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Validation and Error Reporting for Web Database Applications
Server-Side Validation
Client-Side Validation with JavaScript
Validation is essential to web database applications. Ensuring that data meets user and system requirements is important, but ensuring that the database constraints are met by the data is critical. There are three possible data environments in which validation can occur in a three-tiered web database application: in the DBMS, in server-side scripts, and on the client. We discuss the merits and possibilities of these approaches to validation in this chapter.
As the name suggests, client-tier validation occurs at the client browser before a request is sent to the server and is usually validation of <form> data. The most common way to implement client-tier validation is using the scripting language best known as JavaScript. JavaScript isn't a fully fledged programming language, but it's one that can be effectively used for simple tasks such as validation. The drawback of validation at the client is that it depends on the user and his environment: the user can disable JavaScript, and can willfully or passively circumvent the validation, and the client environment isn't usually managed or standardized by the developer of the web database application.
Server-side validation is usually performed in a middle-tier script and is the essential validation tool. When data is inserted, updated, or deleted at the DBMS, it's undesirable to rely on the constraint-checking validation implicitly performed by the DBMS in the database tier. Trapping errors using the PHP MySQL error functions is difficult, has unnecessary network and DBMS overhead, and is hard to present to the user in a meaningful way. A much better approach is to use the middle-tier PHP scripts to validate data and ensure that all constraints of the database are met before modifying the database.
In this chapter, we extend our discussion of validation in PHP. We have already introduced basic validation principles in Chapter 5 with the clean( ) function for security and in Chapter 6 with the field empty( ) checks used before modifying the customer table. We extend those discussions here by introducing the principles of validation and the practice of validating <form> variables and values with PHP. We use the customer <form> we developed in Chapter 6 as our case study. We then consider in more detail the variables and values that are sent from a browser to a server, their variations, and the traps to watch for.
After discussing server-side validation, we discuss client-side JavaScript and how simple validation can be performed at the client to save network costs and improve responsiveness of an application to the user. We also introduce other simple tasks that can be effectively accomplished with JavaScript.
There is nothing worse for a user than annoying, overly persistent, inaccurate, or uninformative validation. For example, error messages that describe an error but don't specify which field contains the error are difficult to correct. However, there is no correct recipe for balancing validation with system requirements: what is pleasing or mandated by requirements in one application might be annoying or useless in another. In this section, we consider practical validation models for web database applications.
Validation is actually two processes: finding errors and presenting error messages. Finding errors can be interactive—where data is checked as it's entered—or it can be post-validation, where the data is checked after entry. Presenting errors can be field-by-field—where a new error message is presented to the user for each error found—or it can be batched, where all errors are presented as a single message. There are other dimensions to validation and error processing, such as the degree of error that is tolerated and the experience level of the user. However, considering only the basic processes, the choice of when to error-check and when to notify the user, leads to four common approaches:
In Chapter 6—without discussing the details—we covered several simple post-validation techniques to check whether mandatory <form> data was entered before inserting or updating data in the database. In addition, we used a batch reporting method, where errors were reported as a list by constructing an error string. In the case study example in this chapter, we discuss additional validation for the customer <form> data to more carefully inspect both mandatory and optional fields. The completed validation code is listed in Chapter 10.
Interactive models are difficult to implement in the web environment. Server-side scripts are impractical for this task, since an HTTP request and response is required to validate each field that's entered. This is usually unacceptable, because the user is required to submit the data after entering each field, response times are likely to be slow, and the server load high.
Client-side scripts can implement an interactive model. However, validation on the client should not be the only method of validation because—as we emphasized in Chapter 5—the user can passively or actively avoid the client-side processes. We discuss the partially interactive solution of including client-side scripts with an HTML <form> later in this chapter.
Post-validation models are practical in web database applications. Both client- and server-side scripts can validate all <form> data during the submission process. In many applications, reasonably comprehensive validation is performed on the client side when the user clicks the <form> submit button. If this validation succeeds, data is submitted to the server and the same—or more comprehensive—validation is performed. Duplicating client validation on the server is essential because of the unreliability of client-side scripts and lack of control over the client environment.
TIP: Client-side validation reduces server and network load, because the user's browser ensures the data is valid prior to the HTTP request. Client-side validation is also usually faster for the user.
The post-validation model can be combined with either field-by-field or batch error reporting. For server-side validation, the batch model is preferable to a field-by-field implementation, as the latter approach has more overhead and is usually slower because each <form> error requires an additional HTTP request and response.
For client-side post-validation, either error-reporting model can be used. The advantage of the field-by-field model is that the cursor can be directed to the field containing the error, making error correction easier. The disadvantage is that several errors require several error messages, and this can be frustrating for the user. The advantage of the batch approach is that all errors are presented in one message. The disadvantage is that the cursor can't easily be directed to the field requiring correction.
WARNING: Server-side validation is essential to secure a web database and to ensure that system and DBMS constraints are met.Client-side validation may be implemented in addition to server-side validation, but all client-side functionality should be duplicated at the server side. Never trust the user or the client browser.
The choice of which reporting model to use depends on the size and complexity of the <form> and on the system requirements.
In the next section, we introduce the practice of server-side post-validation using the batch error reporting method. We introduce client-side scripting as a tool for validation and error reporting in Section 7.3.
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| 6.2. Issues in Writing Data to Databases |  | 7.2. Server-Side Validation |
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