42 lines
1.5 KiB
ReStructuredText
42 lines
1.5 KiB
ReStructuredText
Tailable Cursors
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================
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By default, MongoDB will automatically close a cursor when the client has
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exhausted all results in the cursor. However, for `capped collections
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<https://docs.mongodb.org/manual/core/capped-collections/>`_ you may
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use a `tailable cursor
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<https://docs.mongodb.org/manual/reference/glossary/#term-tailable-cursor>`_
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that remains open after the client exhausts the results in the initial cursor.
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The following is a basic example of using a tailable cursor to tail the oplog
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of a replica set member::
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import time
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import pymongo
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client = pymongo.MongoClient()
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oplog = client.local.oplog.rs
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first = oplog.find().sort('$natural', pymongo.ASCENDING).limit(-1).next()
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print(first)
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ts = first['ts']
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while True:
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# The tailable and await_data options make this a tailable cursor.
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cursor = oplog.find({'ts': {'$gt': ts}},
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tailable=True,
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await_data=True)
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# Enable the OplogReplay cursor option. This enables an optimization
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# to quickly find the 'ts' value we're looking for. It can only be used
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# when querying the oplog.
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cursor.add_option(8)
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while cursor.alive:
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for doc in cursor:
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ts = doc['ts']
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print(doc)
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# We end up here if the find() returned no documents or if the
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# tailable cursor timed out (no new documents were added to the
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# collection for more than 1 second).
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time.sleep(1)
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