Redundancy

Client Redundancy active/active

Both clients are processed active with respect to the server. For the server there seem to be two separated connections. The real active/passive mechanism and the related discarding of values is carried out in WinCC OA. In order to avoid a data loss during the switch-over a general query is recommended (see autoGQ).

The advantage of this redundancy variant is that the server has not to support any redundancy abilities.

The figure below shows the client redundancy architecture in the active/active form.

OPC UA Server Redundancy

The client is always connected to both servers. The subscriptions to both servers are active. By the aid of the timestamp the client compares the values. The client sends the commands to both servers. The command is processed only by the active server. Thereby the client does not have to know which server is the active one. In this case the passive server has to deliver either correct values or none values. It must not deliver faulty values.

The OPC UA driver is establishing a connection to the redundant server as soon as the data point groups have been defined (see Access to WinCC OA Data). All transactions from or to the server are processed for both on main server and on redundant server.

Browsing and the conversion of BrowsePath to NodeId is performed by the server with the higher service level. If both servers have the same level the process is taken over by the main server.

The figure below shows the architecture of the server redundancy (it is assumed that the systems are WinCC OA systems).