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					Two Tier Software Architectures

Purpose and Origin
Two tier software architectures were developed in the 1980s from the file server software
architecture design. The two tier architecture is intended to improve usability by
supporting a forms-based, user-friendly interface. The two tier architecture improves
scalability by accommodating up to 100 users (file server architectures only accommodate
a dozen users), and improves flexibility by allowing data to be shared, usually within a
homogeneous environment [Schussel 96]. The two tier architecture requires minimal
operator intervention, and is frequently used in non-complex, non-time critical information
processing systems. Detailed readings on two tier architectures can be found in Schussel
and Edelstein [Schussel 96, Edelstein 94].

Technical Detail
Two tier architectures consist of three components distributed in two layers: client (requester of
services) and server (provider of services). The three components are

    1. User System Interface (such as session, text input, dialog, and display management
       services)
    2. Processing Management (such as process development, process enactment, process
       monitoring, and process resource services)
    3. Database Management (such as data and file services)

The two tier design allocates the user system interface exclusively to the client. It places
database management on the server and splits the processing management between client and
server, creating two layers. Figure 38 depicts the two tier software architecture.




Figure 38: Two Tier Client Server Architecture Design [Louis 95]

In general, the user system interface client invokes services from the database management
server. In many two tier designs, most of the application portion of processing is in the client
environment. The database management server usually provides the portion of the processing
related to accessing data (often implemented in store procedures). Clients commonly
communicate with the server through SQL statements or a call-level interface. It should be noted
that connectivity between tiers can be dynamically changed depending upon the user's request
for data and services.

As compared to the file server software architecture (that also supports distributed systems), the
two tier architecture improves flexibility and scalability by allocating the two tiers over the
computer network. The two tier improves usability (compared to the file sever software
architecture) because it makes it easier to provide a customized user system interface.
It is possible for a server to function as a client to a different server- in a hierarchical client/server
architecture. This is known as a chained two tier architecture design.

Usage Considerations
Two tier software architectures are used extensively in non-time critical information processing
where management and operations of the system are not complex. This design is used frequently
in decision support systems where the transaction load is light. Two tier software architectures
require minimal operator intervention. The two tier architecture works well in relatively
homogeneous environments with processing rules (business rules) that do not change very often
and when workgroup size is expected to be fewer than 100 users, such as in small businesses.

Three Tier Software Architectures

Purpose and Origin
The three tier software architecture (a.k.a. three layer architectures) emerged in the 1990s to
overcome the limitations of the two tier architecture (see Two Tier Software Architectures). The
third tier (middle tier server) is between the user interface (client) and the data management
(server) components. This middle tier provides process management where business logic and
rules are executed and can accommodate hundreds of users (as compared to only 100 users
with the two tier architecture) by providing functions such as queuing, application execution, and
database staging. The three tier architecture is used when an effective distributed client/server
design is needed that provides (when compared to the two tier) increased performance, flexibility,
maintainability, reusability, and scalability, while hiding the complexity of distributed processing
from the user. For detailed information on three tier architectures see Schussel and Eckerson.
Schussel provides a graphical history of the evolution of client/server architectures [Schussel 96,
Eckerson 95].

The three tier architecture is used when an effective distributed client/server design is needed
that provides (when compared to the two tier) increased performance, flexibility, maintainability,
reusability, and scalability, while hiding the complexity of distributed processing from the user.
These characteristics have made three layer architectures a popular choice for Internet
applications and net-centric information systems.

Technical Detail
A three tier distributed client/server architecture (as shown in Figure 28) includes a user system
interface top tier where user services (such as session, text input, dialog, and display
management) reside.
Figure 28: Three tier distributed client/server architecture depiction [Louis 95]

The third tier provides database management functionality and is dedicated to data and file
services that can be optimized without using any proprietary database management system
languages. The data management component ensures that the data is consistent throughout the
distributed environment through the use of features such as data locking, consistency, and
replication. It should be noted that connectivity between tiers can be dynamically changed
depending upon the user's request for data and services.

