Dataframe是Row对象的RDD，每个对象代表一条记录。一个 Dataframe还知道它的行的模式(即数据字段)。虽然Dataframes 看起来像常规的rdd，它们内部以更有效的方式存储数据，利用它们的模式。此外，它们还提供了rdd上不可用的新操作，例如运行SQL查询的能力。数据帧可以从外部数据源、查询结果或常规rdd中创建。

参考文献:Zaharia M.， et al。学习火花(O'Reilly, 2015)

通过谷歌搜索“DataFrame definition”可以很好地定义一个DataFrame:

数据帧是一种表格，或者是一种二维的类似数组的结构 每一列包含对一个变量的测量，以及每一行 包含一个大小写。

因此，由于其表格格式，DataFrame具有额外的元数据，这允许Spark在最终查询上运行某些优化。

另一方面，RDD只是一个弹性分布式数据集(Resilient Distributed Dataset)，它更像是一个数据黑箱，不能对其进行优化，因为可以对其执行的操作不受约束。

然而，你可以通过RDD方法从一个DataFrame到一个RDD，你也可以通过toDF方法从一个RDD到一个DataFrame(如果RDD是一个表格格式)

一般来说，由于内置的查询优化，建议尽可能使用DataFrame。

大部分答案都是正确的，我只想补充一点

在Spark 2.0中，这两个API (DataFrame +DataSet)将统一为一个API。

统一DataFrame和Dataset:在Scala和Java中，DataFrame和Dataset是统一的，即DataFrame只是Dataset of Row的类型别名。在Python和R中，由于缺乏类型安全，DataFrame是主要的编程接口。”

数据集类似于rdd，但是，它们不使用Java序列化或Kryo，而是使用专门的Encoder来序列化对象，以便在网络上进行处理或传输。

Spark SQL支持两种将现有rdd转换为数据集的方法。第一种方法使用反射来推断包含特定类型对象的RDD的模式。这种基于反射的方法可以生成更简洁的代码，如果在编写Spark应用程序时已经知道模式，这种方法也能很好地工作。

创建数据集的第二种方法是通过编程接口，该接口允许您构造一个模式，然后将其应用于现有的RDD。虽然此方法更详细，但它允许您在运行时之前不知道列及其类型时构造数据集。

在这里你可以找到RDD tof数据帧对话的答案

如何将rdd对象转换为数据帧在火花

Spark RDD(弹性分布式数据集):

RDD is the core data abstraction API and is available since very first release of Spark (Spark 1.0). It is a lower-level API for manipulating distributed collection of data. The RDD APIs exposes some extremely useful methods which can be used to get very tight control over underlying physical data structure. It is an immutable (read only) collection of partitioned data distributed on different machines. RDD enables in-memory computation on large clusters to speed up big data processing in a fault tolerant manner. To enable fault tolerance, RDD uses DAG (Directed Acyclic Graph) which consists of a set of vertices and edges. The vertices and edges in DAG represent the RDD and the operation to be applied on that RDD respectively. The transformations defined on RDD are lazy and executes only when an action is called

Spark DataFrame

Spark 1.3 introduced two new data abstraction APIs – DataFrame and DataSet. The DataFrame APIs organizes the data into named columns like a table in relational database. It enables programmers to define schema on a distributed collection of data. Each row in a DataFrame is of object type row. Like an SQL table, each column must have same number of rows in a DataFrame. In short, DataFrame is lazily evaluated plan which specifies the operations needs to be performed on the distributed collection of the data. DataFrame is also an immutable collection.

