Database Reference
In-Depth Information
discussion. Remember, you are building a menu of services to offer the business and
your job as a technician is to provide the right solution for the business requirement.
We will focus this discussion SQL Server AlwaysOn Failover Cluster Instances and
SQL Server AlwaysOn Availability Groups. These are two unique offerings available
for SQL Server deployments. But, before we get to that, as promised earlier we will
begin our discussion with an overview of ACID, what it is and why it is important, and
what this means for a virtualized SQL Server versus a physical SQL Server.
ACID
an acronym that represents a method by which database transactions are processed
reliably, and defined a transaction as a single logical transaction on the data.
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ACID stands for Atomicity, Consistency, Isolation, and Durability. Atomicity can be
equated to all or nothing. This means that if any part of a transaction fails, then the entire
transaction fails. For SQL Server, this means that if part of a transaction fails, then the
database itself is left unchanged; sometimes this involves rolling back changes made to
the database returns to the original state.
Consistency refers to the state in which the system is in before and after a transaction
begins. If for any reason a transaction would cause the system to enter an invalid state
upon its completion, the transaction is stopped, any changes made to the system are
rolled back, and the system returns to a consistent state once again. The system will start
and end a transaction in a consistent state.
Isolation allows a transaction to believe it is has sole access to the system, which may
not be the case. SQL Server is designed to be accessed by multiple individuals, and it is
imperative these individuals and their transactions believe they have exclusive use of
the system. Transactions may occur at the same time. If these transaction do not believe
they have dedicated use of the system, the system may not be deemed consistent, thus
causing a roll back. It is the isolation property that protects against the consistency
violation.
Durability is the last of the four transaction properties. Durability states that once a
transaction is committed, it is permanent. In other words, no matter what happens to the
system after a successful transaction, the transaction will persist. This includes
hardware failures, software failures, and so on. For SQL Server, this is accomplished
by writing information into a transaction log file prior to releasing the transaction.
Writing the transaction to physical media meets the durability requirement for a
transaction.
