Friday, May 25, 2007

IE 6: gzip + ETag != If-None-Match

ETag + If-None-Match give you the benefit of not having to send unmodified content repeatedly (HTTP 304). IE6 handles this well.

Content-Encoding with gzip gives you the benefit of compressing the content that you send. IE6 handles this well as well.

So gzip + ETag / If-None-Match should give you the combined benefit of sending compressed content when you must and not sending content at all if it's not modified. Well, as you might have already guessed, IE6 does not handle this well. If your content is gzipped and you send an ETag header as well, IE6 does not send an If-None-Match on subsequent requests. Which of course means that you can't leverage HTTP 304.

So if you are servicing IE6 clients beware that it supports either compression or ETags but not both.

Thankfully, this has been fixed in IE7. Firefox of course just works.

Friday, May 18, 2007

More Resources == Scalable

Link: Sam Ruby on Google Maps

Of course, the rest of the iceberg was that Google had simply tiled the Earth. In so doing, they converted a single web service (call me with a bunch of information, and I will provide you with a custom result) into a large number of individually addressable, cacheable, and scalable web resources.
That's it. More the resources more is the opportunity to use Cache-Control headers, to use ETags, to distribute and load-balance the system.

In the same article, Sam also talks about how the web is not a service but a space. And in today's world adding more "space" will scale your system manifold than implementing a state-of-the-art service with the most optimal algorithm. Processor speeds have flattened. Today it's about dual-cores, quad-cores, (your-budget)-cores. The more cores your program can use to get the job done, the more scalable will your system be.

As Brian Goetz puts it: "Tasks must be amenable to parallelization". Parallelization comes for free with every resource you add. So add more resources and see your system scale.

Sunday, May 13, 2007

JavaOne Days 3-4: Mashups and garbage collection

If you are surprised that I am talking about mashups and garbage collection in the same post, well, so was I. But if one can talk about implementing servers in JavaScript, I most definitely can talk about mashups and garbage collection in the same breath.

Day 1 was primarily scripting, day 2 hardcore java. Days 3 and 4 saw sessions on the entire gamut of Java technologies - from garbage collection to mashups. I discuss some of them here.

Blueprints for Mashups

This was arguably the most informative session for me by far at this year's JavaOne. Kudos to Greg, Mark and Sean for putting together a to-the-point, practical and readily-usable session together. Personally, this session validated the REST and JSON concepts I had gathered over the past few months. I'll recommend this session to anybody interested in building mashups, REST services, AJAX and a whole lot more. (No, they haven't paid me to say this.)

Anybody building REST services should design their system with JavaScript in mind. In today's mashup world the browser (and hence JavaScript) is your first class client. XML and JSON are the typical content types returned by REST services. To get over the browser's cross-domain restrictions, there are 2 possible solutions:

  • Server-side proxy: Clients always send requests to the server that is hosting the page. The server in turn acts as a proxy, sends your request to the (remote) mashup service and returns the data it gets from the mashup service to you.
  • Dynamic script tags: Browsers allow script tags to communicate with cross domain servers. This opens up the opportunity for you to issue requests to any mashup service by generating dynamic script tags.
Consider the Atom format if you are serving XML. This opens up your service to the large number of feed readers and other Atom clients out there.

JSON has of course gained immense popularity of late. Although JSON is technically a serialized JavaScript object, the JSON format is highly portable and language independent. In addition to returning JSON, you can also support wrapping the JSON object in a JavaScript callback method (they called it jsonp - JSON with padding). This enables clients to specify a callback method which can readily work with the JSON that the server returns.

Various options are available for securing your REST services:
  • User tokens
  • Session based hash
  • URL based API key
  • Authentication headers
JavaScript best practices:
  • Use namespaces
  • Use CSS for applying styles
  • Don't add to the prototype of common JavaScript objects
  • Setting the innerHTML property is easier / better than DOM manipulation
Components of a ("good") Mashup API / library:
  • A server-side service
  • A client-side JavaScript API / library
  • A client-side CSS for applying styles
  • Document the API
  • Create simple examples enabling a simple cut-and-paste approach to learning your API
As you can see, they have, true to the name, provided blueprints for mashups, best practices, possible hurdles and workarounds, problems and solutions. Once again - highly recommended for anybody who has / will have / may have anything to do with mashups.

