Spark Python Avro Kafka 反序列化器

Question

I have created a kafka stream in a python spark app and can parse any text that comes through it.

            kafkaStream = KafkaUtils.createStream(ssc, zkQuorum, "spark-streaming-consumer", {topic: 1})

I want to change this to be able to parse avro messages from a kafka topic. When parsing avro messages from a file, I do it like:

            reader = DataFileReader(open("customer.avro", "r"), DatumReader())  

I'm new to python and spark, how do I change the stream to be able to parse the avro message? Also how can I specify a schema to use when reading the Avro message from Kafka??? I've done all this in java before but python is confusing me.

Edit:

I tried changing to include the avro decoder

            kafkaStream = KafkaUtils.createStream(ssc, zkQuorum, "spark-streaming-consumer", {topic: 1},valueDecoder=avro.io.DatumReader(schema))

but I get the following error

            TypeError: 'DatumReader' object is not callable

Answer 1

I had the same challenge - deserializing avro messages from Kafka in pyspark and solved it with the Confluent Schema Registry module's Messageserializer method, as in our case the schema is stored in a Confluent Schema Registry.

You can find that module at https://github.com/verisign/python-confluent-schemaregistry

from confluent.schemaregistry.client import CachedSchemaRegistryClient
from confluent.schemaregistry.serializers import MessageSerializer
schema_registry_client = CachedSchemaRegistryClient(url='http://xx.xxx.xxx:8081')
serializer = MessageSerializer(schema_registry_client)


# simple decode to replace Kafka-streaming's built-in decode decoding UTF8 ()
def decoder(s):
    decoded_message = serializer.decode_message(s)
    return decoded_message

kvs = KafkaUtils.createDirectStream(ssc, ["mytopic"], {"metadata.broker.list": "xxxxx:9092,yyyyy:9092"}, valueDecoder=decoder)

lines = kvs.map(lambda x: x[1])
lines.pprint()

Obviously as you can see this code is using the new, direct approach with no receivers, hence the createdDirectStream (see more at https://spark.apache.org/docs/1.5.1/streaming-kafka-integration.html )

Answer 2

As mentioned by @Zoltan Fedor in the comment, the provided answer is a bit old now, as 2.5 years had passed since it was written. The confluent-kafka-python library has evolved to support the same functionality nativly. The only chnage needed in the given code is following.

from confluent_kafka.avro.cached_schema_registry_client import CachedSchemaRegistryClient
from confluent_kafka.avro.serializer.message_serializer import MessageSerializer

And then, you can change this line -

kvs = KafkaUtils.createDirectStream(ssc, ["mytopic"], {"metadata.broker.list": "xxxxx:9092,yyyyy:9092"}, valueDecoder=serializer.decode_message)

I had tested it and it works nicely. I am adding this answer for anyone who may need it in future.

Answer 3

If you don't consider to use Confluent Schema Registry and have a schema in a text file or dict object, you can use fastavro python package to decode Avro messages of your Kafka stream:

from pyspark.streaming.kafka import KafkaUtils
from pyspark.streaming import StreamingContext
import io
import fastavro

def decoder(msg):
    # here should be your schema
    schema = {
      "namespace": "...",
      "type": "...",
      "name": "...",
      "fields": [
        {
          "name": "...",
          "type": "..."
        },
      ...}
    bytes_io = io.BytesIO(msg)
    bytes_io.seek(0)
    msg_decoded = fastavro.schemaless_reader(bytes_io, schema)
    return msg_decoded

session = SparkSession.builder \
                      .appName("Kafka Spark Streaming Avro example") \
                      .getOrCreate()

streaming_context = StreamingContext(sparkContext=session.sparkContext,
                                     batchDuration=5)

kafka_stream = KafkaUtils.createDirectStream(ssc=streaming_context,
                                             topics=['your_topic_1', 'your_topic_2'],
                                             kafkaParams={"metadata.broker.list": "your_kafka_broker_1,your_kafka_broker_2"},
                                             valueDecoder=decoder)