Events and alarms

Before you begin

• Events, alarms and conditionsLearn about events and event notifiers, EventFilter, the states of Alarms and Conditions and the Acknowledge, Confirm and ConditionRefresh methods.
• Subscriptions and the publish cycleUnderstand subscriptions and monitored items, sampling and publishing intervals, Publish requests and keep-alives, sequence numbers and Republish.

Goal

• Subscribe to the events of the Server object (i=2253).
• Activate the simulator’s alarm and receive the alarm event.
• Acknowledge and confirm the alarm, then get the current state again with ConditionRefresh.

What you need

• The apps package and the OPCUA_APPS variable from the lab setup.
• One terminal. The subscriptions tutorial introduces the headless simulator.

1. Start the simulator and connect

"$OPCUA_APPS/bin/pdcomm-opcua-headless"

Start the server and connect:

{"action":"server_start"}

{"action":"connect"}

2. Subscribe to events

{"action":"event_subscribe"}

{"ok":true,"result":{"good":true,"handle":1,"monitored_item":1,"node":"i=2253","status":"Good (0x00000000)","subscription":1}}

Without a notifier, event_subscribe watches the Server object. If no subscription exists, it creates one with a 250 ms publishing interval. Its EventFilter selects EventId, EventType, SourceNode, SourceName, Time, Message, Severity, ConditionId, Retain, ActiveState/Id, AckedState/Id and ConfirmedState/Id.

3. Activate the alarm

Set the alarm source variable Simulator.AlarmSource to true:

{"action":"alarm_set","active":true}

{"ok":true,"result":{"good":true,"node":"ns=2;s=Simulator.AlarmSource","status":"Good (0x00000000)"}}

Receive the event with Publish:

{"action":"publish","count":2}

{"ok":true,"result":{"dropped":0,"notifications":[{"fields":{"AckedState/Id":false,"ActiveState/Id":true,"ConditionId":"ns=2;s=Simulator.Alarm","ConfirmedState/Id":true,"EventId":"43da01187d15378575644e4988945eed","EventType":"i=10637","Message":"Simulator.Alarm","Retain":true,"Severity":700,"SourceName":"PDComm","SourceNode":"ns=2;s=Simulator.AlarmSource","Time":1791997610261},"handle":1,"kind":"event","sequence":1}],"queued":1}}

It’s an OffNormalAlarmType (i=10637) event: the alarm is active (ActiveState true) and not yet acknowledged (AckedState false), with severity 700. Copy the EventId value for the next step; it differs on every run.

4. Acknowledge the alarm

Call Acknowledge with the EventId you copied, replacing the one below:

{"action":"condition","method":"acknowledge","event_id":"43da01187d15378575644e4988945eed","comment":"Checked the pump"}

{"ok":true,"result":{"good":true,"input_results":[],"outputs":[],"status":"Good (0x00000000)"}}

The state changed, so a new event follows:

{"action":"publish"}

{"ok":true,"result":{"dropped":0,"notifications":[{"fields":{"AckedState/Id":true,"ActiveState/Id":true,"ConditionId":"ns=2;s=Simulator.Alarm","ConfirmedState/Id":false,"EventId":"d6a70225556c1a1c53804ec777c6c35f","EventType":"i=10637","Message":"Simulator.Alarm","Retain":true,"Severity":700,"SourceName":"PDComm","SourceNode":"ns=2;s=Simulator.AlarmSource","Time":1791997611262},"handle":1,"kind":"event","sequence":2}],"queued":2}}

AckedState is now true and ConfirmedState false, and the event has a new EventId. Acknowledging again with this EventId fails, because the state is already acknowledged:

{"action":"condition","method":"acknowledge","event_id":"d6a70225556c1a1c53804ec777c6c35f","comment":"Again"}

{"ok":true,"result":{"good":false,"input_results":[],"outputs":[],"status":"BadConditionBranchAlreadyAcked (0x80CF0000)"}}

5. Confirm the alarm

Call Confirm with the latest EventId:

{"action":"condition","method":"confirm","event_id":"d6a70225556c1a1c53804ec777c6c35f","comment":"Pump replaced"}

{"ok":true,"result":{"good":true,"input_results":[],"outputs":[],"status":"Good (0x00000000)"}}

