import asyncio
import nats.aio.errors
from nats.aio.client import Client as NATS

async def example():

   # [begin slow_listener]

   nc = NATS()

   async def error_cb(e):
     if type(e) is nats.aio.errors.ErrSlowConsumer:
       print("Slow consumer error, unsubscribing from handling further messages...")
       await nc.unsubscribe(e.sid)

   await nc.connect(
      servers=["nats://demo.nats.io:4222"],
      error_cb=error_cb,
      )

   msgs = []
   future = asyncio.Future()
   async def cb(msg):
       nonlocal msgs
       nonlocal future
       print(msg)
       msgs.append(msg)

       if len(msgs) == 3:
         # Head of line blocking on other messages caused
	 # by single message proccesing taking long...
         await asyncio.sleep(1)

   await nc.subscribe("updates", cb=cb, pending_msgs_limit=5)

   for i in range(0, 10):
     await nc.publish("updates", "msg #{}".format(i).encode())
     await asyncio.sleep(0)

   try:
     await asyncio.wait_for(future, 1)
   except asyncio.TimeoutError:
     pass

   for msg in msgs:
     print("[Received]", msg)

   await nc.close()

   # [end slow_listener]

loop = asyncio.get_event_loop()
loop.run_until_complete(example())
loop.close()