scala | 易学教程

MongoDB Reactive Streams run-time dependency error with Alpakka Mongo Connector ClassNotFoundException

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问题 I'm trying to integrate the Alpakka Mongo Connector into an application that heavily relies on the Akka libraries for stream processing. The application utilizes Akka HTTP as well. I am encountering a dependency issue at run-time. In particular, I'm getting a NoClassDefFoundError for some kind of Success/Failure wrappers when I try to use the MongoSink.insertOne method provided by the Mongo connector. A full stack-trace: java.lang.NoClassDefFoundError: com/mongodb/reactivestreams/client

In Spark scala, how to check between adjacent rows in a dataframe

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问题 How can I check for the dates from the adjacent rows (preceding and next) in a Dataframe . This should happen at a key level I have following data after sorting on key, dates source_Df.show() +-----+--------+------------+------------+ | key | code | begin_dt | end_dt | +-----+--------+------------+------------+ | 10 | ABC | 2018-01-01 | 2018-01-08 | | 10 | BAC | 2018-01-03 | 2018-01-15 | | 10 | CAS | 2018-01-03 | 2018-01-21 | | 20 | AAA | 2017-11-12 | 2018-01-03 | | 20 | DAS | 2018-01-01 |

Regarding org.apache.spark.sql.AnalysisException error when creating a jar file using Scala

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问题 I have following simple Scala class , which i will later modify to fit some machine learning models. I need to create a jar file out of this as i am going to run these models in amazon-emr . I am a beginner in this process. So i first tested whether i can successfully import the following csv file and write to another file by creating a jar file using the Scala class mention below. The csv file looks like this and its include a Date column as one of the variables. +-------------------+-------

Spark How to Specify Number of Resulting Files for DataFrame While/After Writing

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问题 I saw several q/a's about writing single file into hdfs,it seems using coalesce(1) is sufficient. E.g; df.coalesce(1).write.mode("overwrite").format(format).save(location) But how can I specify "exact" number of files that will written after save operation? So my question is; If I have dataframe which consist 100 partitions when I make write operation will it write 100 files? If I have dataframe which consist 100 partitions when I make write operation after calling repartition(50)/coalsesce

scala issue with reading file from resources directory

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问题 I wrote something like this to read file from resource directory: val filePath = MyClass.getClass.getResource("/myFile.csv") val file = filePath.getFile println(file) CSVReader.open(file) and the result I got was something like this: file:/path/to/project/my_module/src/main/resources/my_module-assembly-0.1.jar!/myFile.csv Exception in thread "main" java.io.FileNotFoundException: file:/path/to/project/my_module/src/main/resources/my_module-assembly-0.1.jar!/myFile.csv (No such file or

How to unit test BroadcastProcessFunction in flink when processElement depends on broadcasted data

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问题 I implemented a flink stream with a BroadcastProcessFunction. From the processBroadcastElement I get my model and I apply it on my event in processElement. I don't find a way to unit test my stream as I don't find a solution to ensure the model is dispatched prior to the first event. I would say there are two ways for achieving this: 1. Find a solution to have the model pushed in the stream first 2. Have the broadcast state filled with the model prio to the execution of the stream so that it

Spark机器学习库（MLlib）指南

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机器学习库（MLlib）指南 MLlib是Spark的机器学习（ML）库。机器学习具有可扩展性和易用性。提供高级API ，它提供了以下工具： ML算法：常见的学习算法，如分类、回归、聚类和协同过滤特征化：特征提取、变换、降维和选择管道：用于构建、评估和调优ML管道的工具持久性：保存和加载算法、模型和管道实用程序：线性代数，统计学，数据处理等。声明：基于DataFrame的API是主要API 基于MLlib RDD的API现在处于维护模式。从Spark 2.0开始，在 spark.mllib 程序包已进入维护模式。Spark的主要机器学习API现在是 DataFrame -based API spark.ml 。有什么影响？ MLlib将支持基于RDD的API spark.mllib 以及错误修复。 MLlib不会为基于RDD的API添加新功能。在Spark 2.x版本中，MLlib将为基于DataFrames的API添加功能，以实现与基于RDD的API的功能奇偶校验。在达到功能奇偶校验（粗略估计Spark 2.3）之后，将弃用基于RDD的API。 The RDD-based API is expected to be removed in Spark 3.0. 预计将在Spark 3.0中删除基于RDD的API。

How do Scala Futures operate on threads? And how can they be used to execute async & non-blocking code?

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问题 To my understanding, there are the 3 ways of doing IO in Scala, which I will try to express in pesudo code. First , synchronous & blocking: val syncAndBlocking: HttpResponse = someHttpClient.get("foo.com") Here the main thread is just idle until the response is back.. Second , async but still blocking: val asyncButBlocking: Future[HttpResponse] = Future { someHttpClient.get("bar.com") } To my understanding, here the main thread is free (as Future executes on a separate thread) but that

Use of abstract type in a concrete class? [duplicate]

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问题 This question already has answers here : Concrete classes with abstract type members (2 answers) Closed 7 years ago . scala> class A { type T <: String; def f(a: T) = println("foo")} defined class A scala> (new A).f("bar") <console>:9: error: type mismatch; found : java.lang.String("bar") required: _1.T where val _1: A (new A).f("bar") ^ Class A has an abstract type T , but is not an abstract class. Creating an object of A (as shown) does not define type T . My first thought was, I am allowed

How to automatically increment version number from my sbt and uploaded to git

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问题 How do I can increment project version number from my build.sbt file so that when you compile it automatically uploads to git? 回答1: The sbt-release plugin will do all of this for you. If you issue the command sbt release from the command line, this plugin will remove the -SNAPSHOT suffix, tag, commit and push the changes to your repository, build, test and release the artifact, then update the version version number (adding the -SNAPSHOT suffix back again), committing the changes once more.