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"…
Month: September 2009
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…
Creativity – Yet another driver towards agile processes
Software developers are often creative people, having entered into software development precisely because of its need for creativity during implementation. While solving a logical problem might sound unrelated to creativity, the ability to solve that problem in many different ways and choosing the right way, is what requires the creativity. This is what turns a mundane data entry job into an exciting and fun job. Not only are there many different ways to implement algorithms or user interfaces, so that they provide the same inputs and outputs as required, there are many different ways depending upon the architectural or design view points taken. For example, should the implementation use as much reusable code as possible, or should the code be as simple, readable and hence maintainable as possible? Or is breaking architectural policy allowed, in order to ensure maintainability, productivity or other aspects, etc... So if programmers, designers and architects are so creative, is it a problem? Well it can be, when requirements specifications are rigid. Developers need a certain amount of leeway in order to feel that they are being creative whilst doing their job. If a specification goes into exact detail about how to implement something (which a requirements specification shouldn't actually need to do, unless the requirement is defining the inputs or outputs of the system), then a developer can feel there is no need for creativity, and the fun part of their job disappears quickly. Once that happens, developer motiviation becomes reduced and project success starts…
NEWS FLASH: Agile does not mean there is no plan
Why aren't all projects run using agile methods? Working in an enterprise where we have maybe 50 applications which all go live at the same time twice a year (as well as another 450 which are also in some stage of the software lifecycle), I was told that it would not be possible to plan such a release without tight control over the planning, and frankly agile methods do not cater for such planning... Erm... I see it a little different. Agile to me means that there is a plan and you know the maximum amount of money that you can to spend. To me it also means there is a fixed deadline. As quality is also fixed (come on, it has to be bug free!), it is scope which changes. Or admittedly to some degree also quality, as you decide that some bugs are tolerable and you can live with them. What is different about agile projects, is that the project plan and scope (functionality) are alive. They are dynamic. They change from day to day as everyone on the project learns and discovers. Just like your day to day plans change as events unfold. It should be illegal for a project manager to state that a project plan is fixed. It shouldn't be allowed that the project manager, upon discovering that a plan needs to change, states that it will not be changed because it has already been communicated to the customer. This view is certainly un-agile. But…
