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9: The Client/Server Database Environment

Charles E. Oyibo

Introduction

In the previous chapters, we discussed the database development activities involved during the systems development life cycle:

We are now at the last phase of the system development life cycle: implementation. At the end of implementation, we expect a functioning system that meets users’ information requirements. In this chapter, we review developments in multi-user database management environments that have led to the development of various client/server strategies for data processing.

Client Server Systems: A networked computing model that distributes processes between clients (front ends) and servers (back ends), which supply the requested services. In a database system, the database generally resides on a server that processes the DBMS. The clients may process the application systems or request services from another server that holds the application programs.

Client/Server Architectures

The server client/server architechtures that have evolved can be distinguished by the distribution of application logic components across clients and servers. There are three components of the application logic:

  1. the input/output (I/O) or presentation logic component: responsible for formatting and presenting data on the user's screen or other output device, and for managing input from a keyboard or other input device.
  2. the processing logic component: which handles:
    1. data processing logic: e.g.: data validation and identification of processing errors,
    2. business rules logic
    3. data management logic: which identifies the data necessary for processing a transaction or query
  3. the storage logic component: which is responsible for data storage and retrieval from the physical storage devices associated with the application. Activities of a DBMS occur in the storage logic component

File Server Architecture

The first client/server architectures developed were file servers. In a basic file/server environment, all data manipulation occurs at the workstations where data is requested: the client handles the presentation logic, processing logic, and much of the storage logic (that part associated with the DBMS). One or more file servers that manage file operations are attached to the LAN, and are shared by the clients.

File server: A device that manages file operations and is shared by each of the client PCs attached to the LAN.

Fat Client: A client PC that is responsible for processing logic, extensive application and business rules logic and many DBMS functions.

Limitations of File Servers

  1. Cosiderable data movement is generated across the network
  2. Each client network must devote memory to a full version of the DBMS; also, each client must be rather powerful to provide suitable response times
  3. The DBMS copy in each workstation must manage the shared database integrity; each application must recognize, for example, locks and take care to initiate the proper locks. Hence, increasing the work of application programmers.

Database Server (Two-Tier) Architectures

Database server: A computer that is responsible for database storage, access, and processing in a client/server environment. Sometimes used to describe a two-tier client/server environment.

In two-tiered approaches, the client workstation is reponsible for managing the user interface, including presentation logic, data processing logic, and business rules logic; the database server is responsible for database storage, access, and processing. Hence, LAN traffic is reduced because only those records that match the requested criteria are transmitted to the client, rather than entire data files. Other advantages:

  1. Only the database servers requires processing power adequate to handle the database; database server can be tuned to optimize data-processing performance. Clients do not generally have to be as powerful.
  2. User authorization, integrity checking, data dictionary maintenance, query and update processing are all performed in one location: the database server.
  3. Possible use of stored procedures.

Three-Tier Architectures

Three-tier architecture: A client/server configuration that includes three layers: a client layer and two server layers. While the nature of the server layers differ, common configuration contains an application server.

Application server: A computer than stores application programs (processing logic and business rules logic).

In some three-tier client architectures, most application code is stored on the application server, and most business processing occurs on the application server rather than on the client workstation or database server, resulting in a thin client

Thin client: a PC configured for handling user interfaces and some application processing, usually with no or limited local data storage.

The three-tier architecture provides the following advantages:

Challenges associated with three- (and n-) tier architectures are enumerated onp. 327-328

Partitioning an Application

Obviously there is no one optimal client/server architecture that is the best solution for all business problems. In most cases though, presentation logic resides on the client, where the user interfaces with the system. Processing logic may be divided across clients and servers. Storage logic usually resides on the database server, close to the physical location of the data. Data integrity control activities, such as constraint checking, are typically placed in the database server; and so are triggers which will always fire when preset conditions (like commands to insert, delete, or update data) are met.

Application partitioning: The process of assigning portions of application code to client or server partitions after it is written, in order to achieve better performance and interoperability (the ability of a component to function on different platforms).

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Using Parallel Computer Architectures

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Using Middleware

Hereunder is a classification system for middleware (based on scalability and recoverability);

In client/server systems, database-oriented middleware provides some sort of application program interface (API) access to a database.

Application program interface (API): Set of routines that an application program uses to direct the performance of procedures by the computer's operating system.

For example, in achieving access to a database, an API calls library routines that transparently route SQL commands from the front-end client application to the database server.

