OPC FAQ

The basic distinction between a client and server is "control." Clients control servers. Clients tell servers what to do, while servers do as they are told.

The following factors will help you determine whether or not an application should be programmed as an OPC client or server:

• Push or pull data: OPC clients can pull data from OPC servers. They can also push data to OPC servers. In both cases they tell the server what to do.
• Synchronous or asynchronous communication: OPC clients can initiate synchronous and asynchronous calls to OPC servers. Again, the client is telling the server what to do.
• Initiating communication: OPC clients typically initiate communication with OPC servers. However, in the case of callbacks, OPC servers actually initiate replies to OPC clients with data changes. In effect, the server behaves like a client. This callback is technically in reply to a client's initial request, and so the client still controls the server.

In contrast to OPC client applications, OPC servers always do as they are told.