CRV2 RevCodePersistentAntiPatterndotNet

=.NET Anti-Pattern: Mishandled Concurrency =

The correct concurrency management techniques is absolutely necessary in order to guarantee data integrity. A way to implement proper concurrency consists in creating a concurrency token which will be checked from the moment the entity object in the database was read until the moment when the submission will be executed. Prior to commit the final changes, the application must execute control where the concurrency token will be compared. If the token differs, conclusions can be drawn that indeed the data has been changed by another user.

The Entity Framework supports optimistic concurrency, unfortunately exceptions derived from errors encountered between the updates are not automatically handled, neither this will protect your data from corrupting.

Fetching Strategy issues
Main purpose of the ORM (Object Relational Mapper) is to create an abstraction, a representation of the database easier to manipulate and access data contained in it than retrieving it using SQL statements. Unfortunately, many developers assume that because the ORM is the middle layer handling the communication and manipulation of records, the data persistence will be handled without issues. The developer should analyze and observe what occurs at the database level once the ORM in question (such as EntityFramework, NHibernate or Ideablade for example) has been implemented. Concurrency issues can arise from incorrect fetching strategy implementations such as for example using ‘CachedOnly’ or ‘DataSourceThenCached’(IdeaBlade). It is highly recommended that the developer revises the proper application depending on the implementation scenario, such as multiple users accessing the same data at the same time in order to avoid data corruption or when data is cached in an action by a certain Entity but the another one is not aware of this change.

Avoid the DTO pattern
Known to many NHibernate developers, the use of DTO pattern seems as something that sounds good but could be highly inconvenient. Main reasons for not implementing are: Contrary to their name, DTO's are no objects but a state which is defined through their behavior. By replicating multiple objects, a developer might end up having to implement multiple changes in the domain model and the DTO classes when for example, introducing a new property in the domain model class.

Avoid Locks
Another anti-pattern approach used by many developers is to lock regions in the database.Most of the time, this is implemented as way to avoid concurrency, however web applications are not properly suited for using locking which will indeed freeze the application. Locking data for the time the request takes will not solve this problem. Using locks in database are absolutely not recommend since they required careful implementation planning and design.(Freeman, pg 179 ,2011)

Race conditions
If the following is run on more than one thread, it will randomly crash. It is not possible to know deterministically whether the code will throw an ArgumentOutOfRangeException. Sometimes it will, sometimes it won’t.(Mclean, 2010)

IList list = new List ; list.Add("Hello"); … // multi-threaded code if(list.Count > 0) {    list.RemoveAt(0); }

In that case a locking can be used, however using locks as mentioned earlier should be consider as an option if it is absolutely necessary.

Example locking code object lockObj = new object; IList list = new List ; list.Add("Hello"); …  // multi-threaded code lock(lockObj) {    if(list.Count > 0) {        list.RemoveAt(0); }   }

Recommendations
completed".(Freeman,pg 179,2011)
 * The best option in this case is to alert the user who initiated the second request that his changes cannot be applied. "This is largely because, by definition, the first request will already have


 * A recommended pattern when using the Entity Framework consists in "making a copy of the entity on the client and send back both the original version unmodified and the modified version or to write the client in such a way that it does not modify the concurrency token".(Simmons, 2009)


 * Keep in mind that Detached objects (such as in the case of NHibernate or IdeaBlade ORM's)may no longer be guaranteed to be synchronized with database state; they’re no longer under the management of NHibernate.However they could still contain persistent data. These ORM's allow you to reuse these instance by associating them with a new Persistence manager.


 * Avoid the use DTO pattern by properly handling detached objects