#27345 removed drift option in favour or recommending setting TTL higher

Author: Joe Bennett

components/lock.rst

@@ -239,16 +239,13 @@ 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
-resource_field  ``_id``         The field name for storing the lock id **MUST be uniquely indexed if you chage it**
-token_field     ``token``       The field name for storing the lock token
-acquired_field  ``acquired_at`` The field name for storing the acquisition timestamp
-expiry_field    ``expires_at``  The field name for storing the expiry-timestamp
-=============== =============== ===================================================================================
+========== ========= =============================================================
+Option     Default   Description
+========== ========= =============================================================
+database             The name of the database [Mandatory]
+collection ``lock``  The name of the collection
+initialTtl ``300.0`` The expiration delay of locks in seconds
+========== ========= =============================================================
 
 .. _lock-store-redis:
 
@@ -469,23 +466,35 @@ method uses the Memcached's ``flush()`` method which purges and removes everythi
 MongoDbStore
 ~~~~~~~~~~~~
 
-The MongoDbStore relies on a unique index on the ``<resource_field>``. By default
-this is ``_id`` which already has a unique index being the collection identifier.
-It is strongly recommended to leave this as is. If you do need to change the
-``<resource_field>`` you **MUST** ensure you place a unique index on your filed.
+An indexed field's value in MongoDB can be a maximum of 1024 bytes in length
+inclusive of structural overhead.
+
+For more details see: https://docs.mongodb.com/manual/reference/limits/#Index-Key-Limit
+
+.. caution::
+
+    This store relies on all client and server nodes to have
+    synchronized clocks for lock expiry to occur at the correct time.
+    To ensure locks don't expire prematurely; the ttl's should be set with enough
+    extra time to account for any clock drift between nodes.
+
+A TTL index MUST BE used on MongoDB 2.2+ to automatically clean up expired locks.
 
-A TTL index can be used on MongoDB 2.2+ to cleanup expired locks automatically.
 Such an index can for example look like this:
 
 .. code-block:: javascript
 
-    db.<lock-collection>.ensureIndex(
-        { "<expiry-field>": 1 },
+    db.lock.ensureIndex(
+        { "expires_at": 1 },
         { "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
 ~~~~~~~~~~