Each Server maintains a list of ServerUrisfor all redundant Serversin the Redundant Server Set. The list is provided together with the Failovermode in the ServerRedundancy Objectdefined in OPC 10000-5. To enable Clients to connect to all Serversin the list, each Serverin the list shall provide the ApplicationDescriptionfor all Serversin the Redundant Server Setthrough the FindServers Service. This information is needed by the Clientto translate the ServerUriinto information needed to connect to the other Serversin the Redundant Server Set. Therefore a Clientneeds to connect to only one of the redundant Serversto find the other Serversbased on the provided information. A Clientshould persist information about other Serversin the Redundant Server Set.

Table 111defines a list of Clientactions for initial connections and Failovers.

Table 111– Redundancy Failover actions

Failover mode and Clientoptions

Cold

Warm

Hot (a)

Hot (b)

HotAndMirrored

On initial connection in addition to actions on Active Server:

Connect to more than one OPC UA Server.

X

X

X

Optional for status check

Create Subscriptionsand add monitored items.

X

X

X

Activate sampling on the Subscriptions.

X

X

Activate publishing.

X

At Failover:

OpenSecureChannel to backup OPC UA Server

X

X

CreateSession on backup OPC UA Server

X

ActivateSession on backup OPC UA Server

X

X

Create Subscriptionsand add monitored items.

X

Activate sampling on the Subscriptions.

X

X

Activate publishing.

X

X

X

Clientscommunicating with a non-transparent Redundant Server Setof Serversrequire some additional logic to be able to handle Server failures and to Failoverto another Server in the Redundant Server Set. Figure 28provides an overview of the steps a Clienttypically performs when it is first connecting to a Redundant Server Set. The figure does not cover all possible error scenarios.

image031.png

Figure 28– Client Start-up steps

The initial Server may be obtained via standard discovery or from a persisted list of Serversin the Redundant Server Set. But in any case the Client needs to check which Server in the Server set it should connect to. Individual actions will depend on the Server Failovermode the Server provides and the Failovermode the Client will use.

Clients once connected to a redundant Server shall be aware of the modes of Failoversupported by a Serversince this support affects the available options related to Client behaviour. A Client may always treat a Server using a lesser Failovermode, i.e. for a Server that provides Hot Redundancy, a Client might connect and choose to treat it as if the Server was running in Warm Redundancyor Cold Redundancy. This choice is up to the Client. In the case of Failovermode HotAndMirrored, the Clientshall not use Failovermode Hotor Warmas it would generate unnecessary load on the Servers.