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For the sake of our discussion a relational database is persuasive essay strategies university of massachusetts lowell umass lowell (navitas), a persistent storage mechanism that enables you to both store data and optionally implement functionality. The goal of top rated essay writing service moreton hall school this article is to overview relational database management system (RDBMS) technology and to explore the practical issues applicable to its use in how to write better essays Downside School, modern organizations; the goal is not to discuss relational theory. how to write better essays Downside School! RDBMSs are used to critical thinking in business ardingly college, store the 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. how to write better essays Downside School! 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 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 how to write better essays Downside School key would be CustomerNumber . To improve access time to a data table you define an critical thinking in business ardingly college, index on the how to write better essays Downside School table. An index provides a quick way to look up data based on one or more columns in the how to write better essays Downside School table, just like the power essay writing trent university index of a book enables you to find specific information quickly. The easiest way to how to write better essays Downside School, manipulate a relational database is to submit Structured Query Language (SQL) statements to 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 get paid to write essays online saxion university ofapplied sciences, 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. Critical thinking in business Ardingly College! 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. Power essay writing Trent University! A very good resource for learning SQL is top rated essay writing service moreton hall school, SQL Queries for Mere Mortals by Micheal J. how to write better essays Downside School! Hernandez and argumentative essay how to write ashbury college John L. Viescas. Code Figure 1. 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 retrieve a row from the "Seminar" table. WHERE SEMINAR_ID = 1701. Code Figure 3. SQL statement to how to write better essays Downside School, update a row in a table in the how to write better essays Downside School "Seminar" table. SET OVERSEER_ID = ‘NCC0001701’, SEMINAR_NUMBER = 3. WHERE SEMINAR_ID = 1701. Code Figure 4. Top rated essay writing service Moreton Hall School! SQL statement to delete data from the "Seminar" table. WHERE SEMINAR_ID > 1701. AND OVERSEER_ID = ‘THX0001138’ Object storage . how to write better essays Downside School! To store an object in a relational database you need to flatten it – create a data representation of how to write better essays Downside School the object – because relational databases only store data. To retrieve the object you would read the data from the critical thinking in business ardingly college database and then create the object, often referred to as restoring the object, based on that data. Although storing objects in 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 object technology are based on different underlying theories, a topic discussed in how to write better essays Downside School, The Object-Relational (O/R) Impedance Mismatch. To store objects successfully in relational databases you need to learn how to map your object schema to your relational database schema. Implementing behavior within the database . how to write better essays Downside School! 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 procedures are operations that run within an RDBMS, the difference being what the operation can return and whether it can be invoked in how to write better essays Downside School, a query. The differences aren’t important for our purposes so the term “stored procedure” will be used to refer to both stored functions and stored procedures. Power essay writing Trent University! In the past stored procedures were written in a proprietary language, such as Oracle’s PL/SQL, although now Java is how to write better essays Downside School, quickly becoming the language of choice for database programming. A stored procedure typically runs some SQL code, massages the persuasive essay strategies university of massachusetts lowell umass lowell (navitas) 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 detail in Implementation Strategies for Persisting Objects in RDBs. Concurrency control . Consider an airline reservation system. Persuasive essay strategies University of Massachusetts Lowell UMass Lowell (Navitas)! There is a flight with one seat left on it, and how to write better essays Downside School two people are trying to reserve that seat at the same time. How to write a brilliant essay Wilbraham and Monson Academy! Both people check the flight status and are told that a seat is how to write better essays Downside School, still available. Both enter their payment information and click the power essay writing trent university reservation button at the same time. Critical thinking in business Ardingly College! What should happen? If the system is working properly only one person should be given a seat and 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 larger transaction fails. Transactions may also be short-lived, running in thousandths of a second, or long-lived, taking hours, days, weeks, or even months to complete. Transaction control is a critical concept for all developers to understand. Enforcing referential integrity . Referential integrity (RI) is the assurance that a reference from one entity to another entity is valid. For example, if a customer references an address, then that address must exist. If the address is deleted then all references to power essay writing trent university, it must also be removed, otherwise your system must not allow the deletion. Critical thinking in business Ardingly College! Contrary to popular belief, RI isn’t just a database issue, it’s an get paid to write essays online saxion university ofapplied sciences, issue for your entire system. A customer is implemented as an object within a Java application and how to write better essays Downside School as one or more records in your database – addresses are also implemented as objects and as rows. To delete an how to write better essays Downside School, address, you must remove the critical thinking in business ardingly college address object from memory, any direct or indirect references to it (an indirect reference to an address would include a customer object knowing the value of the AddressID , the power essay writing trent university primary key of the address in the 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 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. how to write better essays Downside School! Worse yet, if the address is stored in several places (e.g. different databases) you should also consider taking this into account. All developers should understand basic strategies for implementing referential integrity. Table 1 describes the argumentative essay how to write ashbury college 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 how to write better essays Downside School, effectively a handle to the results of a SQL query, enabling you to how to write a brilliant essay wilbraham and monson academy, 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. Persuasive essay strategies University of Massachusetts Lowell UMass Lowell (Navitas)! Performance is improved when a portion of argumentative essay how to write ashbury college a result set is required because less data is transmitted across the how to write better essays Downside School network. Application developers need to understand that the how to write better essays Downside School underlying data can change between the how to write a brilliant essay wilbraham and monson academy 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. Some cursors only allow forward scrolling. Cursors are a resource drain on the argumentative essay how to write ashbury college 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. Critical thinking in business Ardingly College! Development of data intensive behavior that results in a relatively small return value. Encapsulation of database access to support security access control to how to write a brilliant essay wilbraham and monson academy, information. how to write better essays Downside School! Implementation of shared behavior required by many applications. Different version of VMs between application server and database server increases complexity of development. Behavior implemented in the database can easily become a bottleneck. Triggers – A trigger is a procedure that is run either before or after an action (such as a create, update, or delete) is 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 get paid to write essays online saxion university ofapplied sciences best-case scenario – when it is only how to write better essays Downside School, your application code that is coupled to your database schema. This situation is how to write better essays Downside School, traditionally referred to top rated essay writing service moreton hall school, as a stovepipe. how to write better essays Downside School! These situations do exist and are often referred to as stand-alone applications, stovepipe systems, or greenfield projects. Count yourself lucky if this is your situation because it is very rare in how to write better essays Downside School, practice. Other application source code . Figure 3 depicts the worst-case scenario for how to write better essays Downside School relational databases – a wide variety of how to write a brilliant essay wilbraham and monson academy software systems are coupled to how to write better essays Downside School, 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 persuasive essay strategies university of massachusetts lowell umass lowell (navitas) which you don’t, are also coupled to how to write better essays Downside School, your database. Perhaps an online system reads and writes to how to write better essays Downside School, your database. Perhaps a manager has written a spreadsheet, unbeknownst to you, that reads data from critical thinking in business ardingly college, your database that she uses to how to write better essays Downside School, summarize information critical to her job. Data load source code . Argumentative essay how to write Ashbury College! Data loads from other sources, such as government provided tax tables or your own test data, are often coupled to your database schema. how to write better essays Downside School! Data extract source code . 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 how to write better essays Downside School source code as the critical thinking in business ardingly college case may be, which describes the mappings. Itself . Coupling exists within your database. A single column is coupled to argumentative essay how to write ashbury college, any stored procedure that references it, other tables that use the column as a foreign key, any view that references the how to write a brilliant essay wilbraham and monson academy column, and how to write better essays Downside School so on. A simple change could result in several changes throughout your database. Data migration scripts . how to write better essays Downside School! Changes to 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 how to write better essays Downside School that reads the results from the database to compare it against expected results. Clearly this code may need to how to write better essays Downside School, be updated to reflect any database schema changes that you make. Documentation . Some of how to write better essays Downside School the most important documentation that you are likely to keep pertains to your physical database schema, including but not limited to physical data models and 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 that you will need to update your docs at some point. As you can see, coupling is a serious issue when it comes to relational databases. To make matters worse the concept of coupling is virtually ignored within database theory circles. how to write better essays Downside School! Although most database theory books will cover data normalization in excruciating detail, I argue that normalization is the data community’s way of addressing cohesion. My experience is that coupling becomes a serious issue only how to write better essays Downside School, when you start to consider behavioral issues (e.g. how to write better essays Downside School! code), something that traditional database theory chooses not to address. how to write better essays Downside School! 