A WSDL file contains a reference to an XSD document which defines the data structures which can be sent to the service over SOAP. In an XSD, you can define a Type for an element, or things like the elements cardinality, whether its optional or required, etc. When the web server hosting a web service is called, it receives a SOAP envelope which tells it which web service is being called. It could (and you might expect it does) validate the body of the SOAP message against the XSD in the WSDL... but it doesn't. Is this bad? Well, most clients will be generated from the WSDL, so you can assume that the type safety is respected. Saying that, it's not something the server can guarantee, so it needs to check that say a field that is supposed to contain a date, really does contain a date, and not some garbled text that is meant to be a date. But more importantly, something which a client does not guarantee, is whether all required fields in the data structure are actually present. To check this, you can validate incoming SOAP bodies against the XSD. The way to do this, is by using "Handlers". The JAX-WS specification defines two kinds, namely, SOAP Handlers and Logical Handlers. The SOAP kind is useful for accessing the raw SOAP envelope, for example to log the actual SOAP message. The logical kind is useful for accessing the payload as an XML document. To configure a handler,…
Tag: Design
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…
Auto-update of Midlets
Any professional application should be capable of updating itself over the internet. Even Midlets! The idea is fairly easy in that the Midlet needs to make a call to the server to check what version the latest software has, and compare that to the installed version. If there is a newer version available, then it needs to start the (mobile) device's browser and point it at the URL of the new JAD file. The device will take care of the download and installation after that. So, the following article shows you exactly how this can be implemented. First off, refer to the previous article, which described a framework for making server calls. The sequence diagram looks like this: Based on the framework, the following JSP can be installed serverside, which reads the version number out of the JAD file: <%@page import="java.io.FileInputStream"%> <%@page import="java.io.RandomAccessFile"%> <%@page import="java.io.File"%> <%@page import="java.util.List"%> <html> <body> <% RandomAccessFile raf = null; try{ String version = null; String path = request.getSession().getServletContext().getRealPath("index.jsp"); File f = new File(path); f = f.getParentFile(); f = new File(f, "jads/nameOfJad.jad"); raf = new RandomAccessFile(f, "r"); String curr = null; while((curr = raf.readLine()) != null){ if(curr.startsWith("MIDlet-Version:")){ version = curr.substring(16); break; } } %>OK <%=version%>| add other master data like the users details, their roles, etc. here... <% }catch(Exception e){ log.warn("failed to read master data", e); %>ERROR <%=e.getMessage() %> <% }finally{ if(raf != null) raf.close(); } %> </body> </html> This JSP is called when the Midlet starts, and effectively delivers "master data" to the device, such…
Choosing a Model/Event Pattern
The following diagrams show different implementation patterns of models and corresponding events, as typically used in the UI. Depending upon your needs, you can use the following tables to decide which pattern to implement. The patterns are split into model patterns and event patterns. Not all combinations of the patterns make sense, but patterns can be combined to fulfill your unique requirements. At the end I make some recommendations. Model Pattern 1 - Automatic Event Firing With the above pattern every change to the model results in an event being fired. Typically if the focus of a text field is lost, the model gets set (in Eclipse RCP perhaps using databinding). Any views that show this model attribute get notified and update themselves. Works well for cases where multiple views of the same model need to be continuously up to date. A big disadvantage is the number of events which get fired. You also need to watch out for cyclic dependencies whereby an event updates a second view, and that update fires another event which updates the first. UI performance can suffer with this pattern, because its granularity is simply too fine. Model Pattern 2 - Manual Event Firing This pattern lets the caller (typically the controller) fire an event after it has finished updating all parts of the model. Typically the controller performs any validation of multiple fields, may call the server and when everything is good, updates the UI's model. An example might be when an input…
Idempotency and Two Phase Commit
