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POUsing pymongo tailable cursors dies on empty collections
 

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  1. POUsing pymongo tailable cursors dies on empty collections
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    <p>Hoping someone can help me understand if I'm seeing an issue or if I just don't understand mongodb tailable cursor behavior. I'm running mongodb 2.0.4 and pymongo 2.1.1.</p> <p>Here is an script that demonstrates the problem.</p> <pre><code>#!/usr/bin/python import sys import time import pymongo MONGO_SERVER = "127.0.0.1" MONGO_DATABASE = "mdatabase" MONGO_COLLECTION = "mcollection" mongodb = pymongo.Connection(MONGO_SERVER, 27017) database = mongodb[MONGO_DATABASE] if MONGO_COLLECTION in database.collection_names(): database[MONGO_COLLECTION].drop() print "creating capped collection" database.create_collection( MONGO_COLLECTION, size=100000, max=100, capped=True ) collection = database[MONGO_COLLECTION] # Run this script with any parameter to add one record # to the empty collection and see the code below # loop correctly # if len(sys.argv[1:]): collection.insert( { "key" : "value", } ) # Get a tailable cursor for our looping fun cursor = collection.find( {}, await_data=True, tailable=True ) # This will catch ctrl-c and the error thrown if # the collection is deleted while this script is # running. try: # The cursor should remain alive, but if there # is nothing in the collection, it dies after the # first loop. Adding a single record will # keep the cursor alive forever as I expected. while cursor.alive: print "Top of the loop" try: message = cursor.next() print message except StopIteration: print "MongoDB, why you no block on read?!" time.sleep(1) except pymongo.errors.OperationFailure: print "Delete the collection while running to see this." except KeyboardInterrupt: print "trl-C Ya!" sys.exit(0) print "and we're out" # End </code></pre> <p>So if you look at the code, it is pretty simple to demonstrate the issue I'm having. When I run the code against an empty collection (properly capped and ready for tailing), the cursor dies and my code exits after one loop. Adding a first record in the collection makes it behave the way I'd expect a tailing cursor to behave.</p> <p>Also, what is the deal with the StopIteration exception killing the cursor.next() waiting on data? Why can't the backend just block until data becomes available? I assumed the await_data would actually do something, but it only seems to keep the connection waiting a second or two longer than without it.</p> <p>Most of the examples on the net show putting a second While True loop around the cursor.alive loop, but then when the script tails an empty collection, the loop just spins and spins wasting CPU time for nothing. I really don't want to put in a single fake record just to avoid this issue on application startup.</p>
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    1. COinfinitely blocking on something is never a good idea
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    2. COTell that to gobject.MainLoop().run() ;)
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      1. USDaGoodBoy
 

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