birt barcode tool Taming wild EJBs: performance and scalability in Java

Drawer ECC200 in Java Taming wild EJBs: performance and scalability

Taming wild EJBs: performance and scalability
DataMatrix Encoder In Java
Using Barcode encoder for Java Control to generate, create DataMatrix image in Java applications.
www.OnBarcode.com
ECC200 Reader In Java
Using Barcode reader for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
You may recall that we briefly discussed transaction isolation levels in chapter 6, and that we encouraged you to choose the appropriate transactional isolation level supported by the underlying database system(s). Additionally, a highly transactional system must enforce appropriate locking strategies to make sure that users don t step on one another. Now we re ready to cover the known patterns on how to deal with concurrency. Given how important locking is to multiuser applications, let s discuss the various types of locking next. Then we ll focus on optimistic locking with JPA and show how you can obtain locks on entities using the EntityManager API.
Matrix Barcode Printer In Java
Using Barcode encoder for Java Control to generate, create 2D image in Java applications.
www.OnBarcode.com
Universal Product Code Version A Creator In Java
Using Barcode encoder for Java Control to generate, create Universal Product Code version A image in Java applications.
www.OnBarcode.com
13.1.1 Understanding locking types To avoid concurrency issues, applications must use appropriate locking mechanisms. Locking strategies are generally grouped into two camps: optimistic and pessimistic. Optimistic locking can be viewed as a cure it mind-set, whereas pessimistic locking is more of a prevent it concept. This section examines the differences between these two locking approaches.
Code 128B Encoder In Java
Using Barcode drawer for Java Control to generate, create ANSI/AIM Code 128 image in Java applications.
www.OnBarcode.com
Print GS1-128 In Java
Using Barcode drawer for Java Control to generate, create EAN 128 image in Java applications.
www.OnBarcode.com
Pessimistic locking Pessimistic locking is a strategy reflective of a pessimistic state of mind, where you prevent users from doing certain things, assuming that they will go wrong. When using pessimistic locking, you lock all involved database rows for the entire span of time a user shows an interest in modifying an entity.
QR Code JIS X 0510 Creator In Java
Using Barcode printer for Java Control to generate, create QR Code image in Java applications.
www.OnBarcode.com
Industrial 2 Of 5 Drawer In Java
Using Barcode maker for Java Control to generate, create Industrial 2 of 5 image in Java applications.
www.OnBarcode.com
NOTE
Generate DataMatrix In Java
Using Barcode generation for Java Control to generate, create Data Matrix image in Java applications.
www.OnBarcode.com
DataMatrix Creation In Objective-C
Using Barcode printer for iPad Control to generate, create Data Matrix ECC200 image in iPad applications.
www.OnBarcode.com
JPA does not have support for pessimistic locking strategies. However,
PDF 417 Creator In None
Using Barcode printer for Office Excel Control to generate, create PDF-417 2d barcode image in Microsoft Excel applications.
www.OnBarcode.com
Making PDF 417 In None
Using Barcode encoder for Office Word Control to generate, create PDF 417 image in Word applications.
www.OnBarcode.com
some persistence providers will have vendor-specific extensions to use pessimistic locking.
Create QR Code JIS X 0510 In Java
Using Barcode encoder for BIRT Control to generate, create QR Code ISO/IEC18004 image in Eclipse BIRT applications.
www.OnBarcode.com
QR Code JIS X 0510 Creator In Visual Studio .NET
Using Barcode creation for Reporting Service Control to generate, create QR Code image in Reporting Service applications.
www.OnBarcode.com
Figure 13.1 illustrates pessimistic locking. A user may acquire two types of lock on an entity: write or read. A write lock prevents users from attempting any kind of operations such as read, update, or delete, on the locked row(s). A read lock allows others to read the locked data but they are unable to update or delete it. A pessimistic lock is relatively easy to implement, since most databases allow you to lock tables, rows, and so forth. You can use the SELECT FOR UPDATE semantics in the database to implement pessimistic locking. Pessimistic locking has many disadvantages. For instance, it may make your applications slower because all users have to wait for an entity s locks to be released. It may also introduce deadlock conditions into your system. If a system has several users, transactions involve a greater number of objects, or transactions are long-lived, then the chance of having to wait for a lock to be released
EAN128 Printer In Objective-C
Using Barcode encoder for iPad Control to generate, create EAN128 image in iPad applications.
www.OnBarcode.com
Generate Code 3/9 In None
Using Barcode creation for Font Control to generate, create USS Code 39 image in Font applications.
www.OnBarcode.com
Handling entity locking issues
USS-128 Creator In None
Using Barcode creation for Online Control to generate, create USS-128 image in Online applications.
www.OnBarcode.com
Print QR Code In None
Using Barcode generation for Font Control to generate, create QR Code ISO/IEC18004 image in Font applications.
www.OnBarcode.com
Figure 13.1 In pessimistic locking, when a user wants to update the data the underlying records are locked and no other user can perform any operation on the data. It is typically implemented using SELECT ... FOR UPDATE semantics.
Generate QR Code JIS X 0510 In .NET
Using Barcode generation for ASP.NET Control to generate, create Quick Response Code image in ASP.NET applications.
www.OnBarcode.com
Barcode Generator In Visual C#
Using Barcode drawer for VS .NET Control to generate, create Barcode image in .NET framework applications.
www.OnBarcode.com
increases. Pessimistic locking therefore limits the practical number of concurrent users that the system can support. Optimistic locking By contrast, optimistic locking is a strategy that assumes that concurrency problems will occur rarely, and you should detect and resolve each problem when it happens. The optimistic locking mechanism is more difficult to implement, but is a popular approach to enforce control while allowing full data access concurrency. JPA supports the optimistic locking strategy. Although the ActionBazaar system is a high-transaction system, rarely do concurrency problems happen. In the ActionBazaar system, a Seller (as well as the system administrator) is allowed to update her item. Several Bidders may bid for the same item since they are not allowed to change the item. It is less likely that a Seller and an administrator will be updating the same item at the same time, so an optimistic locking strategy makes sense. Figure 13.2 depicts how optimistic locking works. When a user retrieves an entity for update, a copy of the object is provided and changes are made to the copy. After the user makes some changes to the entity, he is ready to save it. The application now obtains a write lock on the object and checks whether data has been updated since the user obtained it. If there has been no update to the original data, then the user is allowed to commit the copy of the entity to the database. Otherwise, the application should throw an exception. The optimistic locking strategy is typically implemented by adding a column to the database table for the entity, and either storing a version number or a
Copyright © OnBarcode.com . All rights reserved.