Middle tier

Use a three-tier client-server architecture. All business logic must be in the middle tier, presentation and dialog on the client, and data services on the server. In this way you can scale the application server processing independently of persistent storage. 

WebLogic is a J2EE middle-tier software suit from the former BEA Systems, now part of Oracle (http //www.oracle.com/ appserver/weblogic/weblogic -suite. html)... 

A three-tiered architecture includes one more node between the client and the database server—the middle tier. In a three-tiered architecture, business logic is offloaded from the client and the database server nodes to the middle tier. In fact, you can choose to further distribute the business logic among more than one middle tier node and still call it a three-tiered (or K-tiered) architecture because the idea is similar. Note that the tiers do not have to be physically separated. You can have both the middle tier server and the database server collocated on the same physical computer but running in different processes with separate memory spaces. Modem hardware architecture can partition a single hardware box into multiple virtually separate computers or domains. Typically, a three-tiered architecture supports a Web-based thin client although it can also work with a rich client. 

The computing power of the middle tier is virtually unlimited. When it reaches its limit, by simply adding more hardware, the system should get back on track assuming the scalability of the software is good. 

Expensive system resources can be more effectively managed. For example, the middle tier can create a pool of database connections that is shared by all clients. It can also implement a complex data caching capability to boost performance. 

Because the middle tier handles concurrent requests from many user sessions, architects and developers are forced to consider threading issues that are considered difficult for many junior developers. 

A more robust solution is to introduce a load balancer between die clients and a farm of identical middle tier servers. The load balancer routes requests to one of the middle tier servers based on the load and netweork topology. If one middle tier server fails, it can reroute the request to another to prevent single point of failure (fail-over). This architecture requires that the servers in the middle tier farm synchronize user sessions with each other in case one has to take over a client from another. 

MDL Core Interface middle tier integration environment... 

Data for a selection of chemistry software vendors are displayed in Figure 21. Shown are the total number of papers that mentioned the names of these companies in CJACS for the entire 12-year period, 1982—1993. At the top, with more than 200 papers each, are the three large modeling companies. Tripos, MSI, and BIOSYM. Note that MDL Information Systems ranks second after Tripos in terms of literature citations. Besides these big four, Serena Software, Chemical Design Ltd., and Cambridge Scientific Computing fall in a middle tier. There is also a fairly broad representation of other software companies receiving mention in the papers of the ACS journals. 

Middle Tier. Large-scale database applications often consist of three "tiers" of programs (1 )the application tier, which interacts... 

Multi-tier Architecture. An expansion of a client-server architecture to include a middle layer of software. The middle tier may run on a computer different from either the client or server computers. The middle tier isolates the client and server programs, so that changes in either of them do not require corresponding changes in the other. The middle tier acts as to receive, authenticate, and transform data as it passes between client and server computers. To make middle tier software easy to change and maintain, it is often written in Java, a modem object-oriented computer language that is available free on most of today s computer platforms. 

Ten years ago the European market was dominated by machines that heated the water and had stainless steel interiors. The North American market was dominated by machines that received hot water from the house plumbing, did not heat the water, and had plastic interiors. Today, high- and middle-tier North American machines are sensor controlled (water temperature and wash time) and come in stainless steel options (Figure 9.1). 

On the liability side, the structure will have tranches whose allocations of income and loss are predefined. From most junior to most senior, the tranches include equity, mezzanine, senior, and super-senior. Equity, or first loss, is analogous to the equity in a company s balance sheet, and represents the residual interest remaining after other liabilities are paid off. The mezzanine, or middle tier of risk, typically represents low-rated tranches all the way up to AA risk. Senior usually refers to AA and AAA risk. Super-senior represents a level of risk that is deeply subordinated (usually by at least 10%) to AAA, and as its name implies is regarded as extremely safe. In a structure that is not uncommon, we will assume all of the liability tranches are unfunded except for the equity. ... 

The scope is easily defined if an analytical tree is used as a feeder document (Chapter 10). The indenture levels can be identified by specifying the number of tiers of the tree to be included in the analysis (limits of resolution). A functional FMEA includes upper and/or middle tiers only a hardware FMEA includes the entire tree. 

Some countries, such as Greece and Portugal, are just introducing natural gas and offer considerable scope for gas consumption, if the required infrastructure can be developed. Finally there is a middle tier of underdeveloped markets, with the share of gas ranging from 6% in Spain to 13-20% in some of the more mature markets listed earlier (France, Germany and Belgium). The market share of gas in Spain is low because gas was only recently introduced but the market should now expand rapidly, with the development of the appropriate infrastructure. 

The main factors of the technogenic changes of the middle tier are creation of pile basements and basements of deep foundations, dewatering, complex of underground structures, change of Stress-Strain State (SSS) of soil massif and formation of natural-technogenic mode of ground water. 

Implemented pilot supply confederations as recommended in the May 2002 policy document Modernizing Supply in the NHS to develop a middle tier between national (PASA) and local (individual NHS trust) level purchasing. 

---