Database Reference
In-Depth Information
A standard behavior when executing atomic operations is that the data will be
locked
and therefore unable to be reached by other queries. However, MongoDB does not
support locking or complex transactions for a number of reasons:
•
In sharded environments (see Chapter 12 for more information
on such environments), distributed locks can be expensive and
slow. MongoDB's goal is to be lightweight and fast, so expensive
and slow goes against the principle.
•
MongoDB developers don't like the idea of deadlocks. In their
view, it's preferable for a system to be simple and predictable
instead.
•
MongoDB is designed to work well for real-time problems. When
an operation is executed that locks large amounts of data, it would
also stop some smaller light queries for an extended period of
time. Again, this goes against the MongoDB goal of speed.
MongoDB includes several update operators (as noted previously), all of which can
atomically update an element:
$set
: Sets a particular value.
•
$unset
: Removes a particular value.
•
$inc
: Increments a particular value by a certain amount.
•
$push
: Appends a value to an array.
•
$pull
: Removes one or more values from an existing array.
•
$pullAll
: Removes several values from an existing array.
•
Using the Update if Current Method
Another strategy that atomic update uses is the
update-if-current
method. This method
takes the following three steps:
1.
It fetches the object from the document.
2.
It modifies the object locally (with any of the previously
mentioned operations, or a combination of them).
3.
It sends an update request to update the object to the new
value, in case the current value still matches the old
value fetched.
