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online article writing jobs for students Windermere Preparatory School

For the sake of online article writing jobs for students Windermere Preparatory School our discussion a relational database is a persistent storage mechanism that enables you to both store data and optionally implement functionality. The goal of critical thinking method university of massachusetts boston — umass boston this article is to overview relational database management system (RDBMS) technology and to explore the practical issues applicable to its use in online article writing jobs for students Windermere Preparatory School modern organizations; the goal is not to discuss relational theory. RDBMSs are used to 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. Because RDBMSs are the what should you do when writing an analytical essay cesar ritz сolleges switzerland dominant persistent storage technology it is critical that all IT professionals understand at least the basics of what do i write my college essay on curtin university singapore 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 key would be CustomerNumber . To improve access time to a data table you define an index on the online article writing jobs for students Windermere Preparatory School table. An index provides a quick way to online article writing jobs for students Windermere Preparatory School look up data based on one or more columns in the table, just like the academic essay writing service the storm king school 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 it. Figure 1 depicts a simple data model, see Data Modeling 101 or better yet Agile Data Modeling, using the online article writing jobs for students Windermere Preparatory School proposed UML data modeling notation. To create a row in the Seminar table you would issue an 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 how to write a good analytical essay the university of canberra (navitas), Code Figure 4 how to delete a row with a DELETE statement. All four of online article writing jobs for students Windermere Preparatory School these examples were taken, as well as the how to write the article ashbury college data model, were adapted from The Object Primer. Critical thinking method University of Massachusetts Boston — UMass Boston! A very good resource for learning SQL is SQL Queries for online article writing jobs for students Windermere Preparatory School, Mere Mortals by Micheal J. Hernandez and John L. Viescas. Code Figure 1. SQL statement to insert a row into the what should you do when writing an analytical essay cesar ritz сolleges switzerland "Seminar" table. (SEMINAR_ID, COURSE_ID, OVERSEER_ID, SEMINAR_NUMBER) VALUES (74656, 1234, РІР‚ВTHX0001138РІР‚в„ў, 2) Code Figure 2. SQL statement to retrieve a row from the "Seminar" table. WHERE SEMINAR_ID = 1701. Code Figure 3. SQL statement to update a row in what should you do when writing an analytical essay cesar ritz сolleges switzerland a table in the "Seminar" table. SET OVERSEER_ID = РІР‚ВNCC0001701РІР‚в„ў, 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 = РІР‚ВTHX0001138РІР‚в„ў Object storage . online article writing jobs for students Windermere Preparatory School! To store an how to write a good analytical essay the university of canberra (navitas), object in a relational database you need to flatten it – create a data representation of the object – because relational databases only store data. online article writing jobs for students Windermere Preparatory School! To retrieve the object you would read the data from the 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 academic essay writing service the storm king school achieve, practice shows that it isnРІР‚в„ўt. This is due to the object-relational impedance mismatch, the critical thinking method university of massachusetts boston — umass boston fact that relational database technology and object technology are based on different underlying theories, a topic discussed in online article writing jobs for students Windermere Preparatory 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. online article writing jobs for students Windermere Preparatory School! Implementing behavior within the database . Behavior is implemented in online article writing jobs for students Windermere Preparatory School a relational database via stored procedures and/or stored functions that can be invoked internally within the database and often by external applications. online article writing jobs for students Windermere Preparatory School! 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 a query. The differences arenРІР‚в„ўt important for our purposes so the term “stored procedure” will be used to refer to both stored functions and online article writing jobs for students Windermere Preparatory School, stored procedures. In the past stored procedures were written in a proprietary language, such as OracleРІР‚в„ўs PL/SQL, although now Java is quickly becoming the what should you do when writing an analytical essay cesar ritz сolleges switzerland language of online article writing jobs for students Windermere Preparatory School choice for database programming. A stored procedure typically runs some SQL code, massages the data, and then hands back a response in the form of online article writing jobs for students Windermere Preparatory School zero or more records, or a response code, or as a database error message. Effective use of online article writing jobs for students Windermere Preparatory School stored procedures is discussed in online article writing jobs for students Windermere Preparatory School detail in Implementation Strategies for Persisting Objects in RDBs. Concurrency control . Critical thinking method University of Massachusetts Boston — UMass Boston! Consider an airline reservation system. There is online article writing jobs for students Windermere Preparatory School a flight with one seat left on it, and two people are trying to reserve that seat at the same time. What do i write my college essay on Curtin University Singapore! Both people check the flight status and 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 system is critical thinking method university of massachusetts boston — umass boston working properly only one person should be given a seat and paul elder critical thinking wilbraham and monson academy, 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 what do i write my college essay on curtin university singapore 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 online article writing jobs for students Windermere Preparatory School, all developers to understand. Enforcing referential integrity . Referential integrity (RI) is the assurance that a reference from how to write a good analytical essay the university of canberra (navitas) one entity to another entity is valid. For example, if a customer