Tailable Cursors

By default, MongoDB will automatically close a cursor when the client has exhausted all results in the cursor. However, for capped collections you may use a tailable cursor that remains open after the client exhausts the results in the initial cursor.

The following is a basic example of using a tailable cursor to tail the oplog of a replica set member:

import time

import pymongo

client = pymongo.MongoClient()
oplog = client.local.oplog.rs
first = oplog.find().sort('$natural', pymongo.ASCENDING).limit(-1).next()
print(first)
ts = first['ts']

while True:
    # For a regular capped collection CursorType.TAILABLE_AWAIT is the
    # only option required to create a tailable cursor. When querying the
    # oplog the oplog_replay option enables an optimization to quickly
    # find the 'ts' value we're looking for. The oplog_replay option
    # can only be used when querying the oplog.
    cursor = oplog.find({'ts': {'$gt': ts}},
                        cursor_type=pymongo.CursorType.TAILABLE_AWAIT,
                        oplog_replay=True)
    while cursor.alive:
        for doc in cursor:
            ts = doc['ts']
            print(doc)
        # We end up here if the find() returned no documents or if the
        # tailable cursor timed out (no new documents were added to the
        # collection for more than 1 second).
        time.sleep(1)