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For the sake of our discussion a relational database is a persistent storage mechanism that enables you to both store data and optionally implement functionality. The goal of this article is to overview relational database management system (RDBMS) technology and to explore the practical issues applicable to its use in modern organizations; the goal is not to discuss relational theory. RDBMSs are used to store the essay writing images Conestoga College, information required by applications built using procedural technologies such as COBOL or FORTRAN, object technologies such as Java and C#, and component-based technologies such as Visual Basic. Because RDBMSs are the dominant persistent storage technology it is critical that all IT professionals understand at least the basics of RDBMSs, the challenges surrounding the essay writing basics university of massachusetts lowell umass lowell (navitas), technology, and when it is appropriate to use them. Tables typically have keys, one or more columns that uniquely identify a row within the table, in the case of the Customer table the key would be CustomerNumber . Essay writing basics University of Massachusetts Lowell UMass Lowell (Navitas)? To improve access time to a data table you define an index on the table. An index provides a quick way to look up data based on paul elder critical thinking university of toronto, new college, one or more columns in the table, just like the index of a book enables you to find specific information quickly. The easiest way to manipulate a relational database is to submit Structured Query Language (SQL) statements to essay writing images Conestoga College it. Figure 1 depicts a simple data model, see Data Modeling 101 or better yet Agile Data Modeling, using the proposed UML data modeling notation. To create a row in the Seminar table you would issue an essay writing about my school robert gordon university (navitas), INSERT statement, an example of which is shown in Code Figure 1. Similarly, Code Figure 2 presents and example of how to read a row by issuing a SELECT statement. Code Figure 3 shows how to update an existing row via an UPDATE statement and Code Figure 4 how to delete a row with a DELETE statement. All four of these examples were taken, as well as the data model, were adapted from The Object Primer. A very good resource for learning SQL is SQL Queries for Mere Mortals by Micheal J. Hernandez and John L. Viescas. Code Figure 1. Paul elder critical thinking University of Toronto, New College? SQL statement to insert a row into the "Seminar" table. (SEMINAR_ID, COURSE_ID, OVERSEER_ID, SEMINAR_NUMBER) VALUES (74656, 1234, ‘THX0001138’, 2) Code Figure 2. SQL statement to critical thinkings university of central florida retrieve a row from the "Seminar" table. WHERE SEMINAR_ID = 1701. Code Figure 3. SQL statement to update a row in a table in the "Seminar" table. SET OVERSEER_ID = ‘NCC0001701’, SEMINAR_NUMBER = 3. WHERE SEMINAR_ID = 1701. Code Figure 4. SQL statement to delete data from the "Seminar" table. WHERE SEMINAR_ID > 1701. AND OVERSEER_ID = ‘THX0001138’ Object storage . To store an object in a relational database you need to flatten it – create a data representation of the essay writing images Conestoga College, object – because relational databases only store data. To retrieve the object you would read the data from the database and essay writing images Conestoga College then create the critical thinkings university of central florida, object, often referred to as restoring the object, based on that data. Although storing objects in essay writing images Conestoga College, a relational database sounds like a simple thing to achieve, practice shows that it isn’t. This is due to the object-relational impedance mismatch, the fact that relational database technology and essay writing images Conestoga College object technology are based on different underlying theories, a topic discussed in essay writing images Conestoga College, The Object-Relational (O/R) Impedance Mismatch. To store objects successfully in essay writing images Conestoga College, relational databases you need to learn how to essay writing images Conestoga College map your object schema to your relational database schema. Implementing behavior within the database . Behavior is implemented in a relational database via stored procedures and/or stored functions that can be invoked internally within the database and often by external applications. Stored functions and essay writing basics university of massachusetts lowell umass lowell (navitas) procedures are operations that run within an RDBMS, the difference being what the operation can return and whether it can be invoked in a query. The differences aren’t important for essay writing images Conestoga College our purposes so the term “stored procedure” will be used to refer to both stored functions and stored procedures. In the past stored procedures were written in a proprietary language, such as Oracle’s PL/SQL, although now Java is quickly becoming the language of choice for harvard bus review dulwich college database programming. A stored procedure typically runs some SQL code, massages the data, and then hands back a response in the form of zero or more records, or a response code, or as a database error message. Effective use of stored procedures is discussed in essay writing images Conestoga College, detail in Implementation Strategies for Persisting Objects in RDBs. Concurrency control . Consider an airline reservation system. There is essay writing images Conestoga College, a flight with one seat left on it, and two people are trying to reserve that seat at essay writing images Conestoga College, the same time. Paul elder critical thinking University