Server Management

The ServerArray Variable defines an array of server URIs. This Variable is also referred to as the server table. Each URI in this array represents a globally-unique logical name for a server within the scope of the network in which it is installed. Each OPC Unified Architecture server instance has a single URI that is used in the server table of other OPC Unified Architecture servers.

Note
Index 0 is reserved for the URI of the local server. The URI of the ServerArray with Index 0 must be identical to the URI of the NamespaceArray with Index 1, since both represent the local server.

Values above 0 are used to identify remote servers and are specific to a server. Topic Services describes discovery mechanism that can be used to resolve URIs into URLs.

The indexes into this table are referred to as server indexes or server names. They are used in OPC Unified Architecture services to identify target nodes of references that reside in remote servers. Clients may read the entire table or they may read individual entries in the table.

Note
The server must not modify or delete entries of this table while any client has an open session to the server, because clients may cache this table. A server may add entries to the table even if clients are connected to the server.

The NamespaceArray Variable defines an array of namespace URIs. This Variable is also referred as namespace table. The indexes into this table are referred to as namespace indexes. Namespace indexes are used in NodeId values provided as OPC Unified Architectureservices parameters, rather than the longer namespace URI. Index 0 is reserved for the OPC Unified Architecture namespace, and index 1 is reserved for the local server. Clients may read the entire table or they may read individual entries in the table.

Note
The server must not modify or delete entries of this table while any client has an open session to the server, because clients may cache this table.

A server may add entries to the table even if clients are connected to the server. It is recommended that servers not change the indexes of this table but only add entries, because the client may cache NodeId values using the indexes. Nevertheless, it might not always be possible for servers to avoid changing indexes in this table. Clients that cache namespace indexes of NodeId values should always check when starting a session to verify that the cached values have not been changed.

The ServerStatus Variable of the ServerStatusType contains elements that describe the status of the server of the ServerStatusDataType.

The ServiceLevel Variable describes the ability of the server to provide its data to the client. The value range is from 0 to 255, where 0 indicates the worst and 255 indicates the best. The concrete values are vendor-specific. The intent is to provide the clients an indication of availability among redundant servers.

The Auditing Variable is a Boolean specifying if the server is currently generating audit events. It is set to TRUE if the server generates audit events, otherwise to false. The Profiles specify what kind of audit events are generated by the server.

The ServerCapabilities Object defines the capabilities supported by the OPC Unified Architecture server.

The ServerDiagnostics Object defines diagnostic information about the OPC Unified Architecture server.

The VendorServerInfo Object represents the browse entry point for vendor-defined server information. This Object is required to be present even if there are no vendor-defined Object nodes beneath it.

The ServerRedundancy Object describes the redundancy capabilities provided by the server. This Object is required even if the server does not provide any redundancy support. If the server supports redundancy, then a subtype of ServerRedundancyType is used to describe its capabilities. Otherwise, it provides an Object of type ServerRedundancyType with an empty array of RedundancySupportArray.

ServerProfileArray defines the conformance profile of the server.

LocaleIdArray is an array of LocaleId values that are known to be supported by the server. The server might not be aware of all LocaleId values that it supports because it may provide access to underlying servers, systems or devices that do not report the values that they support.

MinSupportedSampleRate defines the minimum supported sample rate, including 0, which is supported by the server.

MaxBrowseContinuationPoints is an integer specifying the maximum number of parallel continuation points of the Browse service that the server can support per session. The value specifies the maximum the server can support under normal circumstances, so there is no guarantee the server can always support the maximum. The client should not open more Browse calls with open continuation points than exposed in this Variable. The value 0 indicates that the server does not restrict the number of parallel continuation points the client should use.

MaxQueryContinuationPoints is an integer specifying the maximum number of parallel continuation points of the QueryFirst services that the server can support per session. The value specifies the maximum the server can support under normal circumstances, so there is no guarantee the server can always support the maximum. The client should not open more QueryFirst calls with open continuation points than exposed in this Variable. The value 0 indicates that the server does not restrict the number of parallel continuation points the client should use.

MaxHistoryContinuationPoints is an integer specifying the maximum number of parallel continuation points of the ReadHistory services that the server can support per session. The value specifies the maximum the server can support under normal circumstances, so there is no guarantee the server can always support the maximum. The client should not open more ReadHistory calls with open continuation points than exposed in this Variable. The value 0 indicates that the server does not restrict the number of parallel continuation points the client should use.