{"action":"publish"}

{"ok":true,"result":{"dropped":0,"notifications":[{"fields":{"AckedState/Id":true,"ActiveState/Id":true,"ConditionId":"ns=2;s=Simulator.Alarm","ConfirmedState/Id":true,"EventId":"20f3ef4b7cc43ae65e2983fcfc83561d","EventType":"i=10637","Message":"Simulator.Alarm","Retain":true,"Severity":700,"SourceName":"PDComm","SourceNode":"ns=2;s=Simulator.AlarmSource","Time":1791997611518},"handle":1,"kind":"event","sequence":3}],"queued":3}}

The alarm is still active, so Retain stays true.

6. Refresh the conditions

ConditionRefresh makes the server send the current condition states to the subscription again:

{"action":"condition_refresh"}

{"ok":true,"result":{"good":true,"input_results":[],"outputs":[],"status":"Good (0x00000000)"}}

{"action":"publish","count":2}

{"ok":true,"result":{"dropped":0,"notifications":[{"fields":{"AckedState/Id":null,"ActiveState/Id":null,"ConditionId":null,"ConfirmedState/Id":null,"EventId":"64c6c97bba83b75a5edab0f3ae218f4a","EventType":"i=2787","Message":"Condition refresh started","Retain":null,"Severity":1,"SourceName":"PDComm","SourceNode":"i=2253","Time":1791997611769},"handle":1,"kind":"event","sequence":4},{"fields":{"AckedState/Id":true,"ActiveState/Id":true,"ConditionId":"ns=2;s=Simulator.Alarm","ConfirmedState/Id":true,"EventId":"20f3ef4b7cc43ae65e2983fcfc83561d","EventType":"i=10637","Message":"Simulator.Alarm","Retain":true,"Severity":700,"SourceName":"PDComm","SourceNode":"ns=2;s=Simulator.AlarmSource","Time":1791997611518},"handle":1,"kind":"event","sequence":4},{"fields":{"AckedState/Id":null,"ActiveState/Id":null,"ConditionId":null,"ConfirmedState/Id":null,"EventId":"8e832138bca60fa765e705103240d018","EventType":"i=2788","Message":"Condition refresh ended","Retain":null,"Severity":1,"SourceName":"PDComm","SourceNode":"i=2253","Time":1791997611771},"handle":1,"kind":"event","sequence":4}],"queued":6}}

A RefreshStartEvent (i=2787), the current alarm state and a RefreshEndEvent (i=2788) arrive in that order. The refresh events aren’t conditions, so their condition fields are null.

7. Clear the alarm

{"action":"alarm_set","active":false}

{"action":"publish"}

{"ok":true,"result":{"dropped":0,"notifications":[{"fields":{"AckedState/Id":true,"ActiveState/Id":false,"ConditionId":"ns=2;s=Simulator.Alarm","ConfirmedState/Id":true,"EventId":"609b0e9f70ba2c7229a74725049dc94b","EventType":"i=10637","Message":"Simulator.Alarm","Retain":false,"Severity":100,"SourceName":"PDComm","SourceNode":"ns=2;s=Simulator.AlarmSource","Time":1791997612766},"handle":1,"kind":"event","sequence":5}],"queued":7}}

The alarm is inactive, acknowledged and confirmed, so Retain is false; a client can now remove the condition from its list.

8. Clean up

{"action":"shutdown"}

{"ok":true,"result":{"client_closed":true,"pubsub_stopped":false,"server_stopped":true}}

Common errors

status or error in the reply	Cause and fix
BadEventIdUnknown (0x809A0000)	The EventId is unknown. Copy the EventId of a received event exactly.
BadConditionBranchAlreadyAcked (0x80CF0000)	You acknowledged a state that’s already acknowledged.
BadConditionBranchAlreadyConfirmed (0x80D00000)	You confirmed a state that’s already confirmed.
No subscription exists	You ran condition_refresh or publish before event_subscribe.

The method of condition accepts acknowledge, confirm, add_comment, enable and disable. In the GUI, Acknowledge last and Confirm last on the Events & Conditions tab use the EventId of the last condition event for you.

Next steps

• History