asked on

How to return a DataSet from a DataSnap Server

I Have coded trivial a DataSnap server and client with Embarcadero XE3 C++ builder which works and returns a double when a Sum() method is called.
I now wish to code the server to return a Dataset from a Table which is accessible at the server end and have the client receive the DataSet for local processing.
how do I do this ? I can't code a method at the server such as :

TDataSet TMyClass::ATable(void)
{
return *DataModule1->SQLTable1;
}

so I am guessing that there are DataSnap components that need to be used at the server end and the Client end ?
I attach a screen shot of the server components that I am using.

Roger Alcindor

ASKER

I have found some code (shown below) for the server that would appear to provide an example of the functionality that I need.
What code would I need to receive the Dataset at the client end (Windows 32 App) ?

TDBXReader* TServerMethods1::GetCountries() {

      TDBXCommand *comm;

      try {
            DbConn->Open();
            comm = DbConn->DBXConnection->CreateCommand();
            comm->CommandType = TDBXCommandTypes_DbxSQL;
            comm->Text = "Select * from Country";

            if (!comm->IsPrepared) {
                  comm->Prepare();
            }
            return comm->ExecuteQuery();
      }
      catch (Exception* e) {
            return NULL;

      }

}

sarabande

i don't know much about Embarcadero XE3 but, generally, you would send the data set as a byte array to the client. if the byte array is big - say greater 1k - you would send fixed sized 512 bytes packets until the data was fully sent.

at the client side you probably would wait for the data set in a separate thread what makes your program still responsive. so the general design would be

client: connects to server
server: accepts client
client: sends request to server
server: receives request
checks request and returns TRUE if valid, FALSE otherwise.
client: error on FALSE
starts thread and passes connection handle
returns to user interface
client
thread: waits for server response with timeout
server: query for data set
on success sends data of data set as a byte array to client
thread: receive dataset as byte array.
stores bytearray into a shared dataset
message to ui thread "data received"
exits thread.
client: receive message in a timer handler
show (and/or store) data set

Sara

ASKER CERTIFIED SOLUTION

Roger Alcindor

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sarabande

thanks for your feedback.

i looked at the solution and it looks straight-forward. however, it seems to be a sychronous call to server. if so, the foreground would "freeze" in case of a lengthy query. if that is a problem, you may consider a multi-threaded solution where you keep the user-interface working while waiting for the data set to be returned in a thread. that is not very difficult. you simply would create a new thread in the OnButtonClicked handler and would let do the thread call the server and wait for response while the OnButtonClicked would return.

Sara

Roger Alcindor

ASKER

I will bear that in mind for the future. In this application the query is not a lengthy one and the server is only accessed by the client when the application starts. I anticipate that the foreground freezing would not be an issue as the dataset being returned is only about 30 records.

Roger

Roger Alcindor

ASKER

The issue was solved on another question that I submitted and points were awarded there, see
https://www.experts-exchange.com/questions/28670748/Can't-create-an-instance-of-a-TDBXReader-class.html?anchorAnswerId=40769868#a40769868