The middle tier provides process management services (such as process development, process
enactment, process monitoring, and process resourcing) that are shared by multiple applications.

The middle tier server (also referred to as the application server) improves performance,
flexibility, maintainability, reusability, and scalability by centralizing process logic. Centralized
process logic makes administration and change management easier by localizing system
functionality so that changes must only be written once and placed on the middle tier server to be
available throughout the systems. With other architectural designs, a change to a function
(service) would need to be written into every application [Eckerson 95].

In addition, the middle process management tier controls transactions and asynchronous queuing
to ensure reliable completion of transactions [Schussel 96]. The middle tier manages distributed
database integrity by the two phase commit process (see Database Two Phase Commit). It
provides access to resources based on names instead of locations, and thereby improves
scalability and flexibility as system components are added or moved [Edelstein 95].

Sometimes, the middle tier is divided in two or more unit with different functions, in these cases
the architecture is often referred as multi layer. This is the case, for example, of some Internet
applications. These applications typically have light clients written in HTML and application
servers written in C++ or Java, the gap between these two layers is too big to link them together.
Instead, there is an intermediate layer (web server) implemented in a scripting language. This
layer receives requests from the Internet clients and generates html using the services provided
by the business layer. This additional layer provides further isolation between the application
layout and the application logic.

It should be noted that recently, mainframes have been combined as servers in distributed
architectures to provide massive storage and improve security (see Distributed/Collaborative
Enterprise Architectures).


                  3- and n-Tier Architectures

Contents
       Introduction
       Why 3-tier
       What is 3-tier-architecture
       Advantages
       Critical Success Factors
Introduction
Through the appearance of Local-Area-Networks, PCs came out of their isolation, and
were soon not only being connected mutually but also to servers. Client/Server-
computing was born.

Servers today are mainly file and database servers; application servers are the exception.
However, database-servers only offer data on the server; consequently the application
intelligence must be implemented on the PC (client). Since there are only the
architecturally tiered data server and client, this is called 2-tier architecture. This model is
still predominant today, and is actually the opposite of its popular terminal based
predecessor that had its entire intelligence on the host system.

One reason why the 2-tier model is so widespread, is because of the quality of the tools
and middleware that have been most commonly used since the 90’s: Remote-SQL,
ODBC, relatively inexpensive and well integrated PC-tools (like Visual Basic, Power-
Builder, MS Access, 4-GL-Tools by the DBMS manufactures). In comparison the server
side uses relatively expensive tools. In addition the PC-based tools show good Rapid-
Application-Development (RAD) qualities i.e. that simpler applications can be produced
in a comparatively short time. The 2-tier model is the logical consequence of the RAD-
tools’ popularity : for many managers it was and is simpler to attempt to achieve
efficiency in software development using tools, than to choose the steep and stony path of
"brainware".

Why 3-tier?
Unfortunately the 2-tier model shows striking weaknesses, that make the development
and maintenance of such applications much more expensive.

