Flex MVC options

The set of instructions in rich applications tends to get quite complex. Thus, RIA code needs to be divided up into smaller components to manage complexity. Design patterns such as MVC are loose methodologies for doing this, with limitless implementations in practice. In Flex, MVC are often referred to as microarchitectures. They manage business logic for a client application, allowing decoupling between logic and UI.

The prominent first generation of mature MVC framework for Flash and Flex is called Cairngorm. It is now up to 2.2 with version 3 hovering indefinitely in beta. Cairngorm introduces a new system of event management called the CairngormEvent. It is more analog than extension of a Flex Event, since it has unique dispatch and propagation attributes.

The pros and cons of isolated event management, MVC and FlexEvent, have been largely considered in the developer community, resulting in some rethinking of Cairngorm event methodology. A prominent advance is UniversalMind's UM Cairngorm Extensions. This represents first and foremost an enrichment of the Cairngorm framework. If you plan on going down this path, UM Extensions may allow you to do more with it.

Otherwise, Flex is reaching the second iteration of third party frameworks, as Cairngorm gets more competitors. Mate and Swiz lead the pack, with retooled and reconsidered architectures for events and business logic structures. In some cases, microarchitectures are fully MXML in the second iteration.

By no means is the solution one-sided, as many groups have invested heavily in their extant codebase. In any case, all the options deserve a thorough look see. Some orientation will allow you to consider the best for your project.

Multiple AMF Django apps share a common gateway

A while back I wrote about a library called django-flashpolicies for providing a flexible remoting environment. Essentially, it frees up the root of your django project for whatever content you like while providing a crossdomain policy file there for remoting applications.

That is still one way to go, but as an advance on that, have a look at the Django-amf-gateway library.

The PyAMF team offers info on it as a way to structure a django project such that you can heap on all the apps you want and allow them to share a common http gateway. By simplifying gateway addresses on the server side django-amf-gateway simplifies resource management on the client side.

Works great in applicable cases.

More on mod_wsgi issues

When you add wsgi to Apache it is so you can run Python on Apache. Thus you need, at a minimum, Python and Apache.

Python

If you use a utility such as virtualenv to compartmentalize several domain development environments on one server account, installing mod_wsgi may not and probably will not be associated with the correct Python library. Will not, that is, unless you explicitly point it out in the installation process. Be sure to plan in advance which Python install you are developing for, and be sure to flag it when installing mod_wsgi:

./configure --with-python=/home/django/domains/mydomain.com/bin/python2.5* ***

*** UPDATE (1/31/10): Please read the helpful comment and link from Graham Dumpleton, author of mod_wsgi, below. Per his direction, it is best to install mod_wsgi against the complete base Python installation and add references to the virtual environments from your Apache config or from the WSGI script file. Thanks for taking the time to clarify, and for making mod_wsgi available for general use.

That said, there are many ways to install mod_wsgi, from manual to managed by aptitude, get-apt or the like. The downside to using a managed install is the dependency on the vintage of their managed version. At last check, the aptitude repository of mod_wsgi is still at 1.3 while the current stable version on googlecode is 2.8. Thus, you may want to look into another method, perhaps a manual installation.

Apache

When manually installing mod_wsgi, be sure you have the appropriate development library for apache, in the case of ubuntu hardy heron, apache2_threaded_dev will allow you to install mod_wsgi. Furthermore, if you are using aptitude to install apache2_threaded_dev, go ahead and update the distro as a whole first by running sudo aptitude update. If you do not, you may find the install doesn't work, and there will be no indication as to what went wrong.

Finally, with these measures in place, try ./configure with the appropriate flags and see if you can get the correct library in place. Good luck.

*your location will be different from this example

ORM options for Python server to Flex client

Let's say Python is your solution of choice for backend development and Flash technology is your preferred client. If your project uses a database, you will need a good ORM solution at some point. The options are many and here are a few of them:

• WebORB by MidnightCoders
• Django built-in ORM
• SQLAlchemy ORM

There are surely other options (roll your own, anybody?) but these come to mind readily. Now, how to choose which one? Let's look at the capabilities of each:

WebORB

Pros: MegaOptions, from RTMP Server to Secure Transactions and lots more, turnkey CRUD, GUI admin, GUI performance, plug and play, one-click ORM generator, ORM testing suite. The list goes on...

Cons: Costs a pretty penny last I checked. Could be considered bloaty. I would have to hear some performance testers weigh in on this.

