Errata exists for this version of the document.
The ServiceLevel provides information to a Client regarding the health of a Server and its ability to provide data. See OPC 10000-5 for a formal definition for ServiceLevel. The ServiceLevel is a byte with a range of 0 to 255, where the values fall into the sub-ranges defined in Table 109.
The algorithm used by a Server to determine its ServiceLevel within each sub-range is Server specific. However, all Servers in a Redundant Server Set shall use the same algorithm to determine the ServiceLevel. All Servers, regardless of Redundant Server Set membership, shall adhere to the sub-ranges defined in Table 109.
Table 109 – ServiceLevel Ranges
|
Sub-range |
Name |
Description |
|
0-0 |
Maintenance |
The Failed Server is in maintenance sub-range. Therefore, new Clients shall not connect and currently connected Clients shall disconnect. The Server should expose a target time at which the Clients are able to reconnect. See EstimatedReturnTime defined in OPC 10000-5 for additional information. A Server that has been set to Maintenance is typically undergoing some maintenance or updates. The main goal for the Maintenance ServiceLevel is to ensure that Clients do not generate load on the Server and allow time for the Server to complete any actions that are required. This load includes even simple connections attempts or monitoring of the ServiceLevel. The EstimatedReturnTime indicates when the Client should check to see if the Server is available. If updates or patches are taking longer than expected the Client may discover that the EstimatedReturnTime has been extended further into the future. If the Server does not provide the EstimatedReturnTime, or if the time has lapsed, the Client should use a much longer interval between reconnects to a Server in the Maintenance sub-range than its normal reconnect interval. |
|
1-1 |
NoData |
The Failed Server is not operational. Therefore, a Client is not able to exchange any information with it. The Server most likely has no data other than ServiceLevel, ServerStatus and diagnostic information available. A Failed Server in this sub-range has no data available. Clients may connect to it to obtain ServiceLevel, ServerStatus and other diagnostic information. If the underlying system has failed, typically the ServerStatus would indicate COMMUNICATION_FAULT_6. The Client may monitor this Server for a ServerStatus and ServiceLevel change, which would indicate that normal communication could be resumed. |
|
2-199 |
Degraded |
The Server is partially operational, but is experiencing problems such that portions of the AddressSpace are out of service or unavailable. To understand Client options, see Degraded Servers discussion in this section. An example usage of this ServiceLevel sub-range would be if 3 of 10 devices connected to a Server are unavailable. Servers that report a ServiceLevel in the Degraded sub-range are partially able to service Client requests. The degradation could be caused by loss of connection to underlying systems. Alternatively, it could be that the Server is overloaded to the point that it is unable to reliably deliver data to Clients in a timely manner. If Clients are experiencing difficulties obtaining required data, they shall switch to another Server if any Servers in the Healthy range are available. If no Servers are available in the Healthy range, then Clients may switch to a Server with a higher ServiceLevel or one that provides the required data. Some Clients may also be configured for higher priority data and may check all Degraded Servers, to see if any of the Servers are able to report as good quality the high priority data, but this functionality would be Client specific. In some cases a Client may connect to multiple Degraded Servers to maximize the available information. |
|
200-255 |
Healthy |
The Server is fully operational. Therefore, a Client can obtain all information from this Server. The sub-range allows a Server to provide information that can be used by Clients to load balance. An example usage of this ServiceLevel sub-range would be to reflect the Server’s CPU load where data is delivered as expected. Servers in the Healthy ServiceLevel sub-range are able to deliver information in a timely manner. This ServiceLevel may change for internal Server reason or it may be used for load balancing described in 6.6.2.4.3. Client shall connect to the Server with the highest ServiceLevel. Once connected, the ServiceLevel may change, but a Client shall not Failover to a different Server as long as the ServiceLevel of the Server is accessible and in the Healthy sub-range. |