The requirement for services to be idempotent is often stated as being important for enterprise applications. But what does that mean, and why? Idempotent means that a service can be called multiple times with the same data, and the result will always be the same. For example, if a service call results in a value being written to a database, the same service call made again would result in the same value being written to the database. As such, additative processes where values are incremented cannot be idempotent, for example an insert statement in a database is not idempotent, whereas an update statement usually is. Imagine the case of purchasing a ticket from a web service offering airline tickets. The process probably includes getting an offer to see the price and tarif, reserving an instance of that ticket and finally when the shopping cart is full, confirming that ticket by booking it. Getting an offer would be an idempotent call, since we are just effectively reading data, not writing it. Reserving the ticket cannot be idempotent because each call should result in an individual seat being temporarily reserved - you don't want to reserve the same seat for two passengers. However, should such a reserved ticket not be booked, a background process would need to cancel the temporary reservation, so idempotency is effectively achieved. In the final call, to book a reservation (to guarantee the seat), the call should be idempotent - setting the status of the ticket to "booked"…
Building a Webmail Solution on top of Apache James Mail Server
Part of maxant's offering to small businesses is email hosting. As well as standard POP3/SMTP access, maxant offers webmail access. A quick search on the web shows that there are several open source webmail solutions available. The problem with all of them is that they communicate with the email server through the SMTP protocol. For example, if you wish to preview a list of emails, the web application needs to access the email server and ask for details of each email (while leaving them on the email server, so they can be downloaded at a later time via POP3). Reading all the emails is inefficient and the larger the number of emails in your inbox, the longer it takes to just see a list of emails. The solution built by maxant is based on the Java Mail API from Sun. This API lets you access individual emails in your inbox using an ID. But Apache James Mail Server (James for short) doesn't maintain the index, if a new mail is put in the inbox, so if you have a list of all emails and decide to access one, and in the mean time you have received email, the chances are that you won't be able to read that email! The next problem is how to deal with keeping a copy of sent emails for your "sent items" folder. If you just use the Java Mail API, the only solution for getting a mail into your email server so that it…
Reporting Stats
A few years ago when I was the integration architect in charge of my Global 500 customers EAI projects, the product that we used did not really offer any way to track the number of messages travelling through the system. To get around this, we added a little library that recorded a small amount of information about each message. The idea was that every application (message flow) within the EAI system would be instrumented with this code and we would gain an oversight over which interfaces were the important ones and which ones made a return on their investment. The sort of data we collected was: Interface / Application / Process Name Message Type Message Size Timestamp Status (eg. IN, OUT, SUCCESS, FAIL) Correlation Token (in case several messages belonged together) Error (ie. details of an error) Custom Data (specific to each interface / app / process) There was probably more... it was a few years ago now :-) Anyway, we soon discovered that there were applications that were not even being used, even though our customers had paid many tens of thousands of dollars for them (ok, I admit that isn't a lot of money in the IT world). Since then Google has created its "Analytics" offering which generates some excellent reports for web site statistics. Indeed, maxant has always collected web stats for the websites it runs. It is very important to know your users / customers usage patterns, in order to be able to understand their real…
OSGi – Just another fad?
In the last few weeks I have heard the term OSGi come up more and more, and one blog posting I read suggested that it was the hot topic of 2008. So I started to research a little. I am currently working heavily with the Eclipse Rich Client Platform building applications which use services deployed to IBM WebSphere. Both these platforms are built up on OSGi (the standard) and both use Eclipse Equinox (an implementation of the standard). So it must be important right? Well you don't have to read too much before you start to get the feeling that you have been there and done that before. One aim of OSGi is to provide a micro kernel for deploying and managing services. Well, from a high level, JMX (MBeans) already does that. Not enough? Well there used to be a project called Apache Avalon Phoenix, which was a mirco kernal and although that project died and was resurrected as Loom from Codehaus (which incidentally has a very interesting history of Apache and Phoenix), it is still the basis of some big projects like the Apache James mail server. Other micro kernals? How about JBoss? There is a good blog article dicussing how JBoss has been based on a micro kernel for some time now. The idea is nothing new and in fact in their case, OSGi does not really go far enough that they could be solely based on it. Actually, doesn't the Java EE EJB specification let you…
