#27345 moved clock discrepancy handling to a constructor option 'drift'

Author: Joe Bennett

components/lock.rst

@@ -238,12 +238,13 @@ This store does not support blocking, and expects a TTL to avoid stalled locks::
 
 The ``MongoDbStore`` takes the following ``$options``:
 
-=========== ========= ====================================
-Option      Default   Description
-=========== ========= ====================================
-database              The name of the database [Mandatory]
-collection  ``lock``  The name of the collection
-=========== ========= ====================================
+========== ======== =============================================================
+Option     Default  Description
+========== ======== =============================================================
+database            The name of the database [Mandatory]
+collection ``lock`` The name of the collection
+drift      ``0.0``  Seconds to extend expiries to account for clock discrepancies
+========== ======== =============================================================
 
 .. _lock-store-redis:
 
@@ -469,24 +470,27 @@ inclusive of structural overhead.
 
 For more details see: https://docs.mongodb.com/manual/reference/limits/#Index-Key-Limit
 
+Please be aware, MongoDbStore relies on clock syncronisation between
+the application nodes and mongodb servers. The ``drift`` option can be used
+to extend expiries to account for clock discrepancies.
+
 A TTL index MUST BE used on MongoDB 2.2+ to automatically clean up expired locks.
-Please be aware any clock drift between the application and mongo servers could
-cause locks to be released prematurely. To account for any drift;
-expireAfterSeconds can be set to a value higher than 0. The logical expiry of
-locks is handled by the application so setting a higher ``expireAfterSeconds``
-has no effect other than keeping stale data for longer.
 
 Such an index can for example look like this:
 
 .. code-block:: javascript
 
     db.lock.ensureIndex(
         { "expires_at": 1 },
-        { "expireAfterSeconds": 60 }
+        { "expireAfterSeconds": 0 }
     )
 
 For more details see: http://docs.mongodb.org/manual/tutorial/expire-data/
 
+writeConcern, readConcern and readPreference are not specified by MongoDbStore
+meaning the collection's settings will take effect.
+For more details see: https://docs.mongodb.com/manual/applications/replication/
+
 RedisStore
 ~~~~~~~~~~