Entity Framework and Connection Pooling

Question

I ve recently started to use the Entity Framework 4 0 in my  NET 4 0 application and am curious about a few things relating to pooling    Connection pooling as I know is managed by the ADO NET data provider  in my case that of MS SQL server  Does this apply when you instantiate a new entities context  ObjectContext   i e  the parameterless new MyDatabaseModelEntities    What are the advantages and disadvantages of a  creating a global entities context for the application  i e  one static instance  or b  creating and exposing an entities context for each given operation method  with a using block  Any other recommendations  best practices  or common approaches for certain scenarios that I should know about

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Below code helped my object to be refreshed with fresh database values  The Entry object  Reload   command forces the object to recall database values  GM MEMBERS member   DatabaseObjectContext GM MEMBERS FirstOrDefault p   gt  p Username    username  amp  amp  p ApplicationName    this ApplicationName   DatabaseObjectContext Entry member  Reload

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According to Daniel Simmons      Create a new ObjectContext instance in   a Using statement for each service   method so that it is disposed of   before the method returns    This step is critical for scalability of your service  It makes sure that database connections are not kept open across service calls and that temporary state used by a particular operation is garbage collected when that operation is over  The Entity Framework automatically caches metadata and other information it needs in the app domain  and ADO NET pools database connections  so re-creating the context each time is a quick operation    This is from his comprehensive article here    http   msdn microsoft com en-us magazine ee335715 aspx  I believe this advice extends to HTTP requests  so would be valid for ASP NET   A stateful  fat-client application such as a WPF application might be the only case for a  shared  context

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Connection pooling is handled as in any other ADO NET application  Entity connection still uses traditional database connection with traditional connection string  I believe you can turn off connnection pooling in connection string if you don t want to use it   read more about SQL Server Connection Pooling  ADO NET   Never ever use global context  ObjectContext internally implements several patterns including Identity Map and Unit of Work  Impact of using global context is different per application type   For web applications use single context per request  For web services use single context per call  In WinForms or WPF application use single context per form or per presenter  There can be some special requirements which will not allow to use this approach but in most situation this is enough    If you want to know what impact has single object context for WPF   WinForm application check this article  It is about NHibernate Session but the idea is same   Edit   When you use EF it by default loads each entity only once per context  The first query creates entity instace and stores it internally  Any subsequent query which requires entity with the same key returns this stored instance  If values in the data store changed you still receive the entity with values from the initial query  This is called Identity map pattern  You can force the object context to reload the entity but it will reload a single shared instance   Any changes made to the entity are not persisted until you call SaveChanges on the context  You can do changes in multiple entities and store them at once  This is called Unit of Work pattern  You can t selectively say which modified attached entity you want to save   Combine these two patterns and you will see some interesting effects  You have only one instance of entity for the whole application  Any changes to the entity affect the whole application even if changes are not yet persisted  commited   In the most times this is not what you want  Suppose that you have an edit form in WPF application  You are working with the entity and you decice to cancel complex editation  changing values  adding related entities  removing other related entities  etc    But the entity is already modified in shared context  What will you do  Hint  I don t know about any CancelChanges or UndoChanges on ObjectContext    I think we don t have to discuss server scenario  Simply sharing single entity among multiple HTTP requests or Web service calls makes your application useless  Any request can just trigger SaveChanges and save partial data from another request because you are sharing single unit of work among all of them  This will also have another problem - context and any manipulation with entities in the context or a database connection used by the context is not thread safe   Even for a readonly application a global context is not a good choice because you probably want fresh data each time you query the application

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Accoriding to EF6  4 5 also  documentation  https   msdn microsoft com en-us data hh949853 9  9 3      Context per request  Entity Framework   s contexts are meant to be used as short-lived instances in order to provide the most optimal performance experience  Contexts are expected to be short lived and discarded  and as such have been implemented to be very lightweight and reutilize metadata whenever possible  In web scenarios it   s important to keep this in mind and not have a context for more than the duration of a single request  Similarly  in non-web scenarios  context should be discarded based on your understanding of the different levels of caching in the Entity Framework  Generally speaking  one should avoid having a context instance throughout the life of the application  as well as contexts per thread and static contexts

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