Lifetime of static variable

From MSDN http://msdn.microsoft.com/en-us/library/aa645766%28VS.71%29.aspx

A field declared with the static modifier is called a static variable. A static variable comes into existence before execution of the static constructor (Section 10.11) for its containing type, and ceases to exist when the associated application domain ceases to exist.

The initial value of a static variable is the default value (Section 5.2) of the variable's type.

For purposes of definite assignment checking, a static variable is considered initially assigned.

InstanceContextMode and ConcurrencyMode

There are three InstanceContextMode in WCF.

• PerCall: a new InstanceContext object is created and recycled succeeding each call.
• PerSession: A new InstanceContext object is created per session and instance is not sharable by multiple sessions.
• Single: A single instance is created and used for all incoming calls.
  

There are three ConcurrencyMode supported by WCF.

• Single: The service instance is single threaded and does not accept reentrance calls. If a new message comes when another message is being processed, this message has to wait there until the first one is finished.
• Multiple: The service is multi-threaded. It is fast. No waiting. But developer has to make sure the code is thread-safe.
• Reentrance: The service is single threaded and can accept reentrant calls. It implies that the service processes only one message at a given time. To ensure thread safety, WCF locks the InstanceContext processing a message so that no other messages can be processed. In case of Reentrant mode, the InstanceContext is unlocked just before the service makes an outgoing call thereby allowing the subsequent call to get the lock next time it comes in to the service.

There are many factors we should think about when choosing service behavior.

• Constructor of service. Probably InstanceConxt. Single should be used if there are lots of stuff to load and initialize. InstanceContext.PerSession should be used if there are states to maintain for one client. No thing to maintain for different calls, different clients, InstanceContext.PerCall should be used.
• ConcurrencyMode does not matter if InstanceContextMode.PerCall is adopted. New available thread will be used if new call comes.
  
• InstanceContext.PerCall and InstanceContext.PerSession mean same thing if there is no session maintained for this service
• ConcurrencyMode.Reentrant does not mean multiple thread. It is single thread in most cases except there is outgoing service call.
• Take care of thread-saftety if InstanceContextMode.Single and ConcurrencyMode.Multiple are considered to use. It will be faster and it will also be tricky.
  

We could put some logs to know when the object is initialized and which thread is used in the constructor. Just for confirmation.

OptimisticConcurrencyException-When it is triggered?

It could be triggered in two cases:
1)The entity property is defined in the conceptual layer with an attribute of ConcurrencyMode="fixed"(A property of Entity Framework, Not SQL Server).When this attribute is used, Object Services checks for changes in the database before saving changes to the database. Any conflicting changes will cause an OptimisticConcurrencyException.
By default, however, Object Services saves object changes to the database without checking for concurrency.

2)An OptimisticConcurrencyException can also occur when you define an Entity Data Model that uses stored procedures to make updates to the data source. In this case, the exception is raised when the stored procedure that is used to perform updates reports that zero rows were updated.