of Toronto, New College? Both people check the flight status and critical thinkings university of central florida are told that a seat is still available. Both enter their payment information and click the reservation button at the same time. What should happen? If the essay writing images Conestoga College, system is working properly only essay writing images Conestoga College, one person should be given a seat and critical thinkings university of central florida the other should be told that there is no longer one available. Concurrency control is what makes this happen. Concurrency control must be implemented throughout your object source code and within your database. Transaction control . A transaction is a collection of actions on your database – such as the saving of, retrieval of, or deletion of data – which form a work unit. A flat transactions is an “all-or-nothing” approach where all the actions must either succeed or be rolled back (canceled). A nested transaction is an approach where some of the actions are transactions in their own right. These sub-transactions are committed once successful and are not rolled back if the essay writing about my school robert gordon university (navitas), larger transaction fails. Transactions may also be short-lived, running in critical thinkings university of central florida, thousandths of a second, or long-lived, taking hours, days, weeks, or even months to complete. Transaction control is a critical concept for essay writing images Conestoga College all developers to writing analytical essay braemar college understand. Enforcing referential integrity . Referential integrity (RI) is the assurance that a reference from one entity to another entity is valid. essay writing images Conestoga College? For example, if a customer references an address, then that address must exist. If the address is deleted then all references to it must also be removed, otherwise your system must not allow the deletion. Contrary to popular belief, RI isn’t just a database issue, it’s an issue for your entire system. A customer is implemented as an essay writing basics university of massachusetts lowell umass lowell (navitas), object within a Java application and as one or more records in your database – addresses are also implemented as objects and as rows. To delete an address, you must remove the address object from memory, any direct or indirect references to it (an indirect reference to an address would include a customer object knowing the critical thinkings university of central florida, value of the AddressID , the essay writing basics university of massachusetts lowell umass lowell (navitas), primary key of the essay writing images Conestoga College, address in the critical thinkings university of central florida, database), the address row(s) from your database, and any references to it (via foreign keys) in your database. To complicate matters, if you have a farm of application servers that address object could exist simultaneously on how to write an explanatory essay st. georges school, several machines. Furthermore, if you have other applications accessing your database then it is possible that they too have representations of the address in their memory as well. Worse yet, if the address is stored in paul elder critical thinking university of toronto, new college, several places (e.g. different databases) you should also consider taking this into account. All developers should understand basic strategies for implementing referential integrity. essay writing images Conestoga College? Table 1 describes the common technical features found in RDBMS products, potential ways that developers will use them, and the potential drawbacks associated with their use. Database cursors – A database cursor is essay writing images Conestoga College, effectively a handle to the results of essay writing images Conestoga College a SQL query, enabling you to move forward and backward through the result set one or more records at a time. Accessing large results sets in smaller portions enables your application to display initial results earlier, increasing response time. Harvard bus review Dulwich College? Performance is improved when a portion of a result set is required because less data is how to write an explanatory essay st. georges school, transmitted across the network. Application developers need to writing analytical essay braemar college understand that the underlying data can change between the times that data records are accessed via the cursor: previously retrieved records may have been deleted, records may have been inserted into previously retrieved portions of the result set, or previously retrieved records may have been modified. Not all cursors are created equal. essay writing images Conestoga College? Some cursors only critical thinkings university of central florida, allow forward scrolling. Cursors are a resource drain on the database because they are memory intensive. Java – Most database vendors support a Java VM within the database. Development of relatively platform independent behavior in the database. Development of data intensive behavior that results in a relatively small return value. Encapsulation of database access to support security access control to information. Implementation of shared behavior required by many applications. Different version of VMs between application server and database server increases complexity of paul elder critical thinking university of toronto, new college development. Behavior implemented in the critical thinkings university of central florida, database can easily become a bottleneck. Triggers – A trigger is a procedure that is run either before or after an essay writing images Conestoga College, action (such as a create, update, or delete) is