SOAB is good to go!

Yes, it’s done. I’ve finished putting the finishing touches on the smart client offline application block. All that remains is to figure out a cool name for the project. In any case, I’d post the code and a demo application at tonight. In the meantime, here’s some of the paper work I finished.

Benefits of the new SOAB

  • Simplified Deployment
    No SQL Servers to deploy, no database scripts to run, no logins to create; simply application where you want and run it. The cache and queue databases will be created on the fly the very first time it’s needed. It’s that simple!
  • Inter-dependant queue elements
    Imagine a smart-client application used for cataloging books. The central server maintains a catalog of book and authors. For a book to be cataloged, its author must exist in the database. (Exactly how such an application would be useful is of no concern to us here :). The smart client downloads the list of authors from the server when it goes online. A problem arises when a book along with its author has to be created offline. When an author is created offline a “Create Author” request is queued in the command queue. Likewise, when a book is created a “Create Book” request is queued. The new SOAB provides a mechanism by which the programmer of the smart-client could specify that a “Create Book” request should be executed (transferred to the web-service), only once its related author object has been successfully put on the central database.
  • All the glory of Db4O
    Db4O is a powerful Object Database, with advanced functionality such as native query, SODA query, etc. etc. While the SOAB.Data.Database class attempts to wrap most of the interactions with the underlying Db4O Object Container, it is still accessible to the brave-hearted. This opens up number of additional possibilities such as modifying/deleting individual items from the queue, querying for specific items in the cache or the queue, determining which items are to be modified when the queued elements are executed etc.
  • Tiny Code base, built on .NET 2.0
    The application block has only about 15 classes and interfaces in total: Out of which only 6 you’d need to concern yourselves with. You’d probably take less than 2 hours to identify how to harness the full potential of the SOAB. The new SOAB uses generics, and the new event-based asynchronous invocation methods, breaking away from the old way of doing things.

Working with the SOAB

Working with the SOAB is almost effortless; simply follow these few steps

  • In your smart-client application, add a class derived from SOAB.ServiceAgent. In the sample application I have made this derived class (ServiceClient) a singleton. While this is not always required, it might be a good idea to do so.
  • Create a ServiceContext class and keep it around as it can be used for all service requests. Since, the ServiceClient is a singleton, I’ve made the ServiceContext instance, on of its fields. The ServiceContext class is basically a builder for your web-service proxy class. Make sure that you pass the fully qualified class name of the proxy to its constructor.
  • When you need to invoke a web method, create and populate a ServiceRequest object and enqueue it using QueueManager.Instance.Enqueue(…) method. To populate the ServiceRequest assign;
  • The ServiceContext instance to the ServiceRequest.ServiceContext property.
  • The name of the web-method you are invoking to ServiceRequest.RemoteMethodName.
  • The argument to the web-method to ServiceRequest.Payload.
  • The name of a method accepting a ServiceResponse argument belonging to the ServiceAgent derived class to ServiceRequest.CallbackMethodName.

The method specified in the CallbackMethodName property, is invoked once the ServiceRequest has been executed. You should use the properties in the ServiceResponse object passed on to the method to determine whether the request was successful, cancelled or whether it generated an exception. If you have any object types that might depend on other objects (such as Book that depend on Author), you should implement ICacheDependant interface on the dependant class. Provide the dependency resolution logic in the IsResolved property-getter and return true if the object is ready to be transferred to the web method.

Words to the Wise

Please note that this is my first complete project using Db4O. So, the Db4O-related code might not be optimal. For instance, I’ve used ReaderWriterLocks for guarding object container read/write actions. However, the database itself has built in locking mechanisms which I could’ve used for the same purpose. Hopefully, somebody who’s more experience with Db4o can offer some assistance …?

Alternative Smart Client Offline Application Block


With the advent of the .NET platform, there’s been huge interest in the area of Smart Client applications. While the concepts behind them have existed in pre-.NET days and in non-Microsoft platforms, never has this technology looked more promising than it is today. From WinForms 2.0, WSE 3.0, Composite UI Application Block etc. all the way to WCF and WPF, Microsoft has made sure that if you want to go Smart-Clienting, MS tools are the way to go. Nearly 6 months ago, I got the chance to work on my company’s first ever Smart Client application. Needless to say, being a very successful web development company, it was looked at as high risk – high reward project. If we made it work, we’d have opened up a whole new market; if we did not, it would’ve been back to the old game plan (at least for a while).

So we played it safe; stuck to the recommended application blocks, most stable releases of the development tools etc. etc. Surprisingly enough, we managed to put together our first prototype in record time, using .NET 1.1, 2005 and 2004 releases of MS DAAB (Data Access Application Block) and SOAB (Smart-Client Offline Application Block) and WSE 2.0 SP 3.

This gave us the chance to take a long look at our next milestones. Which were basically, migrate to VS 2005, ditch DIME for MTOM, adopt the new (.NET 2.0) & truly asynchronous remote invocation model and of course migrate the existing application blocks. While rest of the transition was pretty smooth, we never found a new version of the offline application block. I have to say, we never really liked the original SOAB to begin with. It lacked a lot of the functionality we required but came in a huge bundle linking to the DAAB, Caching Application Block, Exception Management Application Block etc. etc. Nearly 140 source code files! Doesn’t make your life any easier when the application starts crashing for no apparent reason…

And by the way, deploying an SQL Server on client machines? Oh, come on! Whatever happened to X-Copy Deployment? Of course, there are other options for storage, but they are hardly worth talking about.

So, it got me thinking. There had to be a better way; and there was.

Db4O – 4 All Your Offline Needs

Now, if you take Microsoft’s Offline Application Block, it serializes the cache objects and command queue items on to its database. Once you do that, you loose lot of flexibility. Of course in the RDBMS world, there’s hardly any alternative. In the MSDE world, there was no way to design a Cache database or a Queue database to hold arbitrary application specific objects. But, with SQL 2005, I’d assume you could use XML columns and along with XQuery to achieve this. Then again, even if you do it, I can still whine about the X-Copy, right?

Last year, while researching for my final year project, I took a look at Db4O ( Db4O is an open-source, object database for .NET/MONO and Java. See, sometimes education does come in handy. I was just playing with the idea of using the object database for the cache/queue database, and it didn’t took 5 minutes before the advantages became evident. The downside is of course, Db4O is distributed under GPL and if you need to use it in a commercial application, you’d need to get a commercial license from the developers.

So hopefully, over the few days, I’d post more details about the Offline Application Block that I have developed. And by Monday, if I could get my code refactored properly and figure out the licensing issues, I’d have the full source code available for you.