       The complete development accumulates on the PC. The PC processes and
        presents information which leads to monolithic applications that are expensive to
        maintain. That’s why it’s called a "fat client".
       In a 2-tier architecture, business-logic is implemented on the PC. Even the
        business-
       logic never makes direct use of the windowing-system, programmers have to be
        trained for the complex API under Windows.
       Windows 3.X and Mac-systems have tough resource restrictions. For this reason
        applications programmers also have to be well trained in systems technology, so
        that they can optimize scarce resources.
       Increased network load: since the actual processing of the data takes place on the
        remote client, the data has to be transported over the network. As a rule this leads
        to increased network stress.
       How to conduct transactions is controlled by the client. Advanced techniques like
        two-phase-committing can’t be run.
       PCs are considered to be "untrusted" in terms of security, i.e. they are relatively
        easy to crack. Nevertheless, sensitive data is transferred to the PC, for lack of an
        alternative.
       Data is only "offered" on the server, not processed. Stored-procedures are a form
        of assistance given by the database provider. But they have a limited application
        field and a proprietary nature.
       Application logic can’t be reused because it is bound to an individual PC-
        program.
       The influences on change-management are drastic: due to changes in business
        politics or law (e.g. changes in VAT computation) processes have to be changed.
        Thus possibly dozens of PC-programs have to be adapted because the same logic
        has been implemented numerous times. It is then obvious that in turn each of
        these programs have to undergo quality control, because all programs are
        expected to generate the same results again.
       The 2-tier-model implies a complicated software-distribution-procedure: as all of
        the application logic is executed on the PC, all those machines (maybe thousands)
        have to be updated in case of a new release. This can be very expensive,
        complicated, prone to error and time consuming. Distribution procedures include
        the distribution over networks (perhaps of large files) or the production of an
        adequate media like floppies or CDs. Once it arrives at the user’s desk, the
        software first has to be installed and tested for correct execution. Due to the
        distributed character of such an update procedure, system management cannot
        guarantee that all clients work on the correct copy of the program.

3- and n-tier architectures endeavour to solve these problems. This goal is achieved
primarily by moving the application logic from the client back to the server.




What is 3- and n-tier architecture?
From here on we will only refer to 3-tier architecture, that is to say, at least 3-tier
architecture.

The following diagram shows a simplified form of reference-architecture, though in
principal, all possibilities are illustrated.



Client-tier

Is responsible for the presentation of data, receiving user events and controlling the user
interface. The actual business logic (e.g. calculating added value tax) has been moved to
an application-server. Today, Java-applets offer an alternative to traditionally written PC-
applications. See our Internet-page for further information.
Application-server-tier

This tier is new, i.e. it isn’t present in 2-tier architecture in this explicit form. Business-
objects that implement the business rules "live" here, and are available to the client-tier.
This level now forms the central key to solving 2-tier problems. This tier protects the data
from direct access by the clients.

The object oriented analysis "OOA", on which many books have been written, aims in
this tier: to record and abstract business processes in business-objects. This way it is
possible to map out the applications-server-tier directly from the CASE-tools that support
OOA.

Furthermore, the term "component" is also to be found here. Today the term pre-
dominantly describes visual components on the client-side. In the non-visual area of the
system, components on the server-side can be defined as configurable objects, which can
be put together to form new application processes.

Data-server-tier

This tier is responsible for data storage. Besides the widespread relational database
systems, existing legacy systems databases are often reused here.

It is important to note that boundaries between tiers are logical. It is quite easily possible
to run all three tiers on one and the same (physical) machine. The main importance is that
the system is neatly structured, and that there is a well planned definition of the software
boundaries between the different tiers.

The advantages of 3-tier architecture
As previously mentioned 3-tier architecture solves a number of problems that are inherent
to 2-tier architectures. Naturally it also causes new problems, but these are outweighed by
the advantages.