how to write an explanatory essay st. georges school, performed on a row in a database table. Enforce referential integrity within your database. These types of triggers can often be automatically generated by your data modeling or database administration tool. Often a lowest common denominator for implementing referential integrity constraints. Perform hand-crafted audit logging. Hand-crafted, or hand-modified, triggers can be difficult to maintain and will increase your dependency on your database vendor. Triggers are typically implemented in a proprietary language requiring an extra skillset on your team. Because triggers are automatically invoked they can be very dangerous (such as “uncontrolled” cascading deletions resulting from chained delete triggers). Behavior implemented in the database can easily become a bottleneck if your database doesn’t scale well. Your application source code . When you change your database schema you must also change the source code within your application that accesses the changed portion of the schema. Figure 2 depicts the best-case scenario – when it is only your application code that is coupled to your database schema. This situation is essay writing images Conestoga College, traditionally referred to as a stovepipe. These situations do exist and are often referred to essay writing images Conestoga College as stand-alone applications, stovepipe systems, or greenfield projects. Count yourself lucky if this is your situation because it is very rare in essay writing images Conestoga College, practice. Other application source code . Figure 3 depicts the worst-case scenario for essay writing images Conestoga College relational databases – a wide variety of software systems are coupled to your database schema, a situation that is quite common with existing production databases. It is quite common to find that in addition to the application that your team is currently working on that other applications, some of which you know about and some of which you don’t, are also coupled to your database. Perhaps an online system reads and writes to your database. Perhaps a manager has written a spreadsheet, unbeknownst to you, that reads data from your database that she uses to summarize information critical to her job. Data load source code . Data loads from other sources, such as government provided tax tables or your own test data, are often coupled to your database schema. Data extract source code . How to write an explanatory essay St. Georges School? There may be data extraction scripts or programs that read data from your database, perhaps to produce an XML data file or simply so your data can be loaded into another database. Persistence frameworks/layers . A persistence framework encapsulates the logic for mapping application classes to persistent storage sources such as your database. When you refactor your database schema you will need to update the meta data, or the source code as the case may be, which describes the essay writing images Conestoga College, mappings. Itself . essay writing images Conestoga College? Coupling exists within your database. A single column is coupled to essay writing basics university of massachusetts lowell umass lowell (navitas) any stored procedure that references it, other tables that use the essay writing images Conestoga College, column as a foreign key, any view that references the column, and so on. A simple change could result in several changes throughout your database. Data migration scripts . Changes to critical thinkings university of central florida your database schema will require changes to your data migration scripts. Test code . Testing code includes any source code that puts your database into a known state, that performs transactions that affect your database, and essay writing images Conestoga College that reads the results from the database to compare it against expected results. Essay writing about my school Robert Gordon University (NAVITAS)? Clearly this code may need to essay writing images Conestoga College be updated to critical thinkings university of central florida reflect any database schema changes that you make. essay writing images Conestoga College? Documentation . Some of the essay writing images Conestoga College, most important documentation that you are likely to how to write an explanatory essay st. georges school keep pertains to your physical database schema, including but not limited to writing analytical essay braemar college physical data models and essay writing images Conestoga College descriptive meta data. When your database schema changes the documentation describing will also need to change accordingly. Although Agile Modeling implores you to Update Only When It Hurts, that your documentation doesn’t have to be perfectly in synch with your schema at all times, the reality is essay writing images Conestoga College, that you will need to update your docs at essay writing images Conestoga College, some point. As you can see, coupling is a serious issue when it comes to relational databases. essay writing images Conestoga College? To make matters worse the concept of coupling is writing analytical essay braemar college, virtually ignored within database theory circles. essay writing images Conestoga College? Although most database theory books will cover data normalization in excruciating detail, I argue that normalization is essay writing images Conestoga College, the data community’s way of addressing cohesion. My experience is that coupling becomes a serious issue only when you start to consider behavioral issues (e.g. code), something that traditional database theory chooses not to critical thinkings university of central florida address. This is another reason to follow Agile Modeling (AM)’s Multiple Models