Basic Database Terms
A session is a single connection to SQL Server, identified by a unique SessionID value. It is initiated through an application when the open method is used on a connection object or through a tool like SSMS when the File | Connect menu item is selected. Even though multiple sessions may originate from the same application (and many query windows opened by the same user using the same SSMS instance), as far as SQL Server is concerned, these are all completely separate SQL Server sessions.
Locking occurs when a SQL Server session takes "ownership" of a resource by acquiring a lock, prior to performing a particular action on that resource, such as reading or updating. Locking will stay in effect until SQL Server releases the locks. Note that locking itself is not a problem; it has very little measurable impact on any aspect of our systems, including performance, except when it results in blocking or deadlocking, or when we are performing excessive monitoring of our system locks.
Blocking occurs when at least two sessions desire concurrent access to the same resource. One session acquires a lock on the resource, in order to perform some action, and so renders that resource temporarily unavailable to other sessions. As a result, other sessions requiring the same resource are temporarily blocked. Typically, the blocked sessions will gain control of the resource after the blocking session releases the locks, so that access to the resource is serialized. Note that not all concurrent access will cause blocking; it is dependent on the operations being performed by the sessions, which determines the type of locks that are acquired.
A deadlock occurs when two sessions mutually block each other. Neither one can release the resources it holds until it acquires a lock on the resource the other session holds. A deadlock can also involve more than two sessions, trapped in a circular chain of dependencies. For example, session A may hold a resource that session B wants, and in turn session A is waiting for session C to release a resource. Session B may also hold a resource that session C wants. So session A is blocking B and is blocked by C, session B is blocking C and is blocked by A, and session C is blocking A and is blocked by B. None of the three sessions can proceed.
• Pressure is a term used to indicate a state where competition for access to a certain resource is causing performance issues. In a database with well-designed tables and queries, SQL Server acquires and releases locks quickly and any blocking is fleeting, and undetectable by the end-user. However, in certain circumstances, such as when long-running transactions hold locks on a resource for a long time, or where a very high number of sessions all require access to the same shared resource, blocking issues can escalate to the point where one session that is blocked, in turn blocks other sessions, which in turn block others. As the "queue" of blocked sessions grows longer, so the load on the system increases and more and more users start to experience unacceptable delays. In such cases, then we say that the resource is experiencing pressure.
Transactions The simplest definition of a transaction is that it is a single unit of work; a task or set of tasks that together form an "all-or-nothing" operation. If some event interrupts a transaction in the middle, so that not all of it was completed, the system should treat the transaction as if it never occurred at all. Transactions can apply to other kinds of systems besides databases, but since this is a database-specific book, we'll be concerned only with database transactions. A transaction can be short, like changing the price of one book in the inventory, or long, like updating the quantity sold of every inventory item at the beginning of an accounting period.