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memory, and yet con icting changes are detected The latter ensures that either all changes are persisted successfully, or nothing is persisted in case of a failure The two mechanisms also illustrate how connections are managed by LINQ to SQL When a user issues a query, LINQ to SQL issues a command As soon as the results are consumed, the conand creates a nection goes back to the connection pool for use by other users of the connection (unless you explicitly change the connection pooling setting on the connection) This pattern repeats for all LINQ to SQL queries Meanwhile, the retrieved objects can be used for potentially long-running computations involving user input (including coffee breaks) or for complex processing is called, LINQ to When all the changes are done and , SQL again opens a connection to the database and uses it to execute , and SQL commands in a transaction LINQ to SQL then returns the connection to the connection pool This minimizes connection usage while providing exibility of operating in disconnected mode and yet making changes to the retrieved objects Thus, precious database resources such as connections are managed carefully to provide a more scalable solution
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In two-tier applications, a single instance handles queries and updates to results of queries However, for applications that have addiinstances for tional tiers, it is often necessary to use separate query and update For example, in the case of an ASPNET application, query and update are executed as separate requests to the web server where most of the code is supposed to operate in a stateless fashion The requests may even go to different machines in a server farm Hence, it is not instance across multiple requests In possible to use the same instance needs to be able to update objects that such cases, a method allows the to deal it has not retrieved The call tells the with entities coming from another tier An to start tracking the entity as if it were retrieved with a query but without actually requiring a database query Just like the
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method, this capability is exposed by for each given entity type The following example shows a typical usage:
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In the absence of , during processing, an entity that was not retrieved in a query is considered to be a new entity for tells the that the entity is insertion into the database not new but merely serialized from another tier needs to preserve the optimistic concurrency capability by ensuring that current and original values are available As a result, three different overloads deal with the current and original values: Original values used for con ict detection: should be used to attach the original values The instance can then be modi ed by playing back the changes serialized from another tier before calling Original and current copies available: does it in one shot This requires two instances with original and current values, respectively Timestamp or no optimistic concurrency members: lets you attach the current value without requiring the original values No playback is needed before
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is called The current version of is attached with updated members but the original timestamp The original timestamp takes care of the optimistic concurrency check If the mapping is set up to skip the optimistic concurrency check, the current version is all that is called is needed for an update when In many multitier applications, the entire entity often is not sent across tiers for simplicity, interoperability, or privacy For example, a supplier may de ne a data contract for a web service that differs from the Order entity used on the middle tier Likewise, a web page may show only a subset of the members of an Employee entity Hence, the multitier support also accommodates such cases Only the members belonging to one or more of the following categories need to be transported between tiers and set before : calling Members that are part of the entity s identity Members that have been changed Members that participate in the optimistic concurrency check LINQ to SQL uses minimal updates It checks which columns are changed from the original to the current version and creates a SQL command to set just the corresponding columns So when you create an entity instance on the mid-tier for attaching, anything that is not changed and not required for the optimistic concurrency check can be skipped The remaining members can have default values in both the original and curuses a timestamp and only the rent versions For example, if an has been changed, you need to set only three properties when up an Order entity , , and the timestamp you property When attaching objects in original state, the entity graph is recursively explored for each attached entity until a known entity is reached By default, no further action is taken for attached entities For example, if you in original state, all related entities are also attach an s to effect an attached You are free to make changes to update, or you can even take action to have them inserted or deleted
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