principle and look beyond data (Ambler 2002a). Performance issues are difficult to predict . Writing analytical essay Braemar College? When you are working with a shared database, such as the situation implied in Figure 3, you may find that the essay writing images Conestoga College, performance characteristics of your database are hard to essay writing images Conestoga College predict because each application accesses the essay writing basics university of massachusetts lowell umass lowell (navitas), database in essay writing images Conestoga College, its own unique way. For example, perhaps one legacy application updates information pertaining to items for sale sporadically throughout the month, enabling a human operator to how to write an explanatory essay st. georges school add new items or update existing ones, activity that doesn’t really affect your application’s performance in a meaningful way. However, this same application also performs batch loads of essay writing images Conestoga College items available for sale via other companies that you have partnered with, items that you want to carry on how to write an explanatory essay st. georges school, your web site as soon as they are available. These batch loads can take several minutes, during which period the how to write an explanatory essay st. georges school, Item table is under heavy load and thus your online application is potentially affected. Data integrity is difficult to ensure with shared databases . Because no single application has control over the harvard bus review dulwich college, data it is essay writing images Conestoga College, very difficult to paul elder critical thinking university of toronto, new college be sure that all applications are operating under the same business principles. For example, your application may consider an essay writing images Conestoga College, order as fulfilled once it has been shipped and a payment has been received. The original legacy application that is still in use by your customer support representatives to take orders over the phone may consider an essay writing images Conestoga College, order as fulfilled once it has been shipped, the how to write an explanatory essay st. georges school, payment received, and the payment deposited into writing analytical essay braemar college, your bank account. A slight difference in the way that a fundamental business rule has been implemented may have serious business implications for any application that accesses the shared databases. Less subtly, imagine what would happen if your online order taking application calculates the total for an order and stores it in the order table, whereas the legacy application calculates the subtotals only for order items but does not total the order. When the order fulfillment application sees an order with no total it calculates the total, and appropriate taxes, whereas if a total already exists it uses the existing figure. If a customer makes an order online and then calls back a few hours later and has one of your customer service representatives modify their existing order, perhaps to add several items to it, the essay writing images Conestoga College, order total is paul elder critical thinking university of toronto, new college, no longer current because it has not been updated properly. Referential integrity issues such as this are covered in detail in the Implementing Referential Integrity article. Essay writing about my school Robert Gordon University (NAVITAS)? Operational databases require different design strategies than reporting databases . The schemas of operational databases reflect the operational needs of the applications that access them, often resulting in a reasonably normalized schema with some portions of it denormalized for performance reasons. Reporting databases, on the other hand, are typically highly denormalized with significant data redundancy within them to support a wide range of reporting needs. Every technology has its strengths and weaknesses, and RDBMS technology is not an exception to essay writing images Conestoga College this rule. Harvard bus review Dulwich College? Luckily there are ways that you can mitigate some of these challenges, and encapsulation is an important technique to do so. By encapsulating access to a database, perhaps through something as simple as data access objects or something as complex as a persistence framework, you can reduce the coupling that your database is involved with. The Implementation Strategies for Persisting Objects in RDBs chapter compares and contrasts various encapsulation strategies that you have available to you. For now assume that it is essay writing basics university of massachusetts lowell umass lowell (navitas), possible to essay writing images Conestoga College hide the details of your database schema from harvard bus review dulwich college, the majority of the developers within your organization while at the same time giving them access to your database. Writing analytical essay Braemar College? Some people, often just the Agile DBA(s) responsible for paul elder critical thinking university of toronto, new college supporting the database, will need to understand and work with the underlying schema to maintain and evolve the encapsulation strategy. One advantage of encapsulating access to your database is essay writing about my school robert gordon university (navitas), that it enables application programmers to focus on the business problem itself. Let’s assume we’re doing something simple such as data access objects that implement the SQL code to access your database schema. The application programmers will work with these data access classes, not the database. This enables your Agile DBA to evolve the database schema as they need to, perhaps via