Thursday, February 23, 2012

From COBOL to DataSnap XE2

Many years ago I had built an automation server for my accounting platform using Delphi 6. It was based on TRemoteDatamodule and the Midas components TDatasetProvider & TClientDataset. It was a discovery for me especially speaking for TClientDataset and its unlimited features not only as a Midas client but as a standalone dataset fulfilled with capabilities that a blog post would not be enough to describe.

A couple of weeks ago, after I finished installing and familiarizing my self with the brand new Delphi XE2, I started studying its DataSnap technology. I have to say that I was somehow impressed by the bunch of features I had seen in demos, videos and webinars. Off course Midas components are still at the first line of building database client/server apps with DataSnap, but also a lot of other technologies have been added or heavily enhanced.

Some of them that I noticed are:
  • Datasnap server with TCP/IP and HTTP(S) protocols
  • Server classes with exposed methods to remote clients
  • JSON object marshalling & un-marshalling
  • Enhanced DBExpress framework (though I have never used it),
  • DataSnap Connectors for Mobile Devices !
  • DataSnap Callbacks
  • Authorization & Authentication
  • And many others that complete a long list to mention.
I was so confused with all these that I decided to give it a real world try by converting a old (from 1989) COBOL application to DataSnap. 
Did I said “Convert” ? Fully rebuild is the right choice of words. 

As database layer I was planning to stick with my well known SQLServer and ADODB in order to focus on Datasnap only. After database schema decided and built into SQL Server, I started the DataSnap server wizard of Delphi XE2. Choose “VCL Forms Application”, enabled all features except HTTPS and selected TDSServerModule as server methods class. 
  • VCL Form selected in order to have a live server interface to do things like message logging, start/stop server, monitor its activity etc.
  • All features selected in order to be able to test most DataSnap capabilities and be ready for mobile clients.
  • TDSServerModule as server methods class provides a TDatamodule surface for VCL non-visual components and IAppServer interface which is essential for Dataset providers and clientdatasets communication (Midas).
The project the wizard  created was a lot different from what I was used using Automation with TypeLib and TRemoteDatamodule, but a few hours later I (think) had a clear understanding of the framework. The major aspects I acknowledge in simple terms are:
  • VCLForm: the main form for monitoring server activity
  • Container: a TDatamodule that is the main server module containing among other components a TDSServer, the class that manages transports from and to remote clients and server classes, a TDSServerClass which in turn used to specify a server-side class.
  • ServerMethods: a TDSServerModule supporting IAppServer interface and contains the published methods that can be called from a remote client using dynamic method invocation.
How these works is pretty simple at its basics. 

Requests are served through the Container’s TDSServer component which coordinates traffic with the help of the other container components. 

As for the TDSServerClass it is the component responsible for creating its associated ServerMethods class. 

ServerMethods class exposes to remote clients its public methods and for the TDSServerModule especially the IAppServer interface for provider/clientdataset communication (Midas).

You can create as many classes as you want, either TComponent, TDataModule, TDSDataModule and expose their published methods, and IAppServer Interface in case of TDSDataModule, by just adding a TDSServerClass component in your server Container and respond to its “OnGetClass” event like this:

procedure TMyServerContainer.DSMyServerMethodsGetClass(DSServerClass
  : TDSServerClass; var PersistentClass: TPersistentClass);
  PersistentClass := TMyClass; //Class of your TComponent, TDatamodule, TDSDataModule

At client side now I had to create a VCL forms application and connect to the server to retrieve and present data for editing to the user. Three components do the base job:
  • TSQLConnection: is the connection component that encapsulates a dbExpress connection to a database server. It can connect to various database server but in the case mentioned it connects to my DataSnap server using tcp/ip protocol.
  • TDSProviderConnection: a component that provides connectivity to a DataSnap server methods class, the above mentioned ServerMethods (TDSServerModule), using dbExpress.
  • TClientDataset: connected to the TDSProviderConnection via RemoteServer property and to the actual provider of the data in ServerMethods via ProviderName property.
I won’t describe further the details of setting the properties of those components as there are a lot of video presentations and articles at the official Embarcadero site & blogs. Also having a close look at the DataSnapXE projects in your XE2 samples folder is a must and I found them to be very much helpful.

Back to now, the business logic of my project is already implemented at the server and test clients have succeeded testing and coordinating them.

TClientDataset works as expected and a few extra communication needed between client and server implemented based on streaming TParams (TParamsStorage) via OwnerData. I use a lot this technique to pass info from one module to another and especially for passing user input from UI forms to datamodule methods that need to build runtime queries and return result sets for browsing, reporting etc. See my article series “A Journey to TParams” for more info.

Client Reporting requests are served from the server as entities of ClientDataset XmlPackets and processed at the client and the only things left are UI enhancements. Hope it soon will be completed so i can dig in more interesting aspects like Mobile connectors.

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