Spark数据集:

作为DataFrame api的扩展，Spark 1.3还引入了DataSet api，在Spark中提供严格类型和面向对象的编程接口。它是不可变的、类型安全的分布式数据集合。像DataFrame一样，DataSet APIs也使用Catalyst引擎来实现执行优化。DataSet是DataFrame api的扩展。

〇其他差异

首先，DataFrame是从SchemaRDD演变而来的。

是的. .Dataframe和RDD之间的转换是绝对可能的。

下面是一些示例代码片段。

df。rdd就是rdd [Row]

下面是一些创建数据框架的选项。

1) yourrddOffrow。toDF转换为DataFrame。 2)使用sql context的createDataFrame Val df = spark。createDataFrame (rddOfRow模式)

where schema can be from some of below options as described by nice SO post.. From scala case class and scala reflection api import org.apache.spark.sql.catalyst.ScalaReflection val schema = ScalaReflection.schemaFor[YourScalacaseClass].dataType.asInstanceOf[StructType] OR using Encoders import org.apache.spark.sql.Encoders val mySchema = Encoders.product[MyCaseClass].schema as described by Schema can also be created using StructType and StructField val schema = new StructType() .add(StructField("id", StringType, true)) .add(StructField("col1", DoubleType, true)) .add(StructField("col2", DoubleType, true)) etc...

事实上，现在有3个Apache Spark api ..

火灾等级:

The RDD (Resilient Distributed Dataset) API has been in Spark since the 1.0 release. The RDD API provides many transformation methods, such as map(), filter(), and reduce() for performing computations on the data. Each of these methods results in a new RDD representing the transformed data. However, these methods are just defining the operations to be performed and the transformations are not performed until an action method is called. Examples of action methods are collect() and saveAsObjectFile().

抽样的例子:

rdd.filter(_.age > 21) // transformation
   .map(_.last)// transformation
.saveAsObjectFile("under21.bin") // action

示例:RDD按属性过滤

rdd.filter(_.age > 21)

DataFrame火

Spark 1.3 introduced a new DataFrame API as part of the Project Tungsten initiative which seeks to improve the performance and scalability of Spark. The DataFrame API introduces the concept of a schema to describe the data, allowing Spark to manage the schema and only pass data between nodes, in a much more efficient way than using Java serialization. The DataFrame API is radically different from the RDD API because it is an API for building a relational query plan that Spark’s Catalyst optimizer can then execute. The API is natural for developers who are familiar with building query plans

示例SQL样式:

df。Filter ("age > 21");

限制: 因为代码是按名称引用数据属性的，所以编译器不可能捕捉到任何错误。如果属性名不正确，则只有在运行时创建查询计划时才会检测到错误。

DataFrame API的另一个缺点是它非常以scala为中心，虽然它确实支持Java，但支持是有限的。

例如，当从现有的Java对象RDD创建DataFrame时，Spark的Catalyst优化器无法推断模式，并假设DataFrame中的任何对象都实现了scala。产品界面。Scala case类解决了这个问题，因为它们实现了这个接口。

数据集火

The Dataset API, released as an API preview in Spark 1.6, aims to provide the best of both worlds; the familiar object-oriented programming style and compile-time type-safety of the RDD API but with the performance benefits of the Catalyst query optimizer. Datasets also use the same efficient off-heap storage mechanism as the DataFrame API. When it comes to serializing data, the Dataset API has the concept of encoders which translate between JVM representations (objects) and Spark’s internal binary format. Spark has built-in encoders which are very advanced in that they generate byte code to interact with off-heap data and provide on-demand access to individual attributes without having to de-serialize an entire object. Spark does not yet provide an API for implementing custom encoders, but that is planned for a future release. Additionally, the Dataset API is designed to work equally well with both Java and Scala. When working with Java objects, it is important that they are fully bean-compliant.

示例数据集API SQL样式:

dataset.filter(_.age < 21);

DataFrame和DataSet之间的评估不同:

阴极级流..(解密spark峰会上的数据框架和数据集演示)

进一步阅读…databricks文章-三个Apache Spark api的故事:rdd vs dataframe和数据集

DataFrame相当于RDBMS中的表，也可以以类似于rdd中的“原生”分布式集合的方式进行操作。与rdd不同，dataframe跟踪模式并支持各种关系操作，从而实现更优化的执行。 每个DataFrame对象表示一个逻辑计划，但由于它们的“惰性”性质，直到用户调用特定的“输出操作”才会执行。