions to only include those from the last year. The resulting dataset is smaller and will be used in subsequent stages. By caching this dataset in memory or on disk, you can avoid recomputing the filter operation each time the data is used in subsequent stages.

Here's an example code snippet to demonstrate caching the dataset:


```
// Load the customer transactions dataset
val transactions = spark.read.format("csv").load("path/to/transactions")

// Stage 1: Filter the transactions to only include those from the last year
val filteredTransactions = transactions.filter(year($"date") === 2022)

// Cache the filtered transactions dataset
filteredTransactions.cache()

// Stage 2: Group the transactions by customer ID
val transactionsByCustomer = filteredTransactions.groupBy($"customer_id")

// Stage 3: Calculate the total transaction amount for each customer
val totalAmountByCustomer = transactionsByCustomer.agg(sum($"amount").as("total_amount"))

// Stage 4: Calculate the average transaction amount for each customer
val avgAmountByCustomer = totalAmountByCustomer.select($"customer_id", $"total_amount" / count($"*").as("avg_amount"))

// Output the result
avgAmountByCustomer.show()
```


By caching the filtered transactions dataset in Stage 1, subsequent stages can access the data from memory or disk, avoiding the need to recompute the filter operation each time. This can significantly reduce the processing time, especially for large datasets or complex operations.