我正在尝试通过Kafka发送一个非常简单的JSON对象，并使用Python和kafka-python从另一侧读取它。然而，我不断看到以下错误：
2017-04-07 10:28:52,030.30.9998989105:kafka.future:8228:ERROR:10620:Error processing callback
Traceback (most recent call last):
File "C:\Anaconda2\lib\site-packages\kafka\future.py", line 79, in _call_backs
f(value)
File "C:\Anaconda2\lib\site-packages\kafka\consumer\fetcher.py", line 760, in _handle_fetch_response
unpacked = list(self._unpack_message_set(tp, messages))
File "C:\Anaconda2\lib\site-packages\kafka\consumer\fetcher.py", line 539, in _unpack_message_set
tp.topic, msg.value)
File "C:\Anaconda2\lib\site-packages\kafka\consumer\fetcher.py", line 570, in _deserialize
return f(bytes_)
File "C:\Users\myUser\workspace\PythonKafkaTest\src\example.py", line 55, in <lambda>
value_deserializer=lambda m: json.loads(m).decode('utf-8'))
File "C:\Anaconda2\lib\json\__init__.py", line 339, in loads
return _default_decoder.decode(s)
File "C:\Anaconda2\lib\json\decoder.py", line 364, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "C:\Anaconda2\lib\json\decoder.py", line 382, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
我已经做了一些研究，发现导致这个错误的最常见原因是JSON格式有误。我尝试通过在我的代码中添加以下内容来打印发送前的JSON，JSON会无误地打印出来。
while True:
json_obj1 = json.dumps({"dataObjectID": "test1"})
print json_obj1
producer.send('my-topic', {"dataObjectID": "test1"})
producer.send('my-topic', {"dataObjectID": "test2"})
time.sleep(1)
这让我怀疑我能够生成JSON，但无法消费它。
以下是我的代码：
import threading
import logging
import time
import json
from kafka import KafkaConsumer, KafkaProducer
class Producer(threading.Thread):
daemon = True
def run(self):
producer = KafkaProducer(bootstrap_servers='localhost:9092',
value_serializer=lambda v: json.dumps(v).encode('utf-8'))
while True:
producer.send('my-topic', {"dataObjectID": "test1"})
producer.send('my-topic', {"dataObjectID": "test2"})
time.sleep(1)
class Consumer(threading.Thread):
daemon = True
def run(self):
consumer = KafkaConsumer(bootstrap_servers='localhost:9092',
auto_offset_reset='earliest',
value_deserializer=lambda m: json.loads(m).decode('utf-8'))
consumer.subscribe(['my-topic'])
for message in consumer:
print (message)
def main():
threads = [
Producer(),
Consumer()
]
for t in threads:
t.start()
time.sleep(10)
if __name__ == "__main__":
logging.basicConfig(
format='%(asctime)s.%(msecs)s:%(name)s:%(thread)d:' +
'%(levelname)s:%(process)d:%(message)s',
level=logging.INFO
)
main()
如果我去掉value_serializer和value_deserializer，我就可以成功地发送和接收字符串。当我运行这段代码时，我可以看到我发送的JSON。以下是一个简短的片段:
ConsumerRecord(topic=u'my-topic', partition=0, offset=5742, timestamp=None, timestamp_type=None, key=None, value='{"dataObjectID": "test1"}', checksum=-1301891455, serialized_key_size=-1, serialized_value_size=25)
ConsumerRecord(topic=u'my-topic', partition=0, offset=5743, timestamp=None, timestamp_type=None, key=None, value='{"dataObjectID": "test2"}', checksum=-1340077864, serialized_key_size=-1, serialized_value_size=25)
ConsumerRecord(topic=u'my-topic', partition=0, offset=5744, timestamp=None, timestamp_type=None, key=None, value='test', checksum=1495943047, serialized_key_size=-1, serialized_value_size=4)
ConsumerRecord(topic=u'my-topic', partition=0, offset=5745, timestamp=None, timestamp_type=None, key=None, value='\xc2Hello, stranger!', checksum=-1090450220, serialized_key_size=-1, serialized_value_size=17)
ConsumerRecord(topic=u'my-topic', partition=0, offset=5746, timestamp=None, timestamp_type=None, key=None, value='test', checksum=1495943047, serialized_key_size=-1, serialized_value_size=4)
ConsumerRecord(topic=u'my-topic', partition=0, offset=5747, timestamp=None, timestamp_type=None, key=None, value='\xc2Hello, stranger!', checksum=-1090450220, serialized_key_size=-1, serialized_value_size=17)
因此，我尝试从消费者中删除value_deserializer，代码可以执行，但是没有反序列化器，消息以String形式输出，这不是我需要的。所以，为什么value_deserializer不起作用？是否有其他方法可以获取Kafka消息中的JSON？
value_deserializer=lambda x: x.decode('utf-8')，然后稍后再执行message_dict = json.loads(message.value)，在那里您可以根据条件进行处理或包装它以进行异常处理。 - MarkHu