What is a health scenario; how does it combine several checks?

Short answer

Health Scenario is the rule that reduces one or more GTM health check results to a single decision, an active or passive result, with and / or groups and, when needed, by reversing them with the UNLESS prefix; the condition is built in the Health Check field of the scenario. It is the scenario, not the health check, that determines the DNS answer: if the scenario is selected in the HC field of an address in the record, the address enters the answer while the scenario is active; if it is selected in the Disaster Recovery Condition field, the answers are given from the backup data center while the scenario is active. By default, the scenario follows the condition (Activation and Deactivation = Automatic); if With Trigger is selected, Trigger Actions are run at the transition, and the transition is held until a separate verification condition is met.

How Does It Work?

In the condition builder in the Health Check field (Health Check):

1. Group: Each row is a group. One or more checks are added to a group; several checks are combined with the and (default, all of them) or or (any one of them) switch in the row.
2. Between groups: The groups are connected with the and / or switch between them (default or in a new group). Groups connected consecutively with and count as a single block, and or starts a new block; the condition is met when one of the blocks is met. At most 10 groups are set up; there are no nested groups.
3. Reversing: In the selector, each check appears in two forms: with its name (met while the check is OK) and with the UNLESS prefix (met while the check is Error). The ready-made Always and Never checks are selected only with their names.
4. Unknown result: Preparing and No Data count as "unknown". In an and group, a single definite false determines the decision, and in an or group, a single definite true; otherwise the scenario keeps its last decision and no health data, last state kept is added to the status text.

Example: in Group 1, UNLESS Ping DC-A and UNLESS HTTP DC-A, then or, and in Group 2, UNLESS Access to DC-A is successful. The condition is met when the two checks on DC-A fail together or when there is no longer access to DC-A; this scenario is selected as the Disaster Recovery Condition.

Activation and Deactivation

Setting	Option	Behavior
Activation	Automatic (default)	When the condition is met, the scenario becomes active.
	With Trigger	When the condition is met, the Activation Trigger → list runs; the scenario stays passive until the Activation Check condition is met.
Deactivation	Automatic (default)	When the condition is broken, the scenario becomes passive.
	With Trigger	When the condition is broken, the Deactivation Trigger ← list runs; the scenario stays active until Deactivation Check is met.
	Off	Once the scenario has become active, it stays active even if the condition is broken (Activation/Deactivation Collision); it does not return to passive until the Deactivation setting is changed.

When the Activation and Deactivation settings resolve to the same result (collision), the scenario keeps its previous decision (Operating Mechanism). The success of a trigger action does not change the decision of the scenario; the transition is completed by the Activation check or Deactivation check condition (What is a Trigger Action; what does it do in which situations?).

Health Scenario in TR7

• Screen: Health Scenarios under Configuration Menu → GTM; the form contains the Scenario Name, Health Check, Activation and Deactivation fields. While the condition is empty, the scenario is not saved.
• Ready-made scenarios: In the Automatic Health Scenarios table, Always is always active and Never is always passive (Automatic Health Scenario Records). They are listed in the Disaster Recovery Condition and Internet Access Health Scenario selectors; they do not appear in the scenario list in the HC field of an address.
• Where it is used: HC Scenario in the HC field of an address, the Disaster Recovery Condition field of a record in Disaster Recovery mode and the Internet Access Health Scenario setting of the local data center (Usage Location).
• Monitoring: The State column in the list (Status Values), the condition and the Used in tab in the preview panel, the Log window and the Health Scenario Status events on the GTM Logs screen.
• Cluster: Each device evaluates the scenario with its own health check results; the decisions of the two devices can differ, and the status circle is split in two.

Common Misconceptions

• "Adding a check to a scenario sets up the 'check failed' condition." A check selected by its name is met while it is OK; for the failure condition, the UNLESS form is selected.
• "An active scenario always indicates a problem." Its meaning depends on where it is used: in the HC field of an address, an active scenario takes the address into the answer; in the Disaster Recovery Condition field, it switches to the backup data center. For an address, a "healthy" condition is set up; for DR, an "unhealthy" (UNLESS) condition.
• "When a trigger action succeeds, the scenario becomes active." While With Trigger is selected, it is the Activation check condition that makes the scenario active.
• "A scenario with Deactivation = Off returns to passive when the condition is broken." It does not; to return to normal, the Deactivation setting of the scenario is changed.

How it is done in the documentation

• Health Scenarios · What Is a Health Scenario?
• Health Scenarios · Health Check
• Health Scenarios · Operating Mechanism
• Health Scenarios · The Status Values

Related questions

• What is a health check in GTM; how does it differ from a Load Balancer health check?
• What is a Trigger Action; what does it do in which situations?
• How does a Health Check result affect the DNS answer?
• What are the Disaster Recovery conditions; is the switchover automatic or manual?