Spark Python Avro Kafka Deserialiser

问题:

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

回答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)

回答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.