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('$natural', pymongo.ASCENDING).limit(-1).next()
print(first)
ts = first['ts']
while True:
# For a regular capped collection CursorType.TAILABLE_AWAIT is the
# only option required to create a tailable cursor. When querying the
# oplog, the oplog_replay option enables an optimization to quickly
# find the 'ts' value we're looking for. The oplog_replay option
# can only be used when querying the oplog. Starting in MongoDB 4.4
# this option is ignored by the server as queries against the oplog
# are optimized automatically by the MongoDB query engine.
cursor = oplog.find({'ts': {'$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)