references an address, then that address must exist. online article writing jobs for students Windermere Preparatory School! If the address is deleted then all references to it must also be removed, otherwise your system must not allow the online article writing jobs for students Windermere Preparatory School deletion. Contrary to online article writing jobs for students Windermere Preparatory School popular belief, RI isnРІР‚в„ўt just a database issue, itРІР‚в„ўs an issue for online article writing jobs for students Windermere Preparatory School, your entire system. A customer is implemented as an how to write the article ashbury college, 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 online article writing jobs for students Windermere Preparatory School memory, any direct or indirect references to it (an indirect reference to online article writing jobs for students Windermere Preparatory School an address would include a customer object knowing the value of the AddressID , the primary key of the address in the paul elder critical thinking wilbraham and monson academy database), the address row(s) from your database, and any references to how to write the article ashbury college it (via foreign keys) in online article writing jobs for students Windermere Preparatory School your database. To complicate matters, if you have a farm of online article writing jobs for students Windermere Preparatory School application servers that address object could exist simultaneously on several machines. Furthermore, if you have other applications accessing your database then it is how to write the article ashbury college possible that they too have representations of the address in what do i write my college essay on curtin university singapore their memory as well. online article writing jobs for students Windermere Preparatory School! Worse yet, if the address is stored in several places (e.g. online article writing jobs for students Windermere Preparatory School! different databases) you should also consider taking this into account. All developers should understand basic strategies for implementing referential integrity. 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 effectively a handle to how to write a good analytical essay the university of canberra (navitas) the results of 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. Performance is online article writing jobs for students Windermere Preparatory School improved when a portion of a result set is online article writing jobs for students Windermere Preparatory School required because less data is transmitted across the network. Application developers need to understand that the underlying data can change between the what do i write my college essay on curtin university singapore times that data records are accessed via the online article writing jobs for students Windermere Preparatory School cursor: previously retrieved records may have been deleted, records may have been inserted into previously retrieved portions of the online article writing jobs for students Windermere Preparatory School 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 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 paul elder critical thinking wilbraham and monson academy database access to support security access control to information. Implementation of shared behavior required by online article writing jobs for students Windermere Preparatory School many applications. Different version of VMs between application server and database server increases complexity of development. online article writing jobs for students Windermere Preparatory School! 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. online article writing jobs for students Windermere Preparatory School! 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 online article writing jobs for students Windermere Preparatory School maintain and will increase your dependency on your database vendor. Triggers are typically implemented in a proprietary language requiring an extra skillset on how to write the article ashbury college, 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 doesnРІР‚в„ўt 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 critical thinking method university of massachusetts boston — umass boston coupled to your database schema. This situation is online article writing jobs for students Windermere Preparatory School traditionally referred to as a stovepipe. These situations do exist and how to write a good analytical essay the university of canberra (navitas), 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 practice. Other application source code . Figure 3 depicts the critical thinking method university of massachusetts boston — umass boston worst-case scenario for 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 online article writing jobs for students Windermere Preparatory School the application that your team is what should you do when writing an analytical essay cesar ritz сolleges switzerland currently working on that other applications, some of which you know about and some of which you donРІР‚в„ўt, are also coupled to your database. Perhaps an online system reads and what should you do when writing an analytical essay cesar ritz сolleges switzerland, 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 critical thinking method university of massachusetts boston — umass boston 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 . 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 critical thinking method university of massachusetts boston — umass boston another database. Persistence frameworks/layers . A persistence framework encapsulates the logic for mapping application classes to online article writing jobs for students Windermere Preparatory School 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 mappings. Itself . Coupling exists within your database. A single column is coupled to any stored procedure that references it, other tables that use the column as a foreign key, any view that references the what do i write my college essay on curtin university singapore column, and so on. A simple change could result in several changes throughout your database. Data migration scripts . online article writing jobs for students Windermere Preparatory 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 online article writing jobs for students Windermere Preparatory School, that reads the results from the database to compare it against expected results. Clearly this code may need to be updated to reflect any database schema changes that you make. online article writing jobs for students Windermere Preparatory School! Documentation . Some of the most important documentation that you are likely to how to write a good analytical essay the university of canberra (navitas) keep pertains to online article writing jobs for students Windermere Preparatory School your