Redundancy in Java

Generator QR in Java Redundancy
Redundancy
Printing QR-Code In Java
Using Barcode printer for Java Control to generate, create QR Code 2d barcode image in Java applications.
Redundancy typically refers to the duplication of hardware or software so that more resources are available for execution For example, it's possible to have redundant Web servers or redundant disk drives, such as RAID With either, the redundancy not only increases the ability of a system to scale, it also increases reliability if one of several Web servers crashes, a Web site remains accessible Note that redundancy, as used here, is not the same as replication The former refers to the duplication of resources, the latter refers to the duplication of data There can be a performance benefit to redundancy, depending on the resource involved Consider a Web server farm By replicating the number of Web servers, we can effectively increase the parallelism of request processing We can achieve near-linear speedups for environments that normally have very busy Web servers, which is why server farms are such a popular scalability solution The only real drawbacks to redundancy are its deployment cost and the data consistency challenge Cost increases not only for the duplicate resources but for the added hardware/software to employ
Creating Barcode In Java
Using Barcode creation for Java Control to generate, create bar code image in Java applications.
them Redundant Web servers, for example, require not only more machines to run them but a load balancer or redirector to control workload distribution Dealing with this extra cost is simply a matter of money: Either you have it or you don't While throwing money at a Web application is generally not the best way to improve scalability, this is one case where it literally pays off Data consistency can be a more complex issue As we discussed previously in greater detail in the section on caching, maintaining consistency requires careful planning and synchronization techniques This can be such an arduous and vexing problem that, in some cases, it demonstrates how the costs of redundancy can outweigh its gains To see how, let's return to our example of caching user account information, this time with redundant write-through caches Initially, the caches were used to reduce the time to access the database accessing a cache located in the same address space is faster than communicating with another process (the database server) However, as Figure 4-4 showed, if we have redundant caches on different machines, we have to update cache B whenever cache A is updated (in addition to eventually updating the database) In short, the round-trip time we saved by not having to access the database server is erased by the round-trip time required for redundant cache synchronization And, as the number of redundant caches increases, the problem obviously gets worse Essentially, when we create a redundant cache we cause the access time to balloon, transforming simple updates into completed transactions The trick with redundancy is knowing when its costs outweigh its benefits In almost all cases where redundancy is successful, the hardware or software is stateless This is why redundant Web serving works so well: HTTP is by nature stateless, and thus Web servers don't need to cache anything and can be reproduced at will Incidentally, Web serving here refers to resolving static pages, not application request As we'll see in later chapters, certain application technologies can cause this stateless deployment strategy to become stateful and thus limit the redundancy of certain application components Remember, redundancy refers to "providing additional independent instances" of hardware or software, whereas replication/caching refers to data "copying" This distinction is important and underscores how replication affects redundancy For example, in revisiting the data consistency issue previously discussed, we see that it isn't the redundancy that causes problems so much as it's the replication of data
Barcode Recognizer In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
Asynchrony
Draw Quick Response Code In C#
Using Barcode maker for .NET Control to generate, create QR image in Visual Studio .NET applications.
Dictionaries define the word synchronous to mean "happening at the same time" For software and hardware systems, asynchronous has a different (but related) meaning: "happening independently of one another" We frequently encounter instances of synchrony throughout computer science For example, clocks are said to be synchronous if they keep the same time (more specifically, if neither moves ahead of the other) Communication is said to be synchronous if one process makes a request of another and waits for a reply In this sense, the exchange is considered synchronous because events happen in a dependent manner a reply always follows a request Similarly, code execution is synchronous: The next instruction comes after the previous one terminates In each of these cases, synchrony is necessary to ensure reasonable activity; for example, if code really did execute out of order, we would get unpredictable results[*]
QR Code JIS X 0510 Encoder In .NET Framework
Using Barcode encoder for ASP.NET Control to generate, create QR image in ASP.NET applications.
Painting QR Code In .NET
Using Barcode drawer for .NET Control to generate, create QR Code image in .NET applications.
EAN13 Creator In Java
Using Barcode creation for Java Control to generate, create EAN13 image in Java applications.
Bar Code Printer In Java
Using Barcode maker for Java Control to generate, create bar code image in Java applications.
Painting Code 128 Code Set B In Java
Using Barcode maker for Java Control to generate, create ANSI/AIM Code 128 image in Java applications.
Encode UCC - 12 In VS .NET
Using Barcode printer for Visual Studio .NET Control to generate, create UCC - 12 image in VS .NET applications.
Recognizing Data Matrix 2d Barcode In .NET Framework
Using Barcode recognizer for .NET Control to read, scan read, scan image in Visual Studio .NET applications.
EAN13 Creation In Visual Studio .NET
Using Barcode printer for VS .NET Control to generate, create UPC - 13 image in Visual Studio .NET applications.
Code39 Creator In Visual Studio .NET
Using Barcode drawer for Visual Studio .NET Control to generate, create Code39 image in Visual Studio .NET applications.