Bridging the Client-Server Divide

webapp-architectureMost software these days is delivered in the form of web applications, and the move towards cloud computing will only emphasize this trend.

Web apps consist of client and server parts, where the client part has been getting bigger lately to deliver a richer user experience.

This split has implications for developers, because the technologies used on the client and server parts are often different.

The client is ruled by HTML, CSS, and JavaScript, while the server is most often developed using JVM or .NET based languages like Java and C#.

Disadvantages of Different Client and Server Technologies

Developers of web applications risk becoming either specialists confined to a single part of the stack or polyglot programmers.

Polyglot programming is the practice of knowing and using many programming languages. There are both advantages and disadvantages associated with polyglot programming. I believe the overriding disadvantage is the context switching involved, which degrades productivity and opens the doors to extra bugs.

Being a specialist has advantages and disadvantages as well. A big disadvantage I see is the “us versus them”, or “not my problem” culture that can arise. In general, Agile teams prefer generalists.

Bringing Server Technologies to the Client

Many attempts have been made at bridging the gap between client and server. Most of these attempts were about bringing server-side technologies to the client.

GWTJava on the client has failed to reached widespread adoption, and now that many people advice to disable Java applets altogether because of security reasons it seems increasingly unlikely that it ever will.

Bringing .NET to the client has likewise failed as Silverlight adoption continues to drop.

Another idea is to translate from server to client technologies. Many languages can now be compiled to JavaScript. The most mature effort is Google Web Toolkit (GWT), which translates from Java. The main problem with GWT is that it supports only a small subset of Java.

All in all I don’t feel there currently is a satisfactory way of using server technologies on the client.

Bringing Client Technologies to the Server

So what about the reverse? There is really only one client-side technology worth looking at today: JavaScript. The only other rival, Flash, is losing out quickly due to lack of support from Apple and the rise of HTML5.

Node.jsJavaScript on the server is starting to make inroads, thanks to the Node.js platform.

It is used by the Cloud9 IDE, for example, and supported by Platform-as-a-Service providers like CloudFoundry and Heroku.

What do you think?

If I had to put my money on any unification approach, it would be Node.js.

Do you agree? What needs to happen to make this a common way of developing web apps? Please let me know your thoughts in the comments.

System dependencies

I’m so happy to be developing in Java. It takes away the drudgery of software development, like memory management. And it frees you of worrying about how certain features are implemented on different platforms: Write Once, Run Anywhere!

You do feel the but coming, don’t you? 😉

Well, sometimes I do run into system dependencies. And since I’m no longer used to it, I don’t expect it anymore. Ah well, this happens only once in a very, very little while. Let me tell you about one such event.

I described in my previous post how I used reflection to extract common code into a base class. I used Class.getMethods(), for which the JavaDoc reads:

public Method[] getMethods() throws SecurityException

Returns an array containing Method objects reflecting all the public member methods of the class or interface represented by this Class object, including those declared by the class or interface and those inherited from superclasses and superinterfaces. Array classes return all the (public) member methods inherited from the Object class. The elements in the array returned are not sorted and are not in any particular order. This method returns an array of length 0 if this Class object represents a class or interface that has no public member methods, or if this Class object represents a primitive type or void.

The interesting part is in italic. This is one of those sentences that you can easily overlook. I know I did.

What does it mean? Nothing more than that the order is undefined in the spec, and so depends on the system (in this case the particular JVM implementation) that you use. We use both Windows and GNU/Linux to test our stuff, but on both we have a Sun JVM. I guess most people will use this one too, since it’s from the makers of Java and it’s free (as in beer and now also as in speech).

But not IBM. For their AIX platform, they have built a custom JVM. And you guessed right: that JVM uses a different order for the methods in the array. Whereas the Sun implementation always gives you methods from the class, then those from it’s base class, etc. the IBM implementation uses the exact reverse order. My code subtly depended on that order, and so it failed on AIX.

BTW, if you want to find out more about which JVM implementation you are using, just issue java -version. This is from my machine at home:

java version "1.6.0_06"
Java(TM) SE Runtime Environment (build 1.6.0_06-b02)
Java HotSpot(TM) Server VM (build 10.0-b22, mixed mode)

The HotSpot part is what gives away that it’s from Sun. This is from our AIX box:

java version "1.5.0"
Java(TM) 2 Runtime Environment, Standard Edition (build
    pap64dev-20080315 (SR7))
IBM J9 VM (build 2.3, J2RE 1.5.0 IBM J9 2.3 AIX
    ppc64-64 j9vmap6423-20080315 (JIT enabled)
J9VM - 20080314_17962_BHdSMr
JIT  - 20080130_0718ifx2_r8
GC   - 200802_08
JCL  - 20080314