physical database schema, including but not limited to what do i write my college essay on curtin university singapore 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 doesnРІР‚в„ўt 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. online article writing jobs for students Windermere Preparatory School! To make matters worse the concept of coupling is virtually ignored within database theory circles. Although most database theory books will cover data normalization in excruciating detail, I argue that normalization is the data communityРІР‚в„ўs way of addressing cohesion. My experience is that coupling becomes a serious issue only when you start to consider behavioral issues (e.g. online article writing jobs for students Windermere Preparatory School! code), something that traditional database theory chooses not to address. This is another reason to follow Agile Modeling (AM)РІР‚в„ўs 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 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 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 doesnРІР‚в„ўt really affect your applicationРІР‚в„ўs 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 online article writing jobs for students Windermere Preparatory School carry on your web site as soon as they are available. These batch loads can take several minutes, during which period the Item table is online article writing jobs for students Windermere Preparatory School under heavy load and how to write the article ashbury college, thus your online application is potentially affected. Data integrity is online article writing jobs for students Windermere Preparatory School difficult to ensure with shared databases . online article writing jobs for students Windermere Preparatory School! Because no single application has control over the data it is very difficult to what should you do when writing an analytical essay cesar ritz сolleges switzerland be sure that all applications are operating under the same business principles. For example, your application may consider an online article writing jobs for students Windermere Preparatory School, order as fulfilled once it has been shipped and a payment has been received. online article writing jobs for students Windermere Preparatory School! The original legacy application that is still in use by critical thinking method university of massachusetts boston — umass boston 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 what should you do when writing an analytical essay cesar ritz сolleges switzerland 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 how to write the article ashbury college, 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 paul elder critical thinking wilbraham and monson academy, appropriate taxes, whereas if a total already exists it uses the existing figure. Academic essay writing service The Storm King School! If a customer makes an order online and academic essay writing service the storm king school, 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 order total is no longer current because it has not been updated properly. Referential integrity issues such as this are covered in what do i write my college essay on curtin university singapore detail in the Implementing Referential Integrity article. Operational databases require different design strategies than reporting databases . The schemas of operational databases reflect the online article writing jobs for students Windermere Preparatory 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. Reporting databases, on the other hand, are typically highly denormalized with significant data redundancy within them to what should you do when writing an analytical essay cesar ritz сolleges switzerland support a wide range of reporting needs. Every technology has its strengths and weaknesses, and RDBMS technology is not an academic essay writing service the storm king school, exception to this rule. Critical thinking method University of Massachusetts Boston — UMass Boston! Luckily there are ways that you can mitigate some of these challenges, and encapsulation is an online article writing jobs for students Windermere Preparatory School, 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 online article writing jobs for students Windermere Preparatory School, Persisting Objects in RDBs chapter compares and contrasts various encapsulation strategies that you have available to you. For now assume that it is possible to 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 paul elder critical thinking wilbraham and monson academy, supporting the online article writing jobs for students Windermere Preparatory School 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 how to write the article ashbury college that it enables application programmers to focus on the business problem itself. LetРІР‚в„ўs assume weРІР‚в„ўre doing something simple such as data access objects that implement the how to write the article ashbury college SQL code to online article writing jobs for students Windermere Preparatory School access your database schema. What should you do when writing an analytical essay Cesar Ritz Сolleges Switzerland! 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 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 concept of online article writing jobs for students Windermere Preparatory School 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. In the best-case scenario your business source code would interact with the data access objects that in turn would interact with the 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. ItРІР‚в„ўs interesting to note that the online article writing jobs for students Windermere Preparatory School business code that your application programmers are writing would still be coupled to the data access objects. Therefore theyРІР‚в„ўd need to change their code whenever the interface of a data access object changed. YouРІР‚в„ўll never get away from coupling. However, from the point of online article writing jobs for students Windermere Preparatory School view of the application programmer this is a much easier change to detect and act on paul elder critical thinking wilbraham and monson academy, – with the database encapsulation strategy in place the application programmers are only academic essay writing service the storm king school, dealing with program source code (e.g. online article writing jobs for students Windermere Preparatory School! Java) and not program source code plus SQL code. Things arenРІР‚в„ўt quite so ideal for the worst-case scenario. Academic essay writing service The Storm King School! 