MySQL 5 Stored Procedures: Relic or Revolution?
It is generally a good practice to separate your data access code from your business logic and presentation logic. Data access routines often are used by multiple program modules and are likely to be maintained by a separate group of developers. A very common scenario requires changes to the underlying data structures while minimizing the impact on higher-level logic. Data abstraction makes this much easier to accomplish.
The use of stored programs provides a convenient way of implementing a data access layer. By creating a set of stored programs that implement all of the data access routines required by the application, we are effectively building an API for the application to use for all database interactions.
Stored programs can improve application performance radically by reducing network traffic in certain situations.
It's commonplace for an application to accept input from an end user, read some data in the database, decide what statement to execute next, retrieve a result, make a decision, execute some SQL and so on. If the application code is written entirely outside the database, each of these steps would require a network round trip between the database and the application. The time taken to perform these network trips easily can dominate overall user response time.
Consider a typical interaction between a bank customer and an ATM machine. The user requests a transfer of funds between two accounts. The application must retrieve the balance of each account from the database, check withdrawal limits and possibly other policy information, issue the relevant UPDATE statements, and finally issue a commit, all before advising the customer that the transaction has succeeded. Even for this relatively simple interaction, at least six separate database queries must be issued, each with its own network round trip between the application server and the database. Figure 1 shows the sequences of interactions that would be required without a stored program.
On the other hand, if a stored program is used to implement the fund transfer logic, only a single database interaction is required. The stored program takes responsibility for checking balances, withdrawal limits and so on. Figure 2 shows the reduction in network round trips that occurs as a result.
Network round trips also can become significant when an application is required to perform some kind of aggregate processing on very large record sets in the database. For instance, if the application needs to retrieve millions of rows in order to calculate some sort of business metric that cannot be computed easily using native SQL, such as average time to complete an order, a very large number of round trips can result. In such a case, the network delay again may become the dominant factor in application response time. Performing the calculations in a stored program will reduce network overhead, which might reduce overall response time, but you need to be sure to take into account the differences in raw computation speed, which I discuss later in this article.
Although it is commonplace for a MySQL database to be at the service of a single application, it is not at all uncommon for multiple applications to share a single database. These applications might run on different machines and be written in different languages; it may be hard, or impossible, for these applications to share code. Implementing common code in stored programs may allow these applications to share critical common routines.
For instance, in a banking application, transfer of funds transactions might originate from multiple sources, including a bank teller's console, an Internet browser, an ATM or a phone banking application. Each of these applications could conceivably have its own database access code written in largely incompatible languages, and without stored programs we might have to replicate the transaction logic, including logging, deadlock handling and optimistic locking strategies, in multiple places and in multiple languages. In this scenario, consolidating the logic in a database stored procedure can make a lot of sense.
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