scala - inferSchema=true 不适用于读取 csv 文件 n Spark Structured Streaming

Question

我收到错误消息

java.lang.IllegalArgumentException: Schema must be specified when creating a streaming source DataFrame. If some files already exist in the directory, then depending on the file format you may be able to create a static DataFrame on that directory with 'spark.read.load(directory)' and infer schema from it.

    at org.apache.spark.sql.execution.datasources.DataSource.sourceSchema(DataSource.scala:251)
    at org.apache.spark.sql.execution.datasources.DataSource.sourceInfo$lzycompute(DataSource.scala:115)
    at org.apache.spark.sql.execution.datasources.DataSource.sourceInfo(DataSource.scala:115)
    at org.apache.spark.sql.execution.streaming.StreamingRelation$.apply(StreamingRelation.scala:35)
    at org.apache.spark.sql.streaming.DataStreamReader.load(DataStreamReader.scala:232)
    at org.apache.spark.sql.streaming.DataStreamReader.load(DataStreamReader.scala:242)
    at org.apache.spark.sql.streaming.DataStreamReader.csv(DataStreamReader.scala:404)
    at io.sekai.core.streaming.KafkaDataGenerator.readFromCSVFile(KafkaDataGenerator.scala:38)

当我加载 csv 文件时

spark2
  .readStream
  .format("csv")
  .option("inferSchema", "true")
  .option("header", "true")
  //.schema(schema)
  .option("delimiter", ",")
  .option("maxFilesPerTrigger", 1)
  .csv(path)

我尝试了另一种格式的选项，例如

spark2
  .readStream
  .format("csv")
  .option("inferSchema", value = true)
  .option("header", value = true)
  //.schema(schema)
  .option("delimiter", ",")
  .option("maxFilesPerTrigger", 1)
  .csv(path)

我想推断架构并注释掉显式架构用法。
csv 文件示例如下:

id,Energy Data,Distance,Humidity,Ambient Temperature,Cold Water Temperature,Vibration Value 1,Vibration Value 2,Handle Movement
1,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
2,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
3,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
4,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
5,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
6,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
7,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
8,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
9,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2
10,0.00 246.47 0.00,4in, 12cm,55.50%,25°C,25°C,0,0,6.08 7.53 0.31m/s^2

这里有什么问题，因为我严格按照选项说明进行操作，但是推断是如何发生的？

Answer 1

您在这里有 2 个选择:

在运行流查询之前，将数据样本写入目的地一次。当您再次运行流式查询时，将推断架构。

套装spark.sql.streaming.schemaInference至 true :

spark.sql("set spark.sql.streaming.schemaInference=true")

来自 docs :

By default, Structured Streaming from file based sources requires you to specify the schema, rather than rely on Spark to infer it automatically. This restriction ensures a consistent schema will be used for the streaming query, even in the case of failures. For ad-hoc use cases, you can reenable schema inference by setting spark.sql.streaming.schemaInference to true.