Hi Arto,

In short, a tier is a piece of software that has its own process on the same or other computer as another. Changing e.g. from a SQL server database to an Oracle (if supported by the application) is therefor easy if the SQL Server datamanagement tier a process that is independed from the rest of the application.

If the SQL server datamanagement layer is in a tier together with e.g. the GUI layer, then you might not be able to exchange the SQL Server datamanagement layer with the Oracle datamanagement layer without a recompile. However there are workarrounds using DLLs or COM objects for each layer.

However, IMO, the difference between tiers and layers is that tiers run in different processes and can be scattered across the network. A single tier itself could consist out of multiple layers. Two layers in the same tier run in the same CPU process space.

Walter,


>Walter,
>
>>I'm sure you know this, but for the lurkers: There is a difference between the definition of tiers and layers. One executable can consist out of many layers. Layers are logical separation of levels of implementation. Each layer communicates with an upper or lower level through a defined interface. Tiers on the other hand are simiular to layers, but they generally run in different processes and even on other machines.
>
>Could you please explain more the next:
>
>>Exchanging a tier is easy, exchanging a layer within a might requires a recompile.
>
>Thanks,
>
>AT