Open database connectivity (ODBC) is similar to API, but for Windows-based client/server applications. It is most useful for accessing relational data, but not well suited for handling other types of data, such as ISAM files. Though it is difficult to program and implement, ODBC has been well accepted because it allow programmers to make connection to almost any vendor's database without learning proprietary code specific to that database. Microsoft's OLE-DB adds value to the ODBC standard by providing a single point of access to multiple databases...

Java Database Connectivity (JDBC) classes can be used to help an applet access any number of databases without understanding the native features of each database. JDBC defines a call-level interface (CLI) for Java development and borrows from ODBC conventions...

Establishing a common language to define the interface between components and a mechanism to mediate will facilitate developing universal middleware. The Object Management Grouo (OMG) is an industry coalition that has produced the Common Object Request Broker Architecture (CORBA), which sets the specification of object-oriented universal middleware. Microsoft has developed a competing model, Distributed Component Object Model (DCOM), but CORBA is a more robust specification because it has been developed to handle many different platforms...

Establishing Client/Server Security

  1. System-level password security: usernames and passwords (to include guidelines for password lenght, password naming conventions, frequency of password changes, etc.)
  2. Database-level password security
  3. Secure client/server communication: encryption, transforming readable data (plain text) into unreadable data (ciphertext)

Client/Server Issues

In order to improve the chances for building a successful client/server application, certain areas should be carefully addressed:

Database Access from Client Applications

Partitioning the environment to create two-, three-, and n-tier architecture means that decisions must be made about the placement of processing logic. (We note that, generally, storage logic (the database engine) is handled by the database server, and the presentation logic is handled by the client.)

Common Logic Distributions

(a) Two-tier client/server environments

Server Storage Logic
Client Processing Logic
Presentation Logic

Fat Client
Server Storage Logic
Processing Logic
Client Presentation Logic

Thin Client
Server Storage Logic
Server & Client Processing Logic
Client Presentation Logic

Distributed

(b) Example: a Web-enabled n-tier client/server environmen t

Database Server Storage Logic DBMS, OS, SQL, TCP/IP
Application Server Processing Logic App/Server, SQL, TCP/IP, OS
Web Server Processing Logic HTTP, CGI, TCP/IP,OS, App/Server API
Client Presentation Logic Browser, HTTP, TCP/IP, OS

Using Query-by-Example

Query-by-Example (QBE): A direct manipulation database language that uses a graphical approach to query construction.

QBE: History and Importance

QBE: The Basics

Microsoft Access™ Usability Hierachy

The hierarchy conveys that Access will be usable at all levels of programming ability and complexity.

  1. At the lowest level are objects, which allow the creation tables, queries, forms, and reports without any specific programming knowledge.
  2. The use of expressions or functions to perform simple processes such as multiplication of fields, validation of data, or enforcement of a specific business rule is possible at the next pyramic level.
  3. At the next level, users can take advantage of stored modules of Visual Basic for Applications (VBA) code, called macros, to automate their applications, again without explicit knowledge of VBA
  4. At the next level, users can program their own modules of VBA code to customer tailor their own applications.
  5. At the top level, Windows API calls to functions or DLLs written in languages such as C, Java, or Visual Basic that can be used to write interfaces to other programs and sources of data.

Other QBE topics discussed:

Using ODBC to Link External Tables Stored on a Database Server

Open database connectivity (ODBC) standard: An application programming interface (API) that provides a common language for application programs to access and process SQL databases independent of the particular RDBMS that is accessed. Such RDBMSs are said to be ODBC-compliant.

Using JDBC to Link External Tables Stored on a Database Server

See sample code to use JDBC to access as JDBC-compliant database on p. 351-353.

Using Visual Basic for Applications (VBA) in Client Applications

Visual Basic for Applications (VBA): The programming language that accompanies Access 2000 (as well as some other Office software, like Excel, right?)

Access 2000 allows us to create a prototype of an application including the database design, menus, forms, and reports. However, there is limit to what we can accomplish using the Access macros that we have access to. Here are the reasons why we would want to learn VBA once we have achieved basic familiarity with Access.

With a few exceptions, converting a prototype from macros to VBA modules for a production version of the application will result in a more robust application with greater capabilities.

As we already know, the Windows OS and Windows applications are event-driven.

Event-driven: Nonprocedural programming that detect an event when it occurs and generates an appropriate response to that event.

Programming in an event -driven environment consists of creating objects and modifying their properties so that they behave as we want them to for each events that affects them.

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Charles E. Oyibo
IDS :: CBA :: UIC