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 . When you are working with a shared database, such as the how to write better essays Downside School situation implied in Figure 3, you may find that the performance characteristics of your database are hard to predict because each application accesses the database in how to write better essays Downside School, 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 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 items available for sale via other companies that you have partnered with, items that you want to carry on your web site as soon as they are available. how to write better essays Downside School! These batch loads can take several minutes, during which period the 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 data it is very difficult to be sure that all applications are operating under the same business principles. For example, your application may consider an how to write better essays Downside School, 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 order as fulfilled once it has been shipped, the payment received, and the payment deposited into your bank account. A slight difference in the critical thinking in business ardingly college way that a fundamental business rule has been implemented may have serious business implications for how to write better essays Downside School any application that accesses the shared databases. Less subtly, imagine what would happen if your online order taking application calculates the get paid to write essays online saxion university ofapplied sciences total for an order and how to write a brilliant essay wilbraham and monson academy stores it in the order table, whereas the legacy application calculates the subtotals only for order items but does not total the how to write better essays Downside School order. Top rated essay writing service Moreton Hall School! 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. How to write a brilliant essay Wilbraham and Monson Academy! If a customer makes an how to write better essays Downside School, 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 get paid to write essays online saxion university ofapplied sciences, it, the order total is 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. Operational databases require different design strategies than reporting databases . The schemas of operational databases reflect the how to write better essays Downside School operational needs of the applications that access them, often resulting in a reasonably normalized schema with some portions of it denormalized for performance reasons. Get paid to write essays online Saxion University ofApplied Sciences! 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 this rule. 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 how to write better essays Downside School contrasts various encapsulation strategies that you have available to you. For now assume that it is possible to how to write better essays Downside School, hide the details of your database schema from the majority of the developers within your organization while at the same time giving them access to your database. Some people, often just the Agile DBA(s) responsible for 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 that it enables application programmers to how to write better essays Downside School, 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 how to write better essays Downside School 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 data access classes up to how to write better essays Downside School, date. This reveals a second advantage to how to write better essays Downside School, this approach – it provides greater freedom to Agile DBAs to do their job. Figure 4 depicts the concept of encapsulating access to your database, showing how the best case scenario of Figure 2 and the worst case scenario of Figure 3 would likely change. how to write better essays Downside School! In the best-case scenario your business source code would interact with the data access objects that in turn would interact with the how to write better essays Downside School 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 how to write better essays Downside School interface of a data access object changed. You’ll never get away from 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 power essay writing trent university, place the application programmers are only dealing with program source code (e.g. how to write better essays Downside School! Java) and not program source code plus SQL code. Things aren’t quite so ideal for the worst-case scenario. Although it is possible that all applications could take advantage of your encapsulation strategy the how to write better essays Downside School reality is that only how to write better essays Downside School, a subset will be able to. 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 power essay writing trent university, use the database encapsulation strategy. Perhaps some applications already have an encapsulation strategy in 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 how to write a brilliant essay wilbraham and monson academy 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 still a benefit to doing this, you are reducing coupling and therefore reducing your development costs and maintenance burden, the problem is that it isn’t a fully realized benefit. Another advantage of how to write better essays Downside School encapsulating access to a database is that it gives you a common place, in addition to the database itself, to power essay writing trent university, implement data-oriented business rules. Object/relational databases . Object/relational databases (ORDBs), or more properly object/relational database management systems (ORDBMSs), add new object storage capabilities to argumentative essay how to write ashbury college, RDBMSs. ORDBs, add new facilities to integrate management of traditional fielded data, complex objects such as time-series and geo-spatial data and diverse binary media such as audio, video, images, and (sometimes) Java applets. ORDBMSs basically add to RDBMSs features such as defined data types, for how to write better essays Downside School example you could define a data type called SurfaceAddress that has all of get paid to write essays online saxion university ofapplied sciences the attributes and behaviors of an address, as well as the ability to navigate objects in how to write better essays Downside School, addition to an RDBMS’s ability to join tables. By implementing objects within the how to write better essays Downside School database, an ORDBMS can execute complex analytical and data manipulation operations to search and transform multimedia and other complex objects. ORDBs support the robust transaction and data-management features of RDBMSs while at the same time offer a limited form of the flexibility of object-oriented databases. Because of how to write better essays Downside School 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 critical thinking in business ardingly college proprietary call interfaces. how to write better essays Downside School! 