--- Aaron wrote:
> No there (traditionally) is no back-channel from the database to
> the client...
>... one of the clear benefits to a 3 tier model
Just to explain a bit further (seeing as I'm apparently on a 3 tier
evangelical mission at the moment):
In a 3-tier model you would have a server application that, say,
places orders. It would have a couple of methods:
interface OrderPlacer {
createOrder( customer : int; product : int; quantity : int );
adviseNewOrder( newOrderClient : OrderClient);
}
Any application that wanted to create a new order would connect to the
OrderPlacer server, and call the 'createOrder' method. It would *not*
insert rows into database tables.
Any application that wanted to know about new orders would need to
implement the 'OrderClient' interface, which includes a 'newOrderMsg'
method, and would need to call the 'adviseNewOrder' method of the
server, passing itself as an argument.
class SomeApplication implements OrderClient {
procedure newOrderMsg( customer : int;
product : int;
quantity : int );
}
The constructor, would call 'OrderServer.adviseNewOrder( self )' and
the .adviseNewOrder method would add the OrderClient object to a TList,
and then whenever 'createOrder' gets called it would execute some code
something like this, to let everybody who's interested in knowing about
new orders:
procedure createOrder( customer : int; product : int; quantity : int )
begin
[ code to update order tables snipped ]
for i := 0 to OrderClientList.Count -1 do
OrderClient( OrderClientList[i] ).newOrderMsg( customer,
product,
quantity )
end;
The client application then does whatever it wants with the information
pushed down to it from the server.
This is, if I recall correctly, an example of the GOF's Observer
pattern.
Hope this helps / enthuses somebody.
Cheers,
Kerry S
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