Figure 72 Clustering involves putting several copies of the same application on different nodes in Java

Drawer QR Code 2d barcode in Java Figure 72 Clustering involves putting several copies of the same application on different nodes
Figure 72 Clustering involves putting several copies of the same application on different nodes
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Where You Have to Distribute
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So you want to minimize distribution boundaries and utilize your nodes through clustering as much as possible The rub is that there are limits to that approach that is, places where you need to separate the processes If you're sensible, you'll fight like a cornered rat to eliminate as many of them as you can, but you won't eliminate them all
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One obvious separation is between the traditional clients and servers of business software PCs on users' desktops are different nodes to shared repositories of data Since they're different machines you need separate processes that communicate The client server divide is a typical inter-process divide A second divide often occurs between server-based application software (the application server) and the database Of course, you don't have to do this You can run all your application software in the
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database process itself using such things as stored procedures But often that's not so practical, so you have to have separate processes They may run on the same machine, but once you have separate processes you immediately have to have to pay most of the costs in remote calls Fortunately, SQL is designed as a remote interface, so you can usually arrange things to minimize that cost Another separation in process may occur in a Web system between the Web server and the application server All things being equal it's best to run the Web and application servers in a single process, but all things aren't always equal You may have to separate because of vendor differences If you're using a software package, it will often run in its own process, so again you're distributing At least a good package will have a coarsegrained interface And finally there may be some genuine reason that you have to split your application server software You should sell any grandparent you can get your hands on to avoid this, but cases do come up Then you just have to hold your nose and divide your software into remote, coarse-grained components
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The overriding theme, in Colleen Roe's memorable phrase, is to be "parsimonious with object distribution" Sell your favorite grandma first if you possibly can
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Working with the Distribution Boundary
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As you design your system you need to limit your distribution boundaries as much as possible, but where you have them you need to take them into account Every remote call travels on the cyber equivalent of a horse and carriage All sorts of places in the system will change shape to minimize remote calls That's pretty much the expected price
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However, you can still design within a single process using fine-grained objects The key is to use them internally and place coarse-grained objects at the distribution boundaries, whose sole role is to provide a remote interface to the fine-grained objects The coarse-grained objects don't really do anything but so they act as a facade for the fine-grained objects This facade is there only for distribution purposes hence the name Remote Facade (388)
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Using a Remote Facade (388) helps minimize the difficulties that the coarse-grained interface introduces This way only the objects that really need a remote service get the coarse-grained method, and it's obvious to the developers that they're paying that cost Transparency has its virtues, but you don't want to be transparent about a potential remote call
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By keeping the coarse-grained interfaces as mere facades, however, you allow people to use the fine-grained objects whenever they know they are running in the same process This makes the whole distribution policy much more explicit Hand in hand with Remote Facade (388) is Data Transfer Object (401) Not only do you need coarse-grained methods, you also need to transfer coarse-grained objects When you ask for an address, you need to send that information in one block You usually can't send the domain object itself, because it's tied in a Web of fine-grained local inter-object references So you take all the data that the client needs and bundle it in a particular object for the transfer hence the term Data Transfer Object (401) (Many people in the enterprise Java community use the term value object for this, but this causes a clash with other meanings of the term Value Object (486)) The Data Transfer Object (401) appears on both sides of the wire, so it's important that it not reference anything that isn't shared over the wire This boils down to the fact that a Data Transfer Object (401) usually only references other Data Transfer Objects (401) and fundamental objects such as strings
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Another route to distribution is to have a broker that migrates objects between processes The idea here is to use a Lazy Load (200) scheme where, instead of lazy reading from a database, you move objects across the wire The hard part of this is ensuring that you don't end up with lots of remote calls I haven't seen anyone try this in an application, but some O/R mapping tools (eg, TOPLink) have this facility, and I've heard some good reports about it
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