database refactorings, and all they need to worry about is keeping the how to write an explanatory essay st. georges school, data access classes up to date. This reveals a second advantage to this approach – it provides greater freedom to Agile DBAs to do their job. Figure 4 depicts the critical thinkings university of central florida, concept of encapsulating access to critical thinkings university of central florida your database, showing how the best case scenario of Figure 2 and the worst case scenario of essay writing about my school robert gordon university (navitas) Figure 3 would likely change. In the best-case scenario your business source code would interact with the essay writing images Conestoga College, data access objects that in turn would interact with the essay writing images Conestoga College, database. The primary advantage would be that all of the data-related code would be in one place, making it easier to modify whenever database schema changes occurred or to support performance-related changes. It’s interesting to note that the business code that your application programmers are writing would still be coupled to the data access objects. Therefore they’d need to change their code whenever the interface of a data access object changed. essay writing images Conestoga College? You’ll never get away from essay writing about my school robert gordon university (navitas), coupling. However, from the point of view of the application programmer this is a much easier change to detect and act on – with the database encapsulation strategy in essay writing about my school robert gordon university (navitas), place the application programmers are only dealing with program source code (e.g. Java) and not program source code plus SQL code. Things aren’t quite so ideal for the worst-case scenario. Critical thinkings University of Central Florida? Although it is possible that all applications could take advantage of your encapsulation strategy the reality is that only a subset will be able to. How to write an explanatory essay St. Georges School? Platform incompatibility can be an issue – perhaps your data access objects are written in Java but some legacy applications are written using technologies that can’t easily access Java. Perhaps you’ve chosen not to rework some of your legacy applications simply to use the database encapsulation strategy. Perhaps some applications already have an encapsulation strategy in how to write an explanatory essay st. georges school, place (if so, you might want to consider reusing the existing strategy instead of building your own). Perhaps you want to use technologies, such as a bulk load facility, that require direct access to the database schema. The point to be made is that a portion of your organization’s application will be able to take advantage of your encapsulation strategy and some won’t. There is essay writing images Conestoga College, still a benefit to doing this, you are reducing coupling and therefore reducing your development costs and maintenance burden, the essay writing images Conestoga College, problem is that it isn’t a fully realized benefit. Another advantage of encapsulating access to a database is that it gives you a common place, in addition to the database itself, to implement data-oriented business rules. Object/relational databases . essay writing images Conestoga College? Object/relational databases (ORDBs), or more properly object/relational database management systems (ORDBMSs), add new object storage capabilities to RDBMSs. ORDBs, add new facilities to integrate management of traditional fielded data, complex objects such as time-series and geo-spatial data and essay writing images Conestoga College diverse binary media such as audio, video, images, and essay writing basics university of massachusetts lowell umass lowell (navitas) (sometimes) Java applets. ORDBMSs basically add to paul elder critical thinking university of toronto, new college RDBMSs features such as defined data types, for example you could define a data type called SurfaceAddress that has all of the attributes and behaviors of an address, as well as the ability to navigate objects in addition to an RDBMS’s ability to join tables. By implementing objects within the database, an ORDBMS can execute complex analytical and data manipulation operations to search and transform multimedia and other complex objects. essay writing images Conestoga College? ORDBs support the robust transaction and data-management features of RDBMSs while at essay writing images Conestoga College, the same time offer a limited form of the flexibility of object-oriented databases. Because of ORDBMSs relational foundation, database administrators work with familiar tabular structures and data definition languages (DDLs) and programmers access them via familiar approaches such as SQL3, JDBC, and proprietary call interfaces. Object databases . Object databases (ODBs), also known as object-oriented databases (OODBs) or object-oriented database management systems (OODBMSs), nearly seamlessly add database/persistence functionality to essay writing images Conestoga College object programming languages. In other words, full-fledged objects are implemented in the essay writing images Conestoga College, database. Essay writing about my school Robert Gordon University (NAVITAS)? They bring much more than persistent storage of programming language objects. How to write an explanatory essay St. Georges School? ODBs extend the semantics of Java to provide full-featured database programming capability, via new class libraries specific to the ODB vendor, while retaining native language compatibility. A major benefit of this approach is the unification of the application and database development into paul elder critical thinking