Verdict: Great at limiting server-side development to a minimum. Great full-service solution. Would be a strong contender in a rich media studio or design shop.

====================================

Django built-in ORM

I'll use it when the time comes. Methods to replace Django's ORM outright with SQLAlchemy have been detailed on some intrepid developer blogs. I would rather not customize the framework too much, as it introduces potential troubleshooting down the line. You can use Django ORM and an addition ORM library like SQLAlchemy interchangeably, so that would be my first choice if going this route.

=====================================

SQLAlchemy

A very robust open source option with a surfeit of detailed documentation, this one regularly gets my vote. It is sufficiently explicit to require familiarity to use, but not too steep a curve to get started quickly. Extensions like Elixir simplify the code at the expense of greater abstraction. This is a case of using the right tool for the right project, so feel free to learn more about both.

MAMP with mod_wsgi

Once you get a database set up, sqlite or otherwise, you can use it with Django applications. You can serve Django directly from a command line or through Apache. In either case you will need Django to be installed. From a command line, simply run the django server and browse to it:

django-admin.py runserver

If your production server is going to be Apache, your best bet is to run Django under Apache locally. To do this you need a helper library for Apache to use. The de facto solution is mod_wsgi.

mod_wsgi

To get started with mod_wsgi, typically you download mod_wsgi from Graham Dumpleton's Google Code repository, you go to your command line, you untar it, you configure it, you make it and you make install it. This makes the code library available to wsgi-enabled services, in effect allowing you to run Django on Apache.

When you make install mod_wsgi, it places the libraries in your /lib directory. Then it links to your Apache directory where OSX keeps it, /usr/libexec/apache2. Installation includes automatic detection, or you can use some optional flags to specify resource paths, detailed in the wsgi installation guidelines.

Now your OSX Apache has modwsgi installed, but the Apache version in MAMP does not. To verify this, try making a Django app and serving it via MAMP. First, make a new django app in an appropriate directory:

django-admin.py startproject myproject

Next, open and modify the config for your MAMP Apache app, located in /Applications/Mamp/conf/apache/httpd.conf. For that matter, have a look at what your OSX Apache instance has in its config file. It is private/etc/apache2/httpd.conf. Searching for wsgi in this file yields:

LoadModule wsgi_module libexec/apache2/mod_wsgi.so

Looks good even if the relative directory location may be an issue. Insert this info into the MAMP httpd.conf and test it. Actually, there is no need because it will not work. This is due to the structure of MAMP and its proprietary use of Apache. Unlike a full install of Apache, the folders in MAMP have a different structure and are optimized to run modularly. Consequently, you cannot append a flag to your configure command in this manner:

./configure --with-apxs=/usr/sbin/apxs2-worker

because the structure available to Apache is not in MAMP, so detection will fail.

This leaves a couple of options. Either reconstruct some of the MAMP Apache folders so detection will succeed or look for another solution. Considering the evident lack of general success from many Google searches, I am letting this fish off the hook. Feel free to leave a comment if you know a way to make it work.

As a final consideration, mod_python appears to be a lot quicker to set up with MAMP. That could be a candidate if you are in a pinch. Otherwise, if serving Python via Apache is your goal, I recommend using the resources included with OS X and skipping MAMP altogether.

MAMP with sqlite

If you want a local enviro to test in, MAMP for Mac is a good way to go. Right out of the box, you get Apache, MySQL, SQLite and PHP support. Also in the mix is python 2.5, which ship with OS X.

With several apps at the ready, how do you get your head around the new environment? Piece by piece. First let's look at SQLite.

SQLite

Have a good read at some typical command line options to get you up and running. Go ahead and try it out. See if you can create a db, add some values, retrieve the values.

When you store a database, it is stored in the current directory. You can see what that is by using the command pwd, aka print working directory. Further documentation is available on the SQLite site. If you decide command line is not your style, one option is the SQLite Manager for Firefox. It's not for remote db admin but it'll do for local scenarios.

In summary, getting up and running with sqlite is simple. MAMP comes with MySQL, so that is another available option.

Send Failed Flex Error in WebORB

Is Flex via WebORB middleware sending a message similar to this?

Error: Server reported an error - Send failed

The resolution will most likely consist of looking over your xml declarations, both for the declared endpoint URI and the service it requests.

If you face such a challenge, read the very useful orientation by WebORB creator and principal Mark Pillar. Check this post to confront the matter head on and move forward.

Good luck.