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:

  1. import time
  3. import pymongo
  5. client = pymongo.MongoClient()
  6. oplog = client.local.oplog.rs
  7. first = oplog.find().sort('$natural', pymongo.ASCENDING).limit(-1).next()
  8. print(first)
  9. ts = first['ts']
  11. while True:
  12.     # For a regular capped collection CursorType.TAILABLE_AWAIT is the
  13.     # only option required to create a tailable cursor. When querying the
  14.     # oplog, the oplog_replay option enables an optimization to quickly
  15.     # find the 'ts' value we're looking for. The oplog_replay option
  16.     # can only be used when querying the oplog. Starting in MongoDB 4.4
  17.     # this option is ignored by the server as queries against the oplog
  18.     # are optimized automatically by the MongoDB query engine.
  19.     cursor = oplog.find({'ts': {'$gt': ts}},
  20.                         cursor_type=pymongo.CursorType.TAILABLE_AWAIT,
  21.                         oplog_replay=True)
  22.     while cursor.alive:
  23.         for doc in cursor:
  24.             ts = doc['ts']
  25.             print(doc)
  26.         # We end up here if the find() returned no documents or if the
  27.         # tailable cursor timed out (no new documents were added to the
  28.         # collection for more than 1 second).
  29.         time.sleep(1)

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