A Grails Porcelain Ajax Framework - Part I: Component Refresh

The Foundation

The foundation of a good porcelain Ajax framework is component refresh, i.e., refreshing an area of the page with new/updated information. Component refresh is equivalent to reloading the page, but provides a better user experience since it happens asynchronously, and reduces the amount of work the server has to do.  Because we are requesting a block of HTML from the server, we don't have to write any additional JS to update the page. jQuery.load does all the heavy lifting. With component refresh it becomes trivial to Ajax enable pagination, sorting, filtering, lazy loading, tabs and many other actions for a better user experience and better performance. In this post, we'll learn how to create a porcelain Ajax framework with component refresh using Grails.

The App

Porcelain frameworks are usually built around an application. For our purposes, lets say we have a simple app for creating, listing, viewing and editing products. The products have a name, price, and description. Additionally, each product can be viewed in four different colors. In our app, the only difference between colors will be the image that is displayed for the product, but the different colors could easily be different tabs of product information, or different configurations of the product.

All our work will be on the product list, which will look like this:

our made up products list

The Project

We start with a standard Grails project and add a Product domain class and generate the controller and views. How to do all that with Grails is out of scope for this discussion, but it's pretty easy.

By default, Grails lists products in a table. Typically products are listed as "cards" to give them more room, so we'll enhance the markup slightly:



At this point we haven't written a line of Ajax code, we're just creating a simple app that will work even with JS disabled. In anticipation of adding Ajax functionality though, we should break the list page up into the components that we plan to refresh. To do this, we'll make use of the g:include tag, which allows us to inline another controller action in our page. We'll create two new controller actions and adjust our templates accordingly:



Notice the original list code has moved into renderList and we return productIds instead of the actual product object.

And we replace the list in list.gsp with g:include controller="product" action="renderList". renderList.gsp is just:



and renderProduct.gsp is the product div above.

For the colors pages, we're going to reuse renderProduct:



and that's it. We now have all our functionality setup and it all works even with JavaScript disabled. Time for some progressive enhancement.

The Framework

The core of our component refresh implementation is going to be g:include. Since g:include is plumbing, we need to ensure that our porcelain covers it very thinly to make future extension easy. One option would be to  use Groovy meta-programming magic to intercept calls to the built in Grails taglib where g:include is. This  has the advantage of being transparent to the user. We don't have to worry about using the right tag, and if we already have a lot of existing code using g:include, this would be the way to go. However, it has the disadvantage of being complicated. If we're starting from scratch, it's much simpler to define our own tag. So enter the FrameworkTaglib:



Instead of g:include, we will use ajax:include. The new tag does two thing. First, it wraps the content of the component in a div with the class "ajax-component" so that out framework JavaScript will know that it's a component. Secondly, it sets the id of that div to the controller action and optional id for the include. That will allow us to call the correct action from our Ajax requests.

The second part of our framework is framework.js:



refresh is also a very thin wrapper, this time around jQuery.load. It has the nice feature of allowing us to use any element inside a component as a representative of the component. Also, we don't have to pass a URL, and can pass a data string or object, just like jQuery.load.

With these two components, our framework is ready! Our first task will be to Ajax enable the color links so we can view the different colors without leaving the list:



With our framework code in place, it couldn't be simpler. We strip the query string off of the link and pass it as data to refresh and we're done. The color links now refresh the individual products in place, for a greatly improved user experience. Also, notice we use jQuery.delegate so that when the component is refreshed we don't have to reattach event handlers.

So that was easy, but what about pagination? Surely it will be a bit more work to Ajax enable pagination.



Or maybe it's the exact same code. It looks like progressively enhancing links in this way might be something to add to framework.js. Notice that we can refresh the entire list as a component, but also have nested components that can be refreshed individually.

In very little code, we've built a framework that has allowed us to progressively Ajax enhance both pagination and content toggle in about seven lines of JS. But lets take an objective look at what this framework does well and what might be problematic.

You can download the complete project to this point from GitHub.

The Good

There is a lot to like about the framework as it stands.

• Very simple usage.
• We take advantage of progressive enhancement. Normal links become refreshes very easily.
• The whole framework is less than 50 lines of code at this point.
• Components can be nested inside other components.

The Bad

Using this code as is on a serious project is probably not the best idea for a few reasons:

• All components are wrapped as divs. That means refreshing something like a table row is impossible.
• In framework.js we assume that the default URL mapping of "/$controller/$action?/$id?" is used, which may not be the case, in which case refresh will fail.
• We have to pass all the parameters to refresh. For just progressively enhancing links, this isn't such a big deal since the link should have all the parameters, but as we get into more advanced features, it will become a pain point.
• Several features of g:include will not work. In particular, the model and params attributes will not be present in a refresh. For simple cases, that's not a problem, but a seriously application will quickly feel the pain of such limitations.
• Two nitpicks
• We use the namespace "ajax" for the taglib, which is not likely to be unique. Another poorly written plugin could cause problems.
• We use the id attribute to store the controller action information. A class might be more appropriate, if a little harder to parse.

