What is the default health check behavior (Period, Timeout, Required Successes, Required Failures)?

Short answer

In a new Health Checks profile, the defaults are Period 1 second, Timeout 1 second, Required Successes 3 and Required Failures 2; Check Type comes as HTTP and Expect Negative Result is off. The check is performed every second, and each check must complete within 1 second; after 2 consecutive failures the server is removed from traffic, and after 3 consecutive successes it returns. If Period and Timeout are set to 120 seconds or more, 1 second is applied.

How Does It Work?

Field	Default	Range accepted in the form	Meaning
Period	1 s	0.5 – 3600 s	Time between checks
Timeout	1 s	0.01 – 3600 s	Longest time waited for a check to complete
Required Successes	3	1 – 10	Consecutive successes required to be considered healthy
Required Failures	2	1 – 10	Consecutive failures required to be considered unhealthy
Expect Negative Result	Off	–	Inverts the result of the check
Check Type	HTTP	–	Check type; cannot be changed after saving

1. When the profile is assigned to a Backend Service, the status becomes Preparing; during this time the Backend Service does not receive traffic.
2. After 3 consecutive successful checks, the status becomes OK and the Backend Service starts receiving traffic.
3. After 2 consecutive failed checks, the status becomes Error and the Backend Service does not receive new traffic; the check continues, and after 3 consecutive successes the status becomes OK again.

Details: Health status transitions.

In TR7

• 120-second limit: The form accepts values up to 3600 seconds for Period and Timeout; however, values of 120 seconds and above are applied as 1 second in the check. These two fields are kept below 120 seconds.
• Shortest interval: In network checks other than Ping, the next check does not start before a check finishes, and at least 1 second is waited between two checks; lowering Period below 1 second does not speed up these types.
• WEB-CLI: The same fields are written with the names --testInterval, --timeout, --requiredSuccess, --requiredFailure and --negate; in WEB-CLI, Required successes and Required failures can be entered up to 100 (Threshold fields found in every type).
• Effect of the change: A change made in the profile is applied to all the Backend Services that use the profile; where the profile is used is seen in the Used in tab of the preview.
• Cluster: A profile change is transferred to the peer device automatically by default; if Manual Sync Mode is on, it is sent with Synchronize.

Common Misconceptions

• "A Period of 0.5 seconds makes the check twice as fast." In network checks other than Ping, at least 1 second is waited between two checks.
• "Timeout cannot be greater than Period." Checks are performed in sequence; Timeout can be greater than Period.
• "A single failed check brings the server down." With the defaults, 2 consecutive failures are required.
• "If I set Period to 300 seconds, the check is performed once every 5 minutes." With values of 120 seconds and above, the check is performed every second.

How it is done in the documentation

• Health Checks · The Default Health Checks Profile Behavior
• Health Checks · Health Status Transitions
• Health Checks · Period
• Health Checks · Timeout Behavior
• WEB-CLI · lb hc · The Threshold Fields Found in Every Type

Related questions

• How many seconds does it take for a server to be considