A detailed example of Selective Message Blocking (SMB) follows. The table below represents the agent's message queue as maintained by the ZIS. The message at the top represents the oldest message in the queue and is the message that is currently being processed by the agent as the example begins.
| Agent Message Queue |
| SIF_Event message containing a StudentSchoolEnrollment object with an Action of "Add." |
| SIF_Event message containing a StudentPersonal object with an Action of "Add." |
| SIF_Request message for a StudentPersonal object from another agent. |
| SIF_Event message containing a StudentSchoolEnrollment object with an Action of "Add." |
When processing the StudentSchoolEnrollmentEvent, the agent requires data from a SchoolInfo object that it doesn't have locally. It would like to request the SchoolInfo object without needing to process subsequent events. To do so, the agent acknowledges the StudentSchoolEnrollmentEvent with an "Intermediate" SIF_Ack indicating that the ZIS will be contacted later to resume delivery of events. It then opens a channel to the ZIS and submits a SIF_Request for the SchoolInfo object.
Upon receipt of the "Intermediate" SIF_Ack, the ZIS freezes the delivery of any SIF_Event messages to this agent until the agent sends a final SIF_Ack releasing the original event. The current state of the queue is now:
| Agent Message Queue |
| SIF_Event message containing a StudentSchoolEnrollment object with an Action of "Add." (blocked) |
| SIF_Event message containing a StudentPersonal object with an Action of "Add." (frozen) |
| SIF_Request message for a StudentPersonal object from another agent. |
| SIF_Event message containing a StudentSchoolEnrollment object with an Action of "Add." (frozen) |
The next message available for delivery to the agent is the SIF_Request for a StudentPersonal object. For our example, the agent will accept the SIF_Request by returning an "Immediate" SIF_Ack indicating that processing is complete and the agent will hand the SIF_Request off to another part of the agent for handling.
Meanwhile, the ZIS has deposited the SIF_Response from the SchoolInfo provider's agent into the queue. The queue now looks like this:
| Agent Message Queue |
| SIF_Event message containing a StudentSchoolEnrollment object with an Action of "Add." (blocked) |
| SIF_Event message containing a StudentPersonal object with an Action of "Add." (frozen) |
| SIF_Event message containing a StudentSchoolEnrollment object with an Action of "Add." (frozen) |
| SIF_Response message containing the SchoolInfo object previously requested. |
The next message the agent receives is the SIF_Response. The agent takes the SIF_Response and uses the information from it along with the data in the original StudentSchoolEnrollment event to update its database. The agent returns (Pull-Mode) or sends (Push-Mode) an "Immediate" SIF_Ack telling the ZIS to discard the SIF_Response message.
The agent has now completed processing of the StudentSchoolEnrollment event and opens a channel to the ZIS and sends a "Final" SIF_Ack with the message identifier for the StudentSchoolEnrollment event. The SIF_Ack says that the agent has completed processing and the ZIS removes the event from the agent queue. The freeze on SIF_Event messages is lifted and the next message to be sent to the agent is the SIF_Event for a StudentPersonal Add:
| Agent Message Queue |
| SIF_Event message containing a StudentPersonal object with an Action of "Add." |
| SIF_Event message containing a StudentSchoolEnrollment object with an Action of "Add." |
The "Immediate" SIF_Ack is a SIF_Ack message with status code of "1."This type of SIF_Ack is returned as a response to a message sent by the ZIS and indicates that the agent has persisted or has processed the message and the ZIS must remove the message from its queue.
The "Intermediate" SIF_Ack is a SIF_Ack message with status code of "2." This type of SIF_Ack is returned as a response to an event message delivered by the ZIS and indicates that the agent has not completed processing of the event and the ZIS must not remove the event message from its queue. The agent will send a "Final" SIF_Ack to the ZIS in the future to signal that the ZIS can discard the event message. An "Intermediate" SIF_Ack message must not be returned by agents in response to messages other than SIF_Event.
A "Final" SIF_Ack is a message with status code of "3." The agent sends this type of SIF_Ack to the ZIS after the agent has completely processed a SIF_Event where it previously sent an "Intermediate" SIF_Ack. When the ZIS receives this message, it must discard the SIF_Event message referenced in the SIF_Ack upon successfully acknowledging the "Final" SIF_Ack.