问题描述
我想从MQTT接收JSON字符串并将其解析为DataFrames df
.我该怎么办?
I want to receive JSON strings from MQTT and parse them to DataFrames df
. How can I do it?
这是我发送到MQTT队列以便在Spark中处理的Json消息的示例:
This is an example of Json message that I send to MQTT queue in order to process in Spark:
{
"id": 1,
"timestamp": 1532609003,
"distances": [2,5,7,8]
}
这是我的代码:
from pyspark.sql import SparkSession
spark = SparkSession \
.builder \
.appName("Test") \
.master("local[4]") \
.getOrCreate()
# Custom Structured Streaming receiver
reader = spark\
.readStream\
.format("org.apache.bahir.sql.streaming.mqtt.MQTTStreamSourceProvider")\
.option("topic","uwb/distances")\
.option('brokerUrl', 'tcp://127.0.0.1:1883')\
.load()\
.selectExpr("CAST(value AS STRING)", "CAST(timestamp AS STRING)")
df = spark.read.json(reader.select("value").rdd)
# Start running the query that prints the running counts to the console
query = df \
.writeStream \
.format('console') \
.start()
query.awaitTermination()
但是此代码失败:
py4j.protocol.Py4JJavaError: An error occurred while calling o45.javaToPython.
: org.apache.spark.sql.AnalysisException: Queries with streaming sources must be executed with writeStream.start();;
mqtt
我尝试如下添加 start
:
df = spark.read.json(reader.select("value").rdd) \
.writeStream \
.format('console') \
.start()
但是出现了同样的错误.我的目标是获得一个DataFrame df
,我可以将其进一步传递给ETL流程.
But got the same error. My goal is to get a DataFrame df
that I can further pass through ETL processes.
更新:
标记为答案的主题并没有帮助我解决问题.首先,当我使用PySpark时,它为Scala提供了解决方案.其次,我测试了答案中提出的解决方案,并返回了空列 json
:
The thread marked as an answer has not helped me solving the problem. First of all it gives the solution for Scala, while I am using PySpark.Secondly, I tested the solution proposed in the answer and it returned me the empty column json
:
reader = spark\
.readStream\
.schema(spark.read.json("mqtt_schema.json").schema) \
.format("org.apache.bahir.sql.streaming.mqtt.MQTTStreamSourceProvider")\
.option("topic","uwb/distances")\
.option('brokerUrl', 'tcp://127.0.0.1:1883')\
.load()\
.selectExpr("CAST(value AS STRING)", "CAST(timestamp AS STRING)")
json_schema = spark.read.json("mqtt_schema.json").schema
df = reader.withColumn('json', from_json(col('value'), json_schema))
query = df \
.writeStream \
.format('console') \
.start()
推荐答案
我想这是因为您的df没有流式传输. reader.select("value").writestream
I guess it is because your df is not streaming.How about try justreader.select("value").writestream
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