university of toronto, new college, a seamless model. As a result, applications require less code, use more natural persistence modeling, and essay writing basics university of massachusetts lowell umass lowell (navitas) code bases are easier to maintain. Object-oriented developers can write complete database applications with a modest amount of additional effort without the need to marshal their objects into flatten data structures for storage, as a result forgoing the how to write an explanatory essay st. georges school, marshalling overhead inherent with other persistence mechanism technologies (such as RDBs). essay writing images Conestoga College? This one-to-one mapping of application objects to essay writing images Conestoga College database objects provides higher performance management of objects and enables better management of the complex interrelationships between objects. Native XML databases . Native XML databases store information as XML documents following one of two approaches: First, a native XML database will either store a modified form of the critical thinkings university of central florida, entire XML document in the file system, perhaps in a compressed or pre-parsed binary form. Second, a native XML database may opt to map the structure of the document to the database, for example mapping the Document Object Model (DOM) to internal structures such as Elements, Attributes, and Text – exactly what is mapped depends on the database. The most important difference between these approaches, from essay writing images Conestoga College, the point of view of an application developer, is the way they are accessed: with the first approach the only interface to the data is paul elder critical thinking university of toronto, new college, XML and related technologies, such as XPath (a language design specifically for addressing parts of an XML document, visit www.w3.org for details) or the DOM whereas with the second approach the database should be accessible via standard technologies such as JDBC. How to write an explanatory essay St. Georges School? The important thing to understand about native XML databases is that they work with the internal structures of the XML documents – they don’t just store them as a binary large object (BLOB) in the database. Flat files . Flat files, such as .TXT or. CSV (comma separated value) files, are commonly used to store data. A single file can be used to how to write an explanatory essay st. georges school store one type of data structure, such as customer information or sales transaction information, or through a coding and formatting strategy the essay writing images Conestoga College, structures of critical thinkings university of central florida several types of data structures. One approach to flat file organization is either to have data values separated by harvard bus review dulwich college a pre-defined character, such as a comma, or tag, such as </FirstName> in an XML document. Another common approach is essay writing images Conestoga College, to delimit data values by size – the first 20 characters of the harvard bus review dulwich college, row represent the first name of the critical thinkings university of central florida, customer, the next 20 characters represent the critical thinkings university of central florida, surname, and essay writing images Conestoga College so on. Hierarchical databases . essay writing images Conestoga College? Hierarchical databases link data structures together like a family tree such that each record type has only one owner, for example an order is essay writing images Conestoga College, owned by only one customer. Hierarchical structures were widely used in the first mainframe database management systems, and are still a very common source of data in essay writing images Conestoga College, many large organizations. Hierarchical databases fell out of favor with the advent of essay writing images Conestoga College relational databases due to paul elder critical thinking university of toronto, new college their lack of flexibility because it wouldn’t easily support data access outside the original design of the data structure. For example, in the customer-order schema you could only access an order through a customer, you couldn’t easily find all the orders that included the sale of a widget because the schema isn’t designed to all that. Prevalence layer . Klaus Wuestefeld defines prevalence as “transparent persistence, fault-tolerance and load-balancing of the execution of the business logic of an information system through the use of state snapshots as well as command and query queuing or logging”. A prevalence layer is effectively a simple persistence framework that serializes objects and essay writing images Conestoga College writes them to harvard bus review dulwich college log files. From the point of view of essay writing about my school robert gordon university (navitas) developers all objects are cached in memory and the persistence of the essay writing images Conestoga College, objects is truly treated as a background task that the developers don’t need to worry about. Table 2 presents a comparison of the various types of persistence mechanism and provides references to vendors where applicable. Paul elder critical thinking University of Toronto, New College? Table 3 presents suggestions for when you might use each type of harvard bus review dulwich college technology. Large organizations will find that they are using several types of persistence mechanism and will even install the products of essay writing images Conestoga College several different vendors. Not only do you have a choice you might be forced to critical thinkings university of central florida work with a wide range of databases whether you want to or not.