Nov 252011
 

Counting semaphores restricts number of concurrent sessions from accessing the same resource. It’s useful when code blocks perform well when number of concurrent users is under a certain number. For instance, SQL Server merge replication implements semaphores to limit number of subscribers to access publication at the same time.

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Nov 162011
 

Semaphore is an abstract for controlling access by multiple threads (sessions) to a common resource. Binary semaphore, it can be called Mutex, or Lock/Unlock, allows only one thread access a resource at a time. Back old days, in C++ programming, critical section object is used for this purpose. In C#, keyword lock(obj){code block} and Monitor class implements binary semaphores. Once a object is locked by one thread, other threads who need to acquire the lock on the object will have to wait until the lock on the object is released by thread previously held. SQL Server is an multi-threading application. It uses semaphores internally to control the access to the record, for instance, while you are modify a record, an exclusive lock is placed on the record and released when modification finishes. This is a binary semaphore. In T-SQL, you can implement binary semaphore by locking row(s) and application locks.

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