MIDAS: a Delphi Three-Tiers approach

Three tiers architecture are generally defined by a Client thier (first thier), an application server (that contains the business logic and the database connection) and a database that stores the persistent information.

Some years ago Borland was proposing a quite easy approach to get benefit from those architectures.

Client computers
They know nothing about the database and database connection. They can access dataset but they don’t know the connection string. (The business logic remains in the Client software, but the dataset are contained in the application)

AppServer
contains the connection string and all the dataset required by the client on the database server.
Advantage : you have to install ADO (MDAC) components on the client machines and more important if configuration changes you have not to reconfigure all the computer park (if you have 2 machines it is ok, but if you have more, it can cost time)=>Zero configuration of thin clients.
You can also safe costs if you have to pay license for connection on the database server per client (because only one server is connected). This depends on the licensing mode.

The database server
Typically it stores the data on secure, transactional and persistent way. It can also do some business logic by using stored procedure.

Technical approach
Generally an application is based on component(s) that implement the interface IAppServer (provided by the Delphi MIDAS Multi tIer Distributed application Service most referred as DataSnap). So to create a Application Server you have to create at least one COM component. In the MIDAS philosophy the COM component (CoAppServer) contains the Dataset. We can create those dataset at design time and configure them with the proper database connection or connection string. The CoAppServer, provide easy way to make the client application communicates with the Datasets contained in the CoAppServer. Practically the client application contains a ClientDataset which is an image of the DataSet contained on the CoAppServer. The communication between the client and server can be done by DCOM, TCP/IP, HTTP, CORBA. The CoAppServer ensure that the communication is bidirectional so you can read and update dataset from the client without having an actual connection on the database itself. The only things you need is a copy of the midas.dll on the client and a valid CoAppServer that you can access.

Oui, je sais ça date un peu, mais bon nostalgie quand tu nous tiens ;-) ... Je n'ai pas eu le temps de traduire ce vieux document qui date de quelques années en français mais après l'avoir lu vous saurez que Midas n'était pas uniquement un roi qui par une grâce divine transformait tout ce qu'il touchait en or... En parlant de grâce divine, ce n'était pas si gracieux puisqu'il ne pouvait plus manger. Il pu heureusement pour lui se débarasser de ce don en se lavant les mains dans le fleuve Pactole qui depuis lors, selon la légende charie de l'or...