- Why This Chapter Is Important
- Understanding the Client/Server Model
- Deciding Whether to Use the Client/Server Model
- The Roles Access Plays in the Application Design Model
- Learning the Client/Server Buzzwords
- Upsizing: What to Worry About
- Proactively Preparing for Upsizing
- Using the Upsizing Wizard
- Defining an ODBC Data Source
- Connecting to a Database Server
Deciding Whether to Use the Client/Server Model
Client/server technology was not as necessary when there was a clear delineation between mainframe applications and personal computer applications. Today, the line of demarcation has blurred. Personal computer applications have taken over many applications that had been relegated to mainframe computers in the past. The problem is that users still are very limited by the bandwidth of network communications. This is one place where client/server technology can really help.
However, many developers are confused about what client/server architecture really is. Some mistakenly believe that an Access MDB database file stored on a file server acts as a database server. This is not the case. (In fact, I have participated in many debates in which other developers have insisted that Access itself is a database server application. Well, it's not.) Access is a front-end application that can process data stored on a back end. In this scenario, the Access application runs on the client machine accessing data stored on a database server running software such as Microsoft SQL Server. Access does an excellent job acting as the client-side, front-end software in this scenario. The confusion is about Access's capability to act as a database server.
The key factor is the way that Access retrieves data when it acts as the front end to a database server versus when the data is stored in an Access MDB file. Suppose that you have a table with 500,000 records. A user runs a query based on the 500,000-record table stored in an Access database on a file server. Suppose that the user wants to see a list of all the Californians who make more than $75,000 per year. With the data stored on the file server in the Access MDB file format, Access sends all records over the network to the workstation and performs the query on the workstation. (See Figure 3.1.) This results in significant network traffic.
Figure 3.1 Access as a front end utilizing data stored in an Access database.
On the other hand, assume that these 500,000 records were stored on a database server such as Microsoft SQL Server. If user runs the same query, SQL Server sends only the names of the Californians who make more than $75,000 per year over the network. In this scenario, SQL Server retrieves only the specific fields requested. (See Figure 3.2.)
What does this mean to you? When should you become concerned with client/server technology and what it can offer you? The following sections present some guidelines on why you might want to upsize from an Access back end to a SQL Server back end.
Figure 3.2 Access as a front end using a true back end.
Dealing with a Large Volume of Data
As the volume of data in your Access database increases, you probably will notice a degradation in performance. Many people say that 100MB is the magical number for the maximum size of an Access database, but many back-end database servers can handle databases containing multiple gigabytes of data. Although a maximum size of 100MB for an Access database is a good general guideline, it is not a hard-and-fast rule. You might find that the need to upsize occurs when your database is significantly larger or smaller than 100MB. The magic number for you depends on all the factors discussed in the following sections, as well as on how many tables are included in the database. Generally, Access performs better with large volumes of data stored in a single table rather than in multiple tables.
Dealing with a Large Number of Concurrent Users
Just as a large volume of data can be a problem, so can a large number of concurrent users. In fact, more than 10 users concurrently accessing an Access database can degrade performance. As with the amount of data, this is not a magic number. I have seen applications with fewer than 10 users whose performance is awful, and I have seen applications with significantly more than 10 users whose performance is acceptable. It often depends on the design of the application as well as what tasks the users are performing.
Demanding Faster Performance
Certain applications demand better performance than other applications. An online transaction processing (OLTP) system generally requires significantly better performance than a decision support system (DSS), for example. Suppose that 100 users are simultaneously taking phone orders. It would not be appropriate for the users of the system to ask their customers to wait 15 seconds between entering each item that is ordered. On the other hand, asking users to wait 60 seconds to process a management report that users run once each month is not a lot to ask (although many still will complain about the minute).
Most back-end database servers can use multithreaded operating systems with multiple processors to handle large volumes of user demand; Access cannot.
Handling Increased Network Traffic
As a file server in an organization experiences increasing demands, the Access application simply might exacerbate an already growing problem. By moving the application data to a database server, the overall reduced demands on the network might provide all users on the network with better performance, regardless of whether they are using the Access application.
Probably one of the most exaggerated situations I have seen is one in which all the workstations were diskless. Windows and all application software were installed on a file server. All the users were concurrently loading Microsoft Word, Microsoft Excel, and Microsoft PowerPoint over the network. In addition, they had large Access applications with many database objects and large volumes of data. All this was stored on the file server as well. Needless to say, performance was abysmal. You can't expect an already overloaded file server to handle sending large volumes of data over a small bandwidth. The benefits offered by client/server technology can help alleviate this problem.
Implementing Backup and Recovery
The backup and recovery options offered with an Access MDB database stored on a file server simply do not rival the options for backup and recovery on a database server. Any database server worth its salt sports very powerful uninterruptible power sources (UPSs). Many have hot-swappable disk drives with disk mirroring, disk duplexing, or disk striping with parity (RAID Level 5). With disk mirroring and duplexing, data can be written to multiple drives at one time, providing instantaneous backups. Furthermore, some database server tape-backup software enables you to complete backups while users are accessing the system. Many offer automatic transaction logging. All these options mean less chance of data loss or downtime. With certain applications, this type of backup and recovery is overkill. With other applications, it is imperative. Although you can mimic some of what database servers have to offer in backup and recovery by using code and replication, it is nearly impossible to get the same level of protection from an Access database stored on a file server that you can get from a database stored on a database server.
Focusing on Security
Access offers what can be considered the best security for a desktop database. However, it cannot compare with the security provided by most database servers. Database server security often works in conjunction with the network operating system. This is the case, for example, with Microsoft SQL Server and Windows NT Server or Windows 2000. The user is given no direct rights to the physical database file; it can be accessed only via an Open Database Connectivity (ODBC) data source or an ADO connection. Remember that, no matter how much security you place on an Access database, this does not prevent a user from seeing or even deleting the entire MDB file from the network disk.
It is very easy to offer protection from this potential problem and others on a database server. Furthermore, many back-end application database server products offer field-level security not offered within an Access MDB file. Finally, many back ends offer integrated security with one logon for both the network and the database.
Sharing Data Among Multiple Front-End Tools
The Access MDB file format is proprietary. Very few other products can read data stored in the Access database format. With a back-end database server that supports ODBC or OLE DB, developers can write front-end applications in a variety of front-end application software, all concurrently using the same back-end data.
Understanding What It All Means
You must evaluate the specific environment in which your application will run:
How many users are there?
How much data exists?
What is the network traffic already like?
What type of performance is required?
How disastrous is downtime?
How sensitive is the data?
What other applications will use the data?
After you answer these and other questions, you can begin to decide whether the benefits of the client/server architecture outweigh the costs involved.
The good news is that it is not an all-or-nothing decision. Various options are available for client/server applications using Access as a front end. Furthermore, if you design your application with upsizing in mind, moving to client/server technology will not require you to throw out what you have done and start again. In fact, Microsoft provides an upsizing wizard that makes upsizing to a SQL Server database a relatively painless process. How painless depends on numerous factors, including how complex your queries are, whether your queries include VBA functions, and other factors that are covered later in this chapter and throughout the book.