PYTHON-1063 - Add an example doc for tailable cursors

Author: behackett

doc/examples/index.rst

@@ -27,4 +27,5 @@ MongoDB, you can start it like so:
  gridfs
  high_availability
  mod_wsgi
+ tailable
  tls

doc/examples/tailable.rst

@@ -0,0 +1,41 @@
+Tailable Cursors
+================
+
+By default, MongoDB will automatically close a cursor when the client has
+exhausted all results in the cursor. However, for `capped collections
+<https://docs.mongodb.org/manual/core/capped-collections/>`_ you may
+use a `tailable cursor
+<https://docs.mongodb.org/manual/reference/glossary/#term-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)
+