N-tier Application Architecture in Java

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N-tier Application Architecture
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Although Figures 2-2 through 2-4 show different ways to split up the business logic, they all contain three basic components: the client, the server, and the network (which is not shown but is implied by the arrows)
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The Client
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We want to cover two types of client: the human one and the automated, or software-based, one Let's start with the human type The client in this case involves a machine with an operating system and network access He typically uses a Web browser to access the Web application, although custom interfaces are also possible This browser speaks the HTTP protocol One of the unique things about this kind of client is that its sessions with the server do not demand constant servicing; there is plenty of "think time" that allows the server to use its resources for clients requesting service The automated client probably runs on a more powerful machine than the human client uses The automated client may use HTTP or a lower-level or proprietary protocol This kind of client may also communicate using technologies like messaging, which may not involve the Web server but can certainly involve the rest of the server-side software (application servers and database) An automated client does not need "think time" and could continually pound a server with requests
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The Network
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The network between client and server is more commonly known as the Internet As we all know, the Internet is made up of many machines and subnetworks distributed across the world When machines communicate with each other, their packets travel through a variety of hardware and software systems en route to their final destination For our purposes, most of their messages are communicated using the TCP/IP protocol IP stands for Internet Protocol and TCP stands for Transmission Control Protocol TCP/IP is a connection-oriented protocol that provides quality of service guarantees Basically, it ensures that bytes can be reliably delivered to parties, even if the underlying network is unreliable TCP/IP is our prime concern because the Internet represents an unreliable network, and this protocol has become its default language However, we should note that there are other transportlayer protocols, most notably the Unreliable Datagram Protocol (UDP) This protocol is not connection-oriented and does not have the same QoS guarantees However, because it does not have such features, it performs better than TCP Although UDP is largely unacceptable for Internet applications, it does make sense for certain types of high-performance Intranet applications Client-Side Network Elements There are three parts of the network worth addressing One is near the client, who is usually not directly connected to the Internet Most have Internet access through a provider service (called an internet service provider, or ISP) Before having a request resolved at the original source on the Internet, clients typically access their browser cache to see if they already have the desired document The browser cache is simply the filesystem on the client machine When a page is requested, if it is not already in the local filesystem, the browser fetches it The page is rendered and, if the page is cacheable, a copy of it is kept in the local filesystem for future access This leads to better client performance, since future access requires network roundtrip time Pages are not cached forever: There are expiration dates and protocols for checking on the updates to a particular Web object The concept of a browser cache, or any cache, to store remote Web page data locally (the avoiding the cost of contacting the originating server for that content) is known as Web caching A proxy cache is either software or hardware that is designed to cache frequently requested Web pages so that an ISP does not have to repeatedly fetch the same pages for its clients When client A fetches Web page X for the first time, the proxy cache requests the page from the original server and stores a copy of the page in its cache (assuming the page is cacheable; more about that later)
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as well as providing a copy to client A When client B requests page X, the proxy cache can simply fetch the page from storage without accessing the network again This leads to better client performance, and what's more, the effect can be shared among clients Figure 2-5 shows the relationship between the client Web browser and the intermediate client-side caches Later, we will discuss more about Web caching and how you can design an application to leverage clientside browser and proxy caches Figure 2-5 Client-side network infrastructure
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Server-Side Network Elements The second part of the network that interests us is the server side Like the client, the server side has an ISP; however, the ISP choice here is one we can control It is important, when designing an application, that you know the bandwidth limits and the backbone network access available to your provider Another important aspect of the server-side part of the network is the way incoming connections are distributed to server resources Local load balancers provide both simple and sophisticated techniques for sharing the workload among multiple machines Requests can be routed based on Web server availability or on the nature of the request For example, image requests can be routed one way and static page requests can be routed another A typical-load balancing hardware device is the Cisco Local Director The balanced load is often distributed among Web server farms Each farm consists of a set of Web servers designed to access the same kind of content Each Web server typically resides on a separate machine, a redundancy that increases Web site reliability Finally, a reverse proxy cache can be set up on the server side to reduce the demand on the server by providing quick access to frequently accessed objects A reverse proxy cache works just like the proxy cache on the client side: It stores frequently requested objects It is useful on the server side because it allows frequently accessed content to be cached even though clients may not share an ISP Figure 2-6 shows one type of deployment strategy that involves a load balancer, a reverse proxy cache, and a Web server farm Figure 2-6 Server-side network infrastructure
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Between Client-Side and Server-Side The third part of the network is the unpredictable mesh of routers and switches that separate client and server We cannot control this mesh any more than we can control the client-side of the network However, like the client side, intermediate caches are strewn along this path They do not exist specifically to help you or your clients; rather, they are trying to provide a general costeffective service by reducing congestion for heavily demanded resources However, if you know how intermediate caches work and how the HTTP protocol works, you can at least inform the network of the nature of your data With enough information, the intermediate network elements can help you by significantly reducing load on your site for static Web pages and the like In general terms, the communication between client and server sides consists of the client ISP, capable of T1 and T3 speeds, forwarding requests through its backbone network service provider (NSP), which speaks in terms of OC-1 and OC-3 speeds These NSPs provide access to network access points (NAPs), which are public exchange facilities where ISPs can reach each other through a process known as ISP-peering NAPs are distributed across the world; collectively, they represent the points where the Internet backbone is "stitched" together Communication at NAPs occurs at very high speeds for example, OC-12 (622 Mbps) and in a point-to-point manner To get a feel for the connection between client and server, see Listing 2-1, output from traceroute, a diagnostic network tool that tracks a packet from client to server This packet goes from Carnegie Mellon University to Yahoo Listing 2-1 traceroute output describing the route from CMU to Yahoo
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1 CAMPUS-VLAN4GWCMUNET (128241) 1744 ms 1052 ms 0992 ms 2 RTRBONE-FA4-0-0GWCMUNET (128202) 37317 ms 54990 ms 75095 ms 3 nss5pscnet (19832224254) 2747 ms 1874 ms 1557 ms 4 1212423573 (1212423573) 11408 ms 22782 ms 21471 ms 5 gbr1-p100wswdcipattnet (12123942) 17880 ms 21404 ms 23662 ms 6 gbr4-p00wswdcipattnet (121221222) 13569 ms 10793 ms 11525 ms 7 ggr1-p370wswdcipattnet (12123953) 11814 ms 10948 ms 10540 ms 8 ibr01-p5-0stng01exodusnet (21632173185) 12872 ms 20572 ms 20885 ms 9 dcr02-g9-0stng01exodusnet (2163396145) 29428 ms 10619 ms 10550 ms
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10 csr21-ve240stng01exodusnet (21633982) 10998 ms 32657 ms 19938 ms 11 21635210122 (21635210122) 11231 ms 20915 ms 32128 ms 12 www7dcxyahoocom (645876176) 36600 ms 10768 ms 12029 ms
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We see that CMU is connected to Yahoo through the backbone providers AT&T and Exodus Communications In recent years, there has also been focus on one approach to optimizing the area between client and server Content distribution has emerged from the Web-caching community as a way for providers to replicate their content through providers that act as reverse proxy caches Content distributors such as Akamai strategically replicate their hosted content so that client access is very fast and does not involve the originating server Content distribution is often a solution for bandwidth-heavy objects, such as images, which quickly clog up server-side bandwidth even though they don't require server-side application logic
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