Phobos - A Server-side JavaScripting framework

While JS clients are ubiquitous and obviously here to stay, for the life of me, I have not yet come to terms to implementing my server in JS as well. The 2 reasons they cited - impedance mismatch and all-you-need-is-an-F5 to redeploy - didn't quite do it for me. May be in another life I'll grow up to JavaScript servers, but not for now. And if at all that day were to come, there should be a JavaScriptOne and no JavaOne.

Garbage collection

If there was one objective of this session, it was to clear the GC myths out there. Some of the finest Java minds were refuting the urban legends out there and when they talk you listen:
  • Object allocation is cheap. Reclaiming young objects is also cheap.
  • Small, short-lived immutable objects are good. Large, long-lived mutable objects are bad.
  • Nulling references rarely helps - except when it comes to arrays.
  • Avoid finalizers - in most cases there are better alternatives possible.
  • Avoid System.gc() - except between well-defined application phases and when the load on your system is low.
  • Object pooling is not required in today's VMs. It is a legacy of older VMs. Exceptions are objects that are expensive to allocate / initialize or objects that represent scarce resources.
  • Consider using reference-objects for limited interactivity with the garbage collector.

Certain memory-leak pitfalls:
  • Objects in wrong scope
  • Lapsed listeners
  • Metadata mismanagement

They strongly advocated FindBugs for finding such pitfalls as well as other potential bugs.

JavaOne Day 2: Hardcore Java and Java EE 5

While Day 1 saw an overdose of scripting, sanity made a return on Day 2. Although JavaScript technology (whoever came up with that) sessions continued, there was a good supply of deep-dive Java and Java EE sessions to keep me busy. And there was also some WADLing.

Generics and Collections

Generics have been around for a while and most Java programmers have some understanding of it. There have been many criticisms against the erasure based implementation of generics - since parameter types are not reified, constructs that require type information at runtime don't work well. However erasures allow 2 most important benefits - migration compatibility and piecewise generification of existing code. Just these benefits make erasures a necessary bane.

Huge additions have been made to the Collections framework in Java 5 and 6. So much so that many recommend programmers to never use arrays - Collections all the way!

Concurrency Practices

The concurrency classes introduced in Java 5 are a great new toolset for the Java programmer. New concurrency related annotations such as @ThreadSafe, @GuardedBy, etc. are being considered. One important aspect to keep in mind - imposing locking requirements on external code is asking for trouble. Ensure that you make your code threadsafe yourself. Performance penalties incurred by synchronized constructs are overblown.

Immutable objects are your friends - final is the new private! They are automatically threadsafe. Object creation is cheap. Aim for less and less mutable state.

Performance is now a function of parallelization - write code such that it can use more cores to get the job done faster. So to improve scalability find serialization in your code and break it up:

  • Hold locks for less time
  • Use AtomicInteger for counters
  • Use more than one lock
  • Consider using ConcurrentMap
  • Consider ThreadLocal for heavyweight mutable objects that don't need to be shared
Effective Java

Builder pattern allows you to construct objects cleanly in a valid state with both required and optional parameters. The basic premise can be expressed in code as such:
MyObject myobj = new MyObject.Builder(requiredParam1,requiredParam2).optionalParam1(opt1).optionalParam2(opt2).build();

Generics - avoid raw types in new code. Don't ignore compiler warnings. Annotate local variables with @SuppressWarnings rather than the entire method / class. Use bounded wildcards to increase applicability of APIs. 3 take home points for paramterized methods:
  • Parameterize using <? extends T> for reading from a Collection
  • Parameterize using <? super T> for writing to a Collection
  • Parameterize using <T> for methods that both read and write
If a type variable appears only once in a method signature then consider using a wildcard. Avoid using bounded wildcards in return types. Generics and arrays don't mix well - consider using generics wherever possible.