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. Platform incompatibility can be an online article writing jobs for students Windermere Preparatory School, issue – perhaps your data access objects are written in Java but some legacy applications are written using technologies that canРІР‚в„ўt easily access Java. Perhaps youРІР‚в„ўve chosen not to rework some of online article writing jobs for students Windermere Preparatory School your legacy applications simply to use the database encapsulation strategy. Perhaps some applications already have an online article writing jobs for students Windermere Preparatory School, 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 database schema. The point to be made is that a portion of your organizationРІР‚в„ўs application will be able to take advantage of your encapsulation strategy and some wonРІР‚в„ўt. 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 isnРІР‚в„ўt 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 . 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 diverse binary media such as audio, video, images, and (sometimes) Java applets. online article writing jobs for students Windermere Preparatory School! ORDBMSs basically add to 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 RDBMSРІР‚в„ўs 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 how to write the article ashbury college, other complex objects. online article writing jobs for students Windermere Preparatory School! ORDBs support the robust transaction and data-management features of academic essay writing service the storm king school RDBMSs while at the same time offer a limited form of the how to write the article ashbury college flexibility of object-oriented databases. Critical thinking method University of Massachusetts Boston — UMass Boston! Because of ORDBMSs relational foundation, database administrators work with familiar tabular structures and critical thinking method university of massachusetts boston — umass boston, 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 object programming languages. In other words, full-fledged objects are implemented in the database. They bring much more than persistent storage of programming language objects. ODBs extend the semantics of academic essay writing service the storm king school Java to provide full-featured database programming capability, via new class libraries specific to the ODB vendor, while retaining native language compatibility. online article writing jobs for students Windermere Preparatory School! A major benefit of how to write a good analytical essay the university of canberra (navitas) this approach is the unification of the application and database development into what should you do when writing an analytical essay cesar ritz сolleges switzerland a seamless model. As a result, applications require less code, use more natural persistence modeling, and 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 online article writing jobs for students Windermere Preparatory School marshal their objects into 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 what should you do when writing an analytical essay cesar ritz сolleges switzerland application objects to 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 entire XML document in the file system, perhaps in what do i write my college essay on curtin university singapore a compressed or pre-parsed binary form. Second, a native XML database may opt to map the online article writing jobs for students Windermere Preparatory School structure of the document to the database, for example mapping the online article writing jobs for students Windermere Preparatory School Document Object Model (DOM) to academic essay writing service the storm king school 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 point of how to write a good analytical essay the university of canberra (navitas) view of an online article writing jobs for students Windermere Preparatory School, application developer, is the paul elder critical thinking wilbraham and monson academy way they are accessed: with the first approach the only interface to online article writing jobs for students Windermere Preparatory School the data is 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. The important thing to understand about native XML databases is that they work with the internal structures of the XML documents – they donРІР‚в„ўt just store them as a binary large object (BLOB) in the database. online article writing jobs for students Windermere Preparatory School! 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 store one type of online article writing jobs for students Windermere Preparatory School data structure, such as customer information or sales transaction information, or through a coding and formatting strategy the structures of 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 online article writing jobs for students Windermere Preparatory School delimit data values by size – the online article writing jobs for students Windermere Preparatory School first 20 characters of the how to write a good analytical essay the university of canberra (navitas) row represent the first name of the how to write a good analytical essay the university of canberra (navitas) customer, the next 20 characters represent the surname, and so on. Hierarchical databases . online article writing jobs for students Windermere Preparatory School! 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 are still a very common source of data in many large organizations. Critical thinking method University of Massachusetts Boston — UMass Boston! Hierarchical databases fell out of favor with the advent of online article writing jobs for students Windermere Preparatory School relational databases due to their lack of flexibility because it wouldnРІР‚в„ўt easily support data access outside the original design of the online article writing jobs for students Windermere Preparatory School data structure. For example, in the customer-order schema you could only access an order through a customer, you couldnРІР‚в„ўt easily find all the orders that included the sale of a widget because the schema isnРІР‚в„ўt 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 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 developers donРІР‚в„ўt need to paul elder critical thinking wilbraham and monson academy worry about. Table 2 presents a comparison of the how to write a good analytical essay the university of canberra (navitas) various types of persistence mechanism and provides references to online article writing jobs for students Windermere Preparatory School vendors where applicable. Critical thinking method University of Massachusetts Boston — UMass Boston! Table 3 presents suggestions for when you might use each type of technology. Large organizations will find that they are using several types of persistence mechanism and will even install the what do i write my college essay on curtin university singapore products of several different vendors. Not only do you have a choice you might be forced to how to write the article ashbury college work with a wide range of databases whether you want to or not.