SupportsAuditing is a Boolean specifying if the server is capable of generating audit events. It is set to TRUE if the server can generate audit events, otherwise it is set to FALSE. It does not specify if the server is currently generating audit events. This information is captured in the Auditing Variable of the Server Object.

ModellingRules Object of the FolderType is an entry point to browse to all modelling rules supported by the server. All modelling rules supported by the server should be able to be browsed starting from this Object.

This ServerDiagnosticsSummary Variable of the ServerDiagnosticsSummaryType contains diagnostic summary information of the ServerDiagnosticsSummaryDataType for the server.

The SamplingRateDiagnosticsArray Variable is an array of diagnostic information per sampling rate. There is one entry for each sampling rate currently used by the server. Its type definition node is the SamplingRateDiagnosticsArrayType, providing a Variable of the SamplingRateDiagnosticsDataType for each entry in the array.

The SubscriptionDiagnosticsArray Variable is an array of subscription diagnostic information per subscription. There is one entry for each notification channel actually established in the server. Its type definition node is the SubscriptionDiagnosticsArrayType, providing a Variable of the SubscriptionDiagnosticsDataType for each entry in the array. Those Variables are also used as Variables referenced by other Variables.

SessionsDiagnosticsSummary of the SessionsDiagnosticsSummaryType contains diagnostic information per session.

EnabledFlag identifies whether or not diagnostic information is collected by the server. It can also be used by a client to enable or disable the collection of diagnostic information of the server. The following settings of the Boolean value apply:

• TRUE indicates that the server collects diagnostic information, and setting the value to TRUE leads to resetting and enabling the collection.

• FALSE indicates that no statistic information is collected, FALSE disables the collection without resetting the statistic values.

SessionDiagnosticsArray Variable of the SessionDiagnosticsArrayType provides an array of the SessionDiagnosticsDataType with an entry for each session in the server having general diagnostic information about a session.

SessionSecurityDiagnosticsArray of the SessionSecurityDiagnosticsArrayType provides an array of the SessionSecurityDiagnosticsDataType with an entry for each active session in the server having security-related diagnostic information about a session. Since this information is security-related, it should not be made accessible to all users, but only to authorised users.

This SessionDiagnostics Variable of the SessionDiagnosticsVariableType contains general diagnostic information of the SessionDiagnosticsDataType about the session.

This SessionSecurityDiagnostics Variable of the SessionSecurityDiagnosticsArrayType provides the SessionSecurityDiagnosticsDataType that contains security-related diagnostic information about client sessions. Since this information is security-related, it should not be made accessible to all users, but only to authorised users.

This SubscriptionDiagnosticsArray Variable is an array of subscription diagnostic information per opened subscription. Its type definition node is the VariableTypeSubscriptionDiagnosticsArrayType providing a Variable of SubscriptionDiagnosticsDataType for each entry in the array.

This RedundancySupport component of the PropertyType indicates what redundancy is supported by the server (value of the RedundancySupport).

RedundancySupport Variable is inherited from the ServerRedundancyType.

Although, in a transparent switchover scenario, all redundant servers serve under the same URI to the client, it may be required to track the exact data source on the client. Therefore, CurrentServerId contains an identifier of the currently-used server in the redundant set. This server is valid only inside a session; if a client opens several sessions, different servers of the redundant set of servers may serve it in different sessions. The value of the CurrentServerId may change due to failover or load balancing, so a client that needs to track its data source shall subscribe to this Variable.

As diagnostic information, the RedundantServerArray of the RedundantServerDataType contains an array of available servers in the redundant set; including their service levels. This array may change during a session.

ServerUriArray is an array with the URI of all redundant servers of the OPC Unified Architecture server. See Concept for the definition of the redundancy concept. Since, in a non-transparent redundancy environment, the client is responsible to subscribe to the redundant servers, it might or might not open a session to one or more redundant servers of this array.

The redundancy support provided by the server is defined in the RedundancySupport component of the the ServerRedundancyType supertype. The client is allowed to access the redundant sever only as described there, however, ”hot” switchover implies the support of “warm” switchover and “warm” switchover implies the support of “cold” switchover.

Note
If the server supports only a “cold” switchover, the ServiceLevelVariable of the server object should be considered to identify the primary server. In this scenario, only the primary server may be able to access the underlying system, because the underlying system may support access only from a single server. In this case, all other servers will be identified with a ServiceLevel of zero.