Atomic 2

source/reference/method/db.collection.findAndModify.txt

@@ -111,7 +111,7 @@ db.collection.findAndModify()
    #. The ``query`` finds a document in the ``people`` collection where
       the ``name`` field has the value ``Tom``, the ``state`` field has
       the value ``active`` and the ``rating`` field has a value
-      : operator:`greater than <$gt>` 10.
+      :operator:`greater than <$gt>` 10.
 
    #. The ``sort`` orders the results of the query in ascending order.
 

source/reference/method/db.collection.remove.txt

@@ -76,7 +76,7 @@ db.collection.remove()
       write operations to that collection. For an unsharded collection,
       you have the option to override this behavior with the
       :operator:`$atomic` isolation operator, effectively isolating the
-      delete operation and blocking all other operations during the
+      delete operation and blocking other write operations during the
       delete. To isolate the query, include ``$atomic: 1`` in the
       ``query`` parameter as in the following example:
 

source/reference/method/db.collection.update.txt

@@ -106,6 +106,15 @@ db.collection.update()
           In version 2.2 of the :program:`mongo` shell, you may also
           specify ``multi`` in the ``options`` parameter.
 
+          .. note::
+
+             The ``multi`` update operation may interleave with other
+             write operations to that collection. For an unsharded
+             collection, you have the option to override this behavior
+             with the :operator:`$atomic` isolation operator,
+             effectively isolating the update operation and blocking
+             other write operations during the update. See the
+             :doc:`isolation operator </reference/operator/atomic>`.
 
    Although the update operation may apply mostly to updating the
    values of the fields, the :method:`update()

source/reference/operator/atomic.txt

@@ -6,14 +6,19 @@ $atomic
 
 .. operator:: $atomic
 
-   In multi-update mode, it's possible to specify an
-   :operator:`$atomic` "operator" that allows you to **isolate** some
-   updates from each other within this operation. Consider the
-   following example:
+   :operator:`$atomic` isolation operator **isolates** a write
+   operation that affect multiple documents from other write operations.
+
+   .. note::
+
+      The :operator:`$atomic` isolation operator does **not** mean
+      "all-or-nothing" atomicity to the write operation.
+
+   Consider the following example:
 
    .. code-block:: javascript
 
-      db.foo.update( { field1 : 1 , $atomic : 1 }, { $inc : { field2 : 1 } } ,  false , true )
+      db.foo.update( { field1 : 1 , $atomic : 1 }, { $inc : { field2 : 1 } } , { multi: true } )
 
    Without the :operator:`$atomic` operator, multi-updates will allow
    other operations to interleave with this updates. If these

source/tutorial.txt

@@ -37,6 +37,7 @@ Development Patterns
    :maxdepth: 1
 
    tutorial/perform-two-phase-commits
+   tutorial/isolate-sequence-of-operations
    tutorial/enforce-unique-keys-for-sharded-collections
    tutorial/aggregation-examples
 

source/tutorial/isolate-sequence-of-operations.txt

@@ -0,0 +1,108 @@
+==============================
+Isolate Sequence of Operations
+==============================
+
+.. default-domain:: mongodb
+
+Background
+----------
+
+Write operations are atomic on the level of a single document: no
+single write operation can atomically affect more than one document or
+more than one collection.
+
+When a single write operation modifies multiple documents, the
+operation as a whole is not atomic, and other operations may
+interleave. The modification of a single document, or record, is always
+atomic, even if the write operation modifies multiple sub-document
+*within* the single record.
+
+No other operations are atomic; however, you can *isolate* a
+single write operation that affects multiple documents using the
+:doc:`isolation operator </reference/operator/atomic>`.
+
+Additionally, the following patterns can manage a sequence of
+operations:
+
+- :method:`findAndModify() <db.collection.findAndModify()>`
+
+- :ref:`tutorial-atomic-update-if-current`
+
+- :doc:`/tutorial/perform-two-phase-commits`
+
+- :method:`ensureIndex() <db.collection.ensureIndex()>` to create a
+  ``unique`` index on a field
+
+.. _tutorial-atomic-update-if-current:
+
+Update if Current
+-----------------
+
+The "Update if Current" pattern queries a document, locally modifies
+various fields of the document, and tries to update the fields of a
+document *if* the fields have not changed in the collection since the
+query.
+
+Consider the following example which attempts to update the ``qty``
+field of a document in the ``products`` collection:
+
+.. code-block:: javascript
+
+   var myCollection = db.products;
+   var myDocument = myCollection.findOne( { sku: 'abc123' } );
+
+   if (myDocument) {
+
+      var oldQty = myDocument.qty;
+
+      if (myDocument.qty < 10) {
+          myDocument.qty *= 4;
+      } else if ( myDocument.qty < 20 ) {
+          myDocument.qty *= 3;
+      } else {
+          myDocument.qty *= 2;
+      }
+
+      myCollection.update( 
+         { 
+           _id: myDocument._id,
+           qty: oldQty
+         },
+         { 
+           $set: { qty: myDocument.qty } 
+         }
+      )
+
+      var err = db.getLastErrorObj();
+
+      if ( err && err.code ) {
+          print("unexpected error updating document: " + tojson( err ));
+      } else if ( err.n == 0 ) {
+          print("No update: no matching document for { _id: " + myDocument._id + ", qty: " + oldQty + " }")
+      }
+
+   }
+
+Consider the following modifications to the "Update if Current" strategy:
+
+- To generalize the strategy to guarantee that the whole document has
+  not changed rather than just certain fields, use the entire document in
+  the query expression.
+
+- Add a version variable that is incremented upon each update operation
+  to the documents. Use this version variable in the query expression.
+
+- Use :operator:`$set` in the update expression to modify only your
+  fields and prevent overriding other fields.
+
+.. Add link to :doc:`/tutorial/create-an-auto-increment-field` once that branch is merged since it's a special case
+
+.. Maybe incorporate the blurb: "MongoDB does not
+   support traditional locking and complex transactions for a number of
+   reasons: First, in sharded environments, distributed locks could be
+   expensive and slow. Mongo DB's goal is to be lightweight and fast. We
+   dislike the concept of deadlocks. We want the system to be simple and
+   predictable without these sort of surprises. We want Mongo DB to work
+   well for realtime problems. If an operation may execute which locks
+   large amounts of data, it might stop some small light queries for an
+   extended period of time."