Add warnings to README, docstring, and logging that async producer does not retry failed messages

Author: Dana Powers

README.md

@@ -51,6 +51,8 @@ producer.send_messages("my-topic", "this method", "is variadic")
 producer.send_messages("my-topic", u'你怎么样?'.encode('utf-8'))
 
 # To send messages asynchronously
+# WARNING: current implementation does not guarantee message delivery on failure!
+# messages can get dropped! Use at your own risk! Or help us improve with a PR!
 producer = SimpleProducer(kafka, async=True)
 producer.send_messages("my-topic", "async message")
 
@@ -63,7 +65,7 @@ producer = SimpleProducer(kafka, async=False,
                           req_acks=SimpleProducer.ACK_AFTER_LOCAL_WRITE,
                           ack_timeout=2000)
 
-response = producer.send_messages("my-topic", "async message")
+response = producer.send_messages("my-topic", "another message")
 
 if response:
     print(response[0].error)

kafka/producer.py

@@ -87,6 +87,9 @@ class Producer(object):
     client - The Kafka client instance to use
     async - If set to true, the messages are sent asynchronously via another
             thread (process). We will not wait for a response to these
+            WARNING!!! current implementation of async producer does not
+            guarantee message delivery.  Use at your own risk! Or help us
+            improve with a PR!
     req_acks - A value indicating the acknowledgements that the server must
                receive before responding to the request
     ack_timeout - Value (in milliseconds) indicating a timeout for waiting
@@ -131,6 +134,9 @@ def __init__(self, client, async=False,
         self.codec = codec
 
         if self.async:
+            log.warning("async producer does not guarantee message delivery!")
+            log.warning("Current implementation does not retry Failed messages")
+            log.warning("Use at your own risk! (or help improve with a PR!)")
             self.queue = Queue()  # Messages are sent through this queue
             self.proc = Process(target=_send_upstream,
                                 args=(self.queue,