In my previous blog posting, I wrote about what I would expect node.js to do in order to become mature. I introduced the idea of having a framework which lets you define a protocol and some handlers in order to let the developer concentrate on writing useful business software, rather than technical code, in a very similar manner to which Java EE does. And through that posting, I came to learn about a thing called Comet. I had stated that using a non-blocking server wouldn't really be any more useful than a blocking one in a typical web application (HTTP), and so I created an example based on my own protocol and a VOIP server, for streaming binary data to many concurrently connected clients. I have now read up on Comet and come to realise there is indeed a good case for having a non-blocking server in the web. That case is pushing data back to the client, like for continuously publishing latest stock prices. While this example could be solved using polling, true Comet uses long-polling or even better, full on push. A great introduction I read was here. The idea is that the client makes a call to the server and instead of the server returning data immediately, it keeps the connection open and returns data at some time in the future, potentially many times. This is not a new idea - the term Comet seems to have been invented in about 2006 and the article I refer…
Tag: Java EE
Java non-blocking servers, and what I expect node.js to do if it is to become mature
node.js is getting a lot of attention at the moment. It's goal is to provide an easy way to build scalable network programs, e.g. build web servers. It's different, in two ways. First of all, it brings Javacript to the server. But more importantly, it's event based, rather than thread based, so relies on the OS to send it events when say a connection to the server is made, so that it can handle that request. The argument goes, that a typical web server "handles each request using a thread, which is relatively inefficient and very difficult to use." As an example they state that "Node will show much better memory efficiency under high loads than systems which allocate 2MB thread stacks for each connection." They go on to state that "users of Node are free from worries of dead-locking the process — there are no locks. Almost no function in Node directly performs I/O, so the process never blocks. Because nothing blocks, less-than-expert programmers are able to develop fast systems". Well, reading those statements makes me think that I have problems in my world which need addressing! But then I look at my world, and realise that I don't have problems. How can that be? Well first of all, because we don't have a thread per request, rather we use a thread pool, reducing the memory overhead, and queuing incoming requests if we can't run them in a thread instantly, until a thread becomes free in the pool. The…
Persistent State Machine with Apache SCXML
The source code for this blog article can be downloaded here. I'm bored of reinventing the wheel. Everytime I need a state machine to ensure my states traverse only valid transitions, I find myself either not bothering, because I trust my coding (and write all the necessary unit tests of course), or writing very similar code over again. So I started wondering if there was a configurable state machine out there somewhere, and in no time at all Google gave me a link to SCXML from Apache. Apache SCXML is an implementation of a configurable state machine based on the SCXML working draft from W3C. I started by taking a look at what it does and how it works, always keeping in mind my requirements based on previous projects. The main question was how I could use a state machine in a persistent entity so that when an attempt is made to change the state, the state machine validated the attempt, ensuring only valid transitions are carried out. That meant two things: The state machine had to be able to have its current state set to any state. If I load an object with state out of the database, I need to be able to set that state in the state machine so that it checks any attempts to change state, based on this starting state. The state machine had to fit into a JPA entity class so that I could persist and load the state. Apache SCXML doesn't come…
Taking Advantage of Parallelism
A while ago some colleagues attended a lecture where the presenter introduced the idea that applications may not take full advantage of the multi-core servers which are available today. The idea was that if you have two cores but a process which is running on a single thread, then all the work is done on one single core. Application servers help in this respect, because they handle multiple incoming requests simultaneously, by starting a new thread for each request. So if the server has two cores it can really handle two requests simultaneously, or if it has 6 cores, it can handle 6 requests simultanously. So multi-core CPUs can help the performance of your server if you have multiple simultaneous requests, which is often the case when your server is running near its limit. But it's not often the case that you want your servers running close to the limit, so you typically scale out, by adding more nodes to your server cluster, which has a similar effect to adding cores to the CPU (you can continue to handle multiple requests simultaneously). So once you have scaled up by adding more cores, and scaled out by adding more servers, how can you improve performance? Some processes can be designed to be non-serial, especially in enterprise scenarios. The Wikipedia article on multi-core processors talks about this. Imagine a process which gathers data from multiple systems while preparing the data which it responds with. An example would be a pricing system. Imagine…
GlassFish 3 In 30 Minutes
The aim: Set up a simple Java EE 6 project on GlassFish v3 in no time at all. The project must include: email, JMS, JPA, web and EJB (session bean, message driven bean and a timer bean). It must also include security and transactions. Sounds like a lot, but thanks to Java Enterprise Edition version 6, setting up a project like this and configuring all the resources in the Application Server are really easy! I chose GlassFish because its open source, has a useful little Admin Console and I've never developed with it before. Before I started I downloaded Java SE 6 (update 20), Mysql Server, the Mysql JDBC Driver and the GlassFish Tools Bundle for Eclipse, which is a WTP Version of Eclipse 3.5.1 with some specific bundles for developing and deploying on GlassFish. The process I wanted to implement was simple: a user goes to a website, clicks a link to a secure page and logs in, after which a message is persisted to the database and an asynchronous email gets sent. The user is shown a confirmation. In the background theres also a task which reads new messages from the database and updates them so they are not processed a second time. The design was to use a servlet for calling a stateless session EJB, which persists a message using JPA and sends a JMS message to a message driven bean for asynchronous processing. The MDB sends an email. A timer EJB processes and updates any messages…
