数据访问：Implementing Efficient Transactions

An OLTP scenario is characterized by a large number of concurrent operations that create, update, and delete data, packaged up as transactions. Most modern RDBMSs implement locking and logging strategies to ensure that the ACID (Atomicity, Consistency, Isolation, and Durability) properties of transactions are maintained. These features aim to guarantee the integrity of the data, but they necessarily have an impact on the performance of your transactions, and you should try and minimize their negative effects wherever possible. The following list provides some suggestions:

• Keep transactions short. A long-running transaction can lock data for an extended period of time, increasing the chances that it will block operations being performed by other concurrent transactions. Therefore, to maximize throughput, it is important to design the business logic carefully, and only perform the operations that are absolutely necessary within the bounds of a transaction.
• Avoid repeating the same work. Poorly designed transactions can lead to deadlock, resulting in operations being undone. Your applications have to detect this situation and may need to repeat the transaction, reducing the performance of the system still further. Design your transactions to minimize this possibility. For example, always access tables and other resources in the same sequence in all operations to avoid the “deadly embrace” form of deadlock.
• Avoid implementing database triggers over data that is updated frequently. Many RDBMSs support triggers that run automatically when specified data is inserted, updated, or deleted. These triggers run as part of the same transaction that fired them, and they add complexity to the transaction. The developer writing the application code to implement the transaction might not be aware that the triggers exist, and might attempt to duplicate their work, possibly resulting in deadlock.
• Do not include interactivity or other actions that might take an indeterminate period of time. If your transactions depend upon input from a user, or data retrieved from a remote source, then gather this data before initiating the transaction. Users may take a long time to provide data, and information received from a remote source may take a long time to arrive (especially if the remote data source is some distant site being accessed across the Internet), give rise to the same consequences as a long-running transaction.

• Implement transactions locally in the database. Many RDBMSs support the concept of stored procedures or other code that is controlled and run by the database management system. You can define the operations that implement a transaction by using a stored procedure, and then simply invoke this stored procedure from your application code. Most RDBMSs are more easily able to refactor and optimize the operations in a stored procedure than they are the individual statements for a transaction implemented by application code that runs outside of the database.
  This approach reduces the dependency that the application has on a particular database schema but it might introduce a dependency on the database technology. If you switch to a different type of database, you might need to completely reimplement this aspect of your system.