      Clear separation of user-interface-control and data presentation from application-
       logic. Through this separation more clients are able to have access to a wide
       variety of server applications. The two main advantages for client-applications are
       clear: quicker development through the reuse of pre-built business-logic
       components and a shorter test phase, because the server-components have already
       been tested.
      Re-definition of the storage strategy won’t influence the clients. RDBMS’ offer a
       certain independence from storage details for the clients. However, cases like
       changing table attributes make it necessary to adapt the client’s application. In the
       future, even radical changes, like let’s say switching form an RDBMS to an
       OODBS, won’t influence the client. In well designed systems, the client still
       accesses data over a stable and well designed interface which encapsulates all the
       storage details.
     Business-objects and data storage should be brought as close together as possible,
      ideally they should be together physically on the same server. This way -
      especially with complex accesses - network load is eliminated. The client only
      receives the results of a calculation - through the business-object, of course.
     In contrast to the 2-tier model, where only data is accessible to the public,
      business-objects can place applications-logic or "services" on the net. As an
      example, an inventory number has a "test-digit", and the calculation of that digit
      can be made available on the server.
     As a rule servers are "trusted" systems. Their authorization is simpler than that of
      thousands of "untrusted" client-PCs. Data protection and security is simpler to
      obtain. Therefore it makes sense to run critical business processes, that work with
      security sensitive data, on the server.
     Dynamic load balancing: if bottlenecks in terms of performance occur, the server
      process can be moved to other servers at runtime.
     Change management: of course it’s easy - and faster - to exchange a component
      on the server than to furnish numerous PCs with new program versions. To come
      back to our VAT example: it is quite easy to run the new version of a tax-object in
      such a way that the clients automatically work with the version from the exact
      date that it has to be run. It is, however, compulsory that interfaces remain stable
      and that old client versions are still compatible. In addition such components
      require a high standard of quality control. This is because low quality components
      can, at worst, endanger the functions of a whole set of client applications. At best,
      they will still irritate the systems operator.
     As shown on the diagram, it is relatively simple to use wrapping techniques in 3-
      tier architecture. As implementation changes are transparent from the viewpoint
      of the object's client, a forward strategy can be developed to replace legacy
      system smoothly. First, define the object's interface. However, the functionality is
      not newly implemented but reused from an existing host application. That is, a
      request from a client is forwarded to a legacy system and processed and answered
      there. In a later phase, the old application can be replaced by a modern solution. If
      it is possible to leave the business object’s interfaces unchanged, the client
      application remains unaffected. A requirement for wrapping is, however, that a
      procedure interface in the old application remains existent. It isn’t possible for a
      business object to emulate a terminal. It is also important for the project planner
      to be aware that the implementation of wrapping objects can be very complex.




Critical Success Factors
System interface

In reality the boundaries between tiers are represented by object interfaces. Due to their
importance they have to be very carefully designed, because their stability is crucial to
the maintenance of the system, and for the reuse of components.

Architecture can be defined as the sum of important long-term system interfaces. They
include basic system services as well as object-meta-information. In distributed object
systems, the architecture is of great importance. The architecture document is a reference
guideline to which all the developers and users must adhere. If not, an expensive and
time- consuming chaos results.

Security

Here we are dealing with distributed systems, so data-protection and access control is the
important thing. For the CORBA-standard, OMG completed the security.service in
different versions in 1995. In the simplest form (level "0") authentication, authorization
and encryption are guaranteed by Netscape’s secure-socket-layer protocol. Level 1
provides authentication control for security unaware applications. Level 2 is much more
fine-grained. Each message invocation can be checked against an access control list, but
programming by the ORB user is required. There are implementations for all levels
available today.

Transactions

For high availability, in spite of fast processing, transaction mechanisms have to be used.
Standardised OMG interfaces are also present here, and many implementations have been
done. The standard defines interfaces to two-phase-commit and offers new concepts like
nested transactions.

Technical infrastructure

There are currently numerous strategies to realise 3-tier architecture. d-tec GmbH sees
distributed object systems, with CORBA in the lead, as the most efficient concept
currently available. It connects the object oriented paradigm (which proved itself by
abstracting complex systems) to tried and trusted communication technologies like
TCP/IP. In addition to basic functions CORBA offers a multitude of useful and
complementing services.

In order to cover these points and to adjust them, it is necessary to have competent
personnel, who’s aim is to develop systems architecture that is tailor-made and has load
capacity. Tool usage should be kept at a minimum. Not only is it important that the
functionality and performance requirements of the new applications system are met, but -
as a whole - also the aspects of change-management, system-management, the
interoperability between components and security. For the realisation of a distributed
system, all parties involved have to meet, the systems engineer, the applications
developers, the operators and the management. The scale of efficiency isn’t based on a
short-term focused, tools-based approach with a questionable long-term result. The pay-
back is in the long run with low maintenance cost through better designed systems.

				
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