Something I've been learning (and teaching) in Software Engineering is that code duplication is the root of all evil. On the other hand, I find it quite hard to explain how this concept should be applied to the development of web apps.

Allow me to clarify... Input & data validation can be an important part of a web app. Sometimes this validation can be quite plex. For example, I worked on a puzzle editor and the validation consisted of checking whether an operation or a move was valid. Non-trivial rules then had to be checked.

Naturally, validation must be done server-side in order to ensure the consistency and quality of the stored data. However, it's a must to do validation client-side to ensure a smooth user experience.

In most instances, client-side and server-side code are written in different languages (i.e. javascript/Python), so validation code has to be written twice. However, in my only experience with GWT/Java (Java on both sides), I found that a large portion of the validation code could be reused. This seemed to make everything easier: maintenance, refactoring, debugging...

So my question to you is: how do you manage issues related to code duplication in projects where the client-side and server-side languages are different?

Something I've been learning (and teaching) in Software Engineering is that code duplication is the root of all evil. On the other hand, I find it quite hard to explain how this concept should be applied to the development of web apps.

Allow me to clarify... Input & data validation can be an important part of a web app. Sometimes this validation can be quite plex. For example, I worked on a puzzle editor and the validation consisted of checking whether an operation or a move was valid. Non-trivial rules then had to be checked.

Naturally, validation must be done server-side in order to ensure the consistency and quality of the stored data. However, it's a must to do validation client-side to ensure a smooth user experience.

In most instances, client-side and server-side code are written in different languages (i.e. javascript/Python), so validation code has to be written twice. However, in my only experience with GWT/Java (Java on both sides), I found that a large portion of the validation code could be reused. This seemed to make everything easier: maintenance, refactoring, debugging...

So my question to you is: how do you manage issues related to code duplication in projects where the client-side and server-side languages are different?

• javascript
• gwt
• client-server

• 2 With respect to validation, see stackoverflow./questions/39628/… and stackoverflow./questions/778726/… – Roatin Marth Commented Dec 15, 2009 at 18:34
• 2 Thanks, very relevant posts, which I somehow missed. – Philippe Beaudoin Commented Dec 15, 2009 at 18:38

3 Answers 3

The way I usually handle this is to write the validation code on the server side and expose it via a web method (in .NET, similar functionality exists in most other languages) so that it can be called from javascript. As a result, you have a single method that can be called both synchronously and asynchronously from the client side and also called from the server side. This isn't applicable in every case but it's worked very well for me so far.

Typically, it's really hard to avoid duplicating the generated code, but a mon approach is to use a code generator to build either the server or client side code so you only code one half of it. The most popular approach is writing the server-side mon and then having the code generator build the JavaScript code for you. For instance, the language we use at my pany is Coldfusion and Form-o-matic solves that problem for us. People have also approach the problem from the opposite direction by writing JavaScript which can be executed server side. I'd look for a framework that will do this for you.

A possible solution is that you abstract the actual validation in a validation description file (in XML or any other means) using a DSL. This way you only need to implement the validation engine in both client and server language and base the validations on the same description file.