The very first paper on RDD Resilient Distributed Datasets: A Fault-Tolerant Abstraction for In-Memory Cluster Computing described it as follows:
ResilientDistributedDatasets (RDDs), a distributed memory abstraction that lets programmers perform in-memory computations on large clusters in a fault-tolerant manner. As Spark is written in a functional programming paradigm, one of the key concepts of functional programming is immutable objects. Resilient Distributed Dataset is also an immutable dataset.
Formally, we can define an RDD as an immutable distributed collection of objects. It is the primary data type of Spark. It leverages cluster memory and is partitioned across the cluster.
The following is the logical representation of RDD:
RDDs can consist of (key, value) pairs as well. The following is the logical representation of pair of RDDs:
Also, as mentioned, RDD can be partitioned across the cluster. So the following is the logical representation of partitioned RDDs in a cluster:
An RDD supports only two types of operations. One is called transformation and the other is called action. The following are the explanations of both of these:
- Transformation: If an operation on an RDD gives you another RDD, then it is a transformation. Consider you have an RDD of strings and want to filter out all values that start with H as follows:
So, a filter operation on an RDD will return another RDD with all the values that passes through the filter condition. So, a filter is an example of a transformation
- Action: If an operation on an RDD gives you a result other than an RDD, it is called an action: for example, the sum of all values in an RDD, or the count of all the values or retrieving all values of RDD in form of a list, and so on. The following is the logical representation of an action sum of an RDD:
So, the rule is if after an operation on an RDD, you get an RDD then it is a transformation; otherwise, it is an action. We will discuss all the available transformations and actions that can be performed on an RDD, with the coding examples, in Chapter 4, Understanding the Spark Programming Model and Chapter 7, Spark Programming Model - Advanced.
Another important thing to understand about RDD is Lazy evaluation. Spark creates a DAG, also called the lineage graph, of all the operations you perform on an RDD. Execution of the graph starts only when an action is performed on RDD. Let's consider an example of DAG operations on RDD:
Here, first an RDD is calculated by reading data from a stable storage and two of the transformations are performed on the RDD and then finally an action is performed to get the result.
Look at the previous diagram; one would infer that RDD1 will be created as soon as a Spark job finds the step to create it from the database and then it will find the transformation steps, so it will perform transformations. Then it finds an action and so it will run the given action to calculate the result. However, this is not true.
In reality, a Spark job will start creating DAG steps until it finds a step that asks it to perform action on RDD. When the job finds this step, it starts executing the DAG from the first vertex.
The following are the benefits of this approach:
- Fault tolerance: The lineage graph of the operations on an RDD, makes it fault tolerant. Since Spark is well aware of the steps it needs to perform to create an RDD, it can recalculate the RDD or its partitions in case of failure of the previous step instead of repeating the whole process again. For example, with DAG, if a partition of RDD is lost while processing, it can be calculated from RDD2, instead of repeating the process of calculating it from the database and performing two transformations. This gives a huge benefit of saving time and resources in case of failures.
- Optimizing resource usage: As Spark knows all the steps to be performed to calculate the end result in advance, it can leverage this information to use the cluster resources in a most optimized manner.
Following are some benefits that Spark RDD model provides over Hadoop MapReduce Model:
- Iterative processing: One of the biggest issue, with MapReduce processing is the IO (Input/Output) involved. It really slows down the process of MapReduce if you are running iterative operations where you would basically chain MapReduce jobs to perform multiple aggregations.
Consider running a MapReduce job that reads data from HDFS and performs some aggregation and writes the output back to HDFS. Now, mapper jobs will read data from HDFS and write the output to the local filesystem after completion and Reduce pulls that data and runs the reduce process on it. After which, it writes the output to HDFS (not considering the spill mechanism of mapper and reducer).
Now, let's say you want to perform another aggregation on the output data so you will execute another MapReduce job on the output data which will go through a similar I/O process. So the following is the logical representation of how iterative operations will run in MapReduce.
On the other hand, Spark will not perform such I/O in most of the cases for the job previously described. Data will be read from HDFS once and then Spark will perform in memory transformation on RDD for every iteration. The output of every step (that is, another RDD) will be stored in the distributed cluster memory. The following is the logical representation of the same job in Spark:
Now, here is a catch. What if the size of the intermediate results is more than the distributed memory size? In that case, Spark will spill that RDD to disk.
- Interactive Processing: Another benefit of the data structure of Spark over MapReduce or Hadoop can be seen when the user wants to run some ad-hoc queries on the data placed on some stable storage.
Let's say you are trying to run some MapReduce jobs (or Hive queries) on the data to do some analysis. If you are running multiple queries on same input data, MapReduce will read the data from storage, let's say HDFS, every time you run the query. A logical representation of that can be as follows:
On the other hand, Spark provides a mechanism to persist an RDD in memory (different mechanisms of persisting RDD will be discussed later in Chapter 4, Understanding the Spark Programming Model). So, you can execute one job and save RDD in memory. Then, other analytics can be executed on the same RDD without reading the data from HDFS again. The following is the logical representation of that:
When a Spark job encounters Spark Action 1, it executes the DAG and calculates the RDD. Then the RDD will be persisted in memory and Spark Action 1 will be performed on the RDD. Afterwards, Spark Action 2 and Spark Action 3 will be performed in the same RDD. So, this model helps to save lot of I/O from the stable storage in case of interactive processing.
