The Session-less Service invocation is introduced for Services, such as Read, Write or Call, that do not require any caller specific state information. It is accessible through the SessionlessInvoke Service which provides the context information required to call Services without a Session.
Session-less invocation is limited to Services of the View Service Set (with exception of RegisterNodes and UnregisterNodes), Attribute Service Set, Method Service Set, NodeManagement Service Set and Query Service Set. All Services belonging to these Service Sets that are supported by a Server via a Session shall also be supported via the SessionlessInvoke Service.
Session-less Services can be invoked via a SecureChannel by using the Access Token returned from the Authorization Service as the authenticationToken in the requestHeader. The SecureChannel shall have encryption enabled to prevent eavesdroppers from seeing the Access Token. The Access Token provides the user authentication. If application authentication through the SecureChannel is sufficient, Servers may not require the Access Token and assume an anonymous user. In this case the authenticationToken shall be null.
The SessionlessInvoke Messages are just an envelope for the Service to invoke and do not have a RequestHeader and ResponseHeader like other Services. Those parameters are already part of the body which contains the Message for the Service to invoke.
Any Endpoint used for normal communication could be used for Session-less invocation provided the Endpoint supports encryption. The Server returns Bad_ServiceUnsupported if it does not support Session-less invocation for the request specified in the body. If it supports invocation but not with the combination of Endpoint and security settings used it returns Bad_SecurityModeInsufficient.
Servers may expose Endpoints which are only for use with Session-less invocation. These Endpoints shall support GetEndpoints and FindServers in addition to the SessionlessInvoke Service. The Server returns Bad_ServiceUnsupported for the other Services.
A Session ensures that a namespace index or a server index does not change during the lifetime of a Session. This cannot be ensured between Session-less Services invocations. There are two options to ensure the namespace indices in the call match the expected namespace URIs in the Server. One option for the caller is to provide the list of namespace URIs used to build the namespace indices. This works best for single Session-less Service invocations. The second option is to provide the UrisVersion to ensure consistency of namespace arrays between Client and Server. The UrisVersion is first read from the Server together with the NamespaceArray and ServerArray. This reduces the overhead per call for a sequence of Session-less Service invocations.