The Conclusion

The core idea of a good component refresh is being able to pair together the view (GSP) and the code that generates the model (the controller action). This pairing allows us to treat each component as it's own entity, making refresh and other advanced features possible. Grails and jQuery provided us with some very nice pipes to start with, so creating the component refresh porcelain was perhaps easier than it would normally be, but I think the simplicity helps the underlying concepts to show more easily.

Next time we will build multi-component refresh and try to fix some of the problems with our framework. It's not good from much beyond basic enhancement at this point, but we're off to a good start.

Remember, you can download the complete project to this point from GitHub.

How to Build an Ajax Framework

Plumbing & Porcelain

I've built three or four Ajax "frameworks" in my career. I don't know if you would call them frameworks because there are really two kinds of things I mean when I say "framework". There is the extremely general purpose plumbing framework, which can do anything, but as a result leaves you to write a good bit of code and maybe pick and choose which parts you need. Some examples of plumbing type Ajax frameworks/components would include sever side frameworks like DWR, Ajax4JSF, etc. but also client side frameworks like dojo, Scriptaculous, jQuery, etc. Also most of the Rails type server frameworks have Ajax support built in: e.g., Grails and Django.

Then there are the porcelain frameworks. The problem with the plumbing frameworks is that they can do anything, so you're presented with a big toolbox full of neat things and a bunch of pipes, but you have to put it all together. A porcelain Ajax framework is usually built by an organization for their apps, or even for a single application. Sometimes they become an entire platform internal to the application and sometimes no thought is put into developing the porcelain and you end up with each developer doing Ajax in their own way.

Inner Platforms & Thoughtless Scaffolding

Some developers recoil at the thought of internal platforms. They tend to make a few use cases easy but then require horrible black voodoo workarounds when trying to do something the original architects didn't think of.  Other developers are completely frustrated with the thoughtlessly thrown together and generally rickety Ajax platform they've been trying to maintain.

An inner platform sometimes arises when otherwise highly competent enterprise (read: Java or .Net) developers first encounter front-end programming and dynamic languages.  They are used to structure and standards and ACID properties and so forth. This lack of static types and compilers instills a certain sense of fear and the only way to conquer that fear is to create "coding standards" and, then when those are not followed, to create an internal UI framework that forces everyone to code in a certain way, without closures, functional programming, overloaded operators, dynamic dispatch, and all those other ugly hacks that are bad for performance.

Other times, an inner platform is really just the result of well intentioned senior developers trying to make things easier for the more junior members of the team. Unfortunately, they often do so by hiding important information and you end up with a very leaky abstraction that is buggy and that the users (programmers other than the authors) are completely helpless to fix because they have no experience with the underlying framework. The points is that there are many ways to end up with an inner platform that does more harm than good.

Thoughtless scaffolding on the other hand is just the result of... thoughtlessness. Most often it's because a programmer without much experience and no real computer science training is writing all the code and whatever tool he's read about today is the perfect tool for the job. But it also happens when a new project is started and the technical leadership fails to identify the patterns that should be used. Maybe they start off with some vague notion like, "JSON is the answer," and become committed to a bad strategy early on and never have the time or the courage to go back and clean it up.

Both extremes have problems, but they also have advantages. For the tasks it was built for, the inner platform makes adding  new Ajax features extremely simple. On the other hand, the thoughtless scaffolding never gets in the way. Because very little of it is connected, it's easy to rip part of it out and build something new entirely. 

We want to find a middle ground between over engineered and thrown together, but where that middle ground lies can depend a lot on whether maintenance or new features are your priority. Instead we will focus on the core principles and features that any porcelain Ajax framework should encompass.

Core Principles

These are the core principles I have learned through trial an error over the past 5 years, building and using frameworks across the spectrum from somewhat thoughtless to a massive inner platform:

1. Remember it's a web app - It's not a desktop app. You're probably not build the next Gmail, so stick with what works. Use semantic markup, keep the pages simple and light, and use progressive enhancement so the app works without JS. For public facing apps this is a must obviously, but even for internal apps where you know that your users have a decent browser with JS enabled, you will stay out of trouble by keeping it simple.
2. Do everything you can on the server - Especially rendering HTML. JSON is overrated. jQuery's load method can do 95% of what you need in one line of JS. For the most part, JS should not be producing HTML. Things go much more smoothly when all HTML is produced server side because then the logic to generate the markup doesn't need to be duplicated. There are exceptions, like if you have a feature using 3rd party code, like a Google Map, that simply will not work without JavaScript. Then it might be necessary  to generate some of the UI in JS. If you must generate HTML in JS, use a templating framework. If you are concatenating anything more than 100 characters of HTML together, you're making a mess of your code.
3. Keep the porcelain thin - Do not under any circumstances create more than one layer of abstraction between the code and the core web technologies. Most of the HTML output of your application should correspond to HTML in a template file. Taglibs are fine, but try to use your templating language even inside of them. Do not create a system where the developer doesn't have to know HTML. The same goes for JavaScript and CSS. Do not write code to generate CSS ever. You may use a framework that generates CSS to reduce repetition, but try to only use the big ones with a good community. Do not generate JavaScript code ever. Generate inline JSON instead and have your scripts consume it to determine behavior dynamically. Use stateful CSS rules.
4. Cover all the pipes - When you realize that to make it all work you need every load() call to add some parameter, or check some condition, you better hope you can go to one place in your code and make that change. Hopefully the plumbing provides good hooks, or there is some AOP/IoC magic that can cover it. Otherwise, you may have to create very very thin wrappers around plumbing functionality. Not in the hopes of portability (you will never port this app to a different framework, it will be rewritten), but so that you can funnel all calls to that feature through one place.

Core Features

So what features does a HTML based, server oriented porcelain framework provide?

1. Component Refresh - This is the core. There needs to be a simple way to designate an area of the page as a component and an easy way to make a call to refresh its HTML. This may be as simple as using jQuery's load with a selector to get the desired page fragment, although it's nice if the server can do the minimal amount of work necessary to render the HTML you need.
2. Multi-Component Refresh - Sometimes you need to refresh several parts of the page whose only common parent is the body element. You could just reload the whole page, but for an action that only affects part of the page that's both inefficient and bad UX. You could also use multiple requests, but if there are more than 2 components, then many browsers will block the 3rd request until one of the others return. There is probably also a server part of the framework that allows the components to share data instead of making redundant calls.
3. JSON - Not to be consumed by Ajax calls, but to inline as script tags in the page. Most frameworks can take the same data model you pass to your templates and convert it to JSON automagically for your JS to consume too. You probably will want to include it inside your components' HTML so you can update the JS state when a component is refreshed.

Next Time Let's Build One

Soon, I hope to have some example code for a simple Grails porcelain framework. Actual code can be a lot more informative and illuminating than pithy maxims.

When to go dynamic

The most typical complaint I hear from Java developers about Groovy is, "I spent X hours trying to figure out this problem that would've been a compile time error in Java." And they're right. Very little compile time checking is done in Groovy. It can't be done because methods may be available at runtime that aren't resolvable at compile time. That's part of what it means to be a dynamic language.

Some developers seem to think using the optional types will save them. e.g.,

int foo (String bar) {
 return bar.size()
}

Looks good, right? Try running this script:

int foo (String bar) {
 return bar.size()
}
if(false) {
 String s = foo(123)
 println s
}
println "Done"

Go ahead. Run that script. No error. Types aren't checked at compile time. If we change false to true, we get a groovy.lang.MissingMethodException at runtime, but that's it. Your compiler can't save you here. Good unit tests will catch some errors if you use the optional types, but you have to know what the error is before you can test for it, so you've already spent the X hours trying to track it down.

What makes Java so good for enterprise work is the static types. Static types enforce a contract. Static types make it possible for Eclipse to take you to the exact method that will be invoked by a particular line in the source. If the method is on an interface, Eclipse can tell you all types that implement that interface method and take you to any implementation you choose immediately. This is extremely useful when you're trying to navigate a complex, unfamiliar system. While a human may be able to easily figure out what file to open, having a tool that can do it for you automatically allows your concentration to remain on what the code does and lets you work at a higher mental level of abstraction.

If I have to learn a new framework that someone has written, I'll get through it 10x faster if it's in Java. No/bad documentation? Not a problem, I'll just look at the code for that method. Whereas if I come upon an undocumented Groovy method, what do I do? Ugh, I have to grep through all of the source to figure out where it's defined, how it's used, yada yada. Not quick. Breaks my concentration.

Java is very good at making large systems manageable. However, the trade off is that
any system that has been under active development for more than 2 weeks quickly becomes a large system. There is nothing concise about Java. There are very few shortcuts. The simplicity of the language and the rigidity of types means you have to write more code and produce more classes to get things done and God help you if you want anything remotely flexible. Code that needs to do something even slightly dynamic can quickly become unreadable and strewn with all kinds of reflection objects, odd exceptions and other nastiness. Or worse. You could end up in XML hell.

Enter Groovy. Dynamic languages do best when there are conventions and consistent ways of doing things. Grails is proof of this. Controllers are in the controllers folder, services in the services folder, etc. Each class follows a certain arechtype and there are good idioms in place that imply what is going on. However, when you start writing plugins, or doing things outside of the normal scope of Grails, things can get messy fast.

Anytime you're coding in uncharted territory, consider going back to Java. Writing a plugin? Implement as much of it as you can in Java. Only go back to Groovy if the equivalent Java code gets too ugly or would be many times longer. Most of your plugin/framework/new invention's plumbing should be in Java. The plumbing is where you connect everything together, and it's important to verify that thingDoer is of type ThingDoer, and that all interceptors properly implement the ThingDoerInterceptor interface. You want your API to fail fast so your users don't end up with a NPE exception deep in your code that they have no hope of debugging.

Keep Groovy at the fringes, handling the nasty implementation details, and protected by static types and good assertions. Make sure your Groovy class implements an interface, or better yet, have a Java class implement the interface and wrap the Groovy implementation so you can jump to it in Eclipse. Use Java to provide enforce the type contracts and provide a well defined framework. Use Groovy to make the implementation readable.

Grails, DWR, and the dynamic web

This post used to be titled "Using DWR with Grails", because the Grails DWR plugin lacked support for custom converters and creators. I wrote this patch (EDIT: Nabble screws up the patch formatting so that it doesn't take. You can download the patch here) adding this functionality and I was going to blog about how to apply and use it. However, before I posted this, I did a bit of searching, and apparently my patch got applied the same day I submitted it. It would have been nice if someone had told me they took my patch, but presumably this means that the next release will have the functionality. If you can't wait, I presume you can build from the trunk, or use my forum post. Enough of that.

I'm not big on making predictions, but this one seems pretty obvious to me. The reason there are so many Java web frameworks is that they all suck at being web frameworks. There is no clear winner in the Java space. We started with Servlets, which were terrible for web development because you couldn't tell what the heck the final HTML would look like. Then we had JSP, which is better, but once you start trying to apply DRY, get into a templating system and tag libraries, you're suddenly just as removed from the final result as you were with Servlets. In both cases, if I want to know what is being produced, I have to fire up my JEE container and do a request. Not to mention that creating templates and tags requires some lovely combination of XML and Java code. I don't know about you, but that feels off to me.

Why? XML doesn't give me much flexibility. DRY gets violated all the time because there is no mechanism in the design of XML for reasonable reuse. Java on the other hand lets me refactor and reuse to my heart's content, but as we've already established, it's crap for actually producing HTML.

And what about Ajax? JSP is fine if your site uses tables for layout and all links result in a full page refresh or occasional popup window, but if you want to build a responsive, dynamic site (using say, Ext) you'll end up having rendering logic in two places, WhateverCrapFramework and JavaScript. The best possible outcome is that you hardly use JSP (or whatever your 'controller' is) and rely on your Ajax toolkits.

Again why? Because static typing everything is not appropriate for user interfaces. A user only has one input type available to them: text. They can't enter FooBarDtos directly, they don't even know what one is. You have to take some text and convert and validate it. Granted, they can also click things, but HTTP seals the deal for the web because all data submitted must be in text form. Your types cannot help you until you have already converted and validated the data, at which point it's ready to pass on to your service layer. So why the heck do you need types everywhere in your controller? You don't, but Java forces them on you anyways.

I think Java is great for web applications by the way. Spring+Hibernate remains my first choice for implementing a solid service and data access layer (I like Guice too). Static typing is great for helping ensure that your services are solid, and your data remains valid.

Back to Ajax. I made a big investment in JSF in graduate school. I nearly killed myself and my wife building a framework for creating Ajax enabled components for JSF. It took me a few months after finishing to realize that JSF still doesn't make Java web development any less like wearing plate mail while figure skating.

When you get right down to it, the best language for developing a web UI is the language that was always meant for the web: JavaScript. DWR completely bridges the gap between Java and JavaScript. It converts all that pesky string data into nice typed data for your Java service layer and then turns around and converts those types that the user doesn't know about into pleasant little strings for web browsers to gobble up. You can expose your Spring service beans directly, secure them with Acegi (you better be doing that anyway, if you're relying on your controller to secure access, you're using a sieve as an umbrella), and call them from JavaScript without even having to know how HTTP requests work, much less Ajax.

In conclusion, we need real developers on the client side. Good designers/UX folk don't usually make the best technical developers, and the dynamic web (especially for enterprise) requires a solid, coherent client side codebase, and things like DWR make that easy. If you're a Java web application developer that's still afraid of JavaScript, you'd better learn to face your fears.