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 object programming languages. Critical thinking in business Ardingly College! In other words, full-fledged objects are implemented in the database. They bring much more than persistent storage of how to write better essays Downside School programming language objects. Top rated essay writing service Moreton Hall 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. how to write better essays Downside School! A major benefit of this approach is the unification of the application and database development into a seamless model. As a result, applications require less code, use more natural persistence modeling, and code bases are easier to how to write better essays Downside School, maintain. Object-oriented developers can write complete database applications with a modest amount of additional effort without the need to marshal their objects into critical thinking in business ardingly college flatten data structures for storage, as a result forgoing the marshalling overhead inherent with other persistence mechanism technologies (such as RDBs). This one-to-one mapping of application objects to database objects provides higher performance management of objects and how to write better essays Downside School 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 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 how to write a brilliant essay wilbraham and monson academy example mapping the how to write better essays Downside School 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 the how to write better essays Downside School point of argumentative essay how to write ashbury college view of an application developer, is the way they are accessed: with the first approach the only interface to the data is XML and related technologies, such as XPath (a language design specifically for argumentative essay how to write ashbury college addressing parts of an XML document, visit www.w3.org for details) or the DOM whereas with the second approach the how to write better essays Downside School database should be accessible via standard technologies such as JDBC. Get paid to write essays online Saxion University ofApplied Sciences! 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 . How to write a brilliant essay Wilbraham and Monson Academy! Flat files, such as .TXT or. how to write better essays Downside School! CSV (comma separated value) files, are commonly used to store data. A single file can be used to store one type of data structure, such as customer information or sales transaction information, or through a coding and formatting strategy the structures of critical thinking in business ardingly college several types of data structures. One approach to flat file organization is either to have data values separated by a pre-defined character, such as a comma, or tag, such as </FirstName> in an XML document. Another common approach is to delimit data values by size – the first 20 characters of persuasive essay strategies university of massachusetts lowell umass lowell (navitas) the row represent the first name of the customer, the next 20 characters represent the surname, and how to write better essays Downside School so on. Hierarchical databases . Hierarchical databases link data structures together like a family tree such that each record type has only one owner, for example an order is owned by only one customer. Hierarchical structures were widely used in the first mainframe database management systems, and how to write better essays Downside School are still a very common source of data in many large organizations. Hierarchical databases fell out of how to write better essays Downside School favor with the advent of relational databases due to their lack of flexibility because it wouldn’t easily support data access outside the original design of how to write better essays Downside School 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 top rated essay writing service moreton hall school load-balancing of the execution of power essay writing trent university the business logic of an information system through the power essay writing trent university use of how to write better essays Downside School state snapshots as well as command and critical thinking in business ardingly college query queuing or logging”. A prevalence layer is effectively a simple persistence framework that serializes objects and how to write better essays Downside School writes them to log files. From the point of view of developers all objects are cached in memory and the persistence of the objects is truly treated as a background task that the how to write better essays Downside School developers don’t need to worry about. Table 2 presents a comparison of the various types of persistence mechanism and critical thinking in business ardingly college provides references to vendors where applicable. Table 3 presents suggestions for when you might use each type of technology. how to write better essays Downside School! Large organizations will find that they are using several types of persistence mechanism and will even install the products of several different vendors. Top rated essay writing service Moreton Hall School! Not only how to write better essays Downside School, do you have a choice you might be forced to work with a wide range of databases whether you want to top rated essay writing service moreton hall school, or not.