Java EE 5 Blueprints

Filter.doFilter() may run in a different thread than the Servlet.service() method. The Servlet API does not use NIO, however it is possible to implement it yourself. New annotations may make web.xml optional.

The JSP and JSF EL have been unified. # is now reserved in JSP 2.1. The javax.faces.ViewState hidden field (for client side state) will be standardized. Include this field in your AJAX postback. Application-wide configuration of JSF resource bundles will be possible in faces-config.xml. The <f:verbatim> tag is no longer needed to interleave HTML and JSF content. @PostConstruct and @PreDestroy annotations will be supported for JSF managed beans.


WADL - Web Application Description Language. As a colleague of mine put it - it's WSDL for REST! And IMO that's almost what it is. It's all in good intent to introduce WADL - a formal definition of your REST resources, allows you to automatically stub out code in your favorite language, aids testing. However WADL has many rough edges and I don't see myself using it anytime soon until those have been addressed - overloaded / loosely typed query parameters, security, non-standard content negotiation, et al.

Tuesday, May 8, 2007

JavaOne Day 1: Scripting Everywhere

If you were new to JavaOne, you might be forgiven for thinking that the message was - Strongly typed languages are so-90s, scripting is in! And why wouldn't you - every other word you heard was - JRuby, Groovy, Server-side JavaScripting. And if that wasn't enough, Sun introduced a half-baked, half-after-thought, all put-together-in-a-last-minute-sprint JavaFX. Ok so I don't want to disregard the hard work put together by folks to get JavaFX ready for JavaOne but it all seemed so unconvincing. The demos were clunky, some didn't work and those that did were hardly "Wow!".

Nothing to "Wow!"

If you like showtime, there was none. No big announcements, no new features, no gennext projects. But IMO that was a good thing. Java has seen many evolutions, many innovations. It's time to make it just a little better, a little more robust, a little more performant. Java is a mature technology now and any radical change might actually be shaky news for the massive Java community.

Oh so Groovy

As I mentioned - scripting languages were the talk of the day. And Groovy is one of them. It has been around for a while and the rich set of features it offers are testimony to that - Java-like syntax, pre-compiled or runtime compilation, "duck typing", annotation support and a whole pageful of other features.

I wonder what has lead to this sudden wave of dynamic languages? Wasn't it only yesterday when everything-should-be-strongly-typed was the hallowed rule of programming. Maybe it's the advent of mashups. Maybe the uptake in AJAX. Or maybe we just need something new to keep us interested!

Web as a platform

"Integrated rich clients" - or what we know as mashups - are the killer apps of the day. Things (technologies, techniques, hacks) that allow for mashups to be assembled with minimal code will be on the victory parade. More code is executed on the client but most of that code is still sent to the client by the server. No wonder scripting is big today and getting bigger. But what I do wonder is where does that leave EJBs / ESBs?

JSR 311 - Java REST API

I had questioned the need for frameworks to build RESTful services earlier. And after having sat thru a session explaining the new Java REST API spec I am no more close to getting an answer to that. Ok so they showed a few annotations which got rid of a few lines of code. But does the same simplicity work if I have even a slightly complicated URI pattern? Or will it work if I don't use the "Accepts" header for conent negiotiation and use query params or path extensions? And from what I saw it seemed they were generating XML and JSON from Java code. Aren't JSPs (or any template technology) a whole lot easier to do that?

I got Closure

Neal Gafter gave a presentation on Closures in Java. Very well done, no gimmicks, to the point. A great presentation for a potentially very powerful feature in the Java language. If Neal needs more voices to move this up the JCP and into Sun's plans for Java SE 7, count me at +1.

Take home point

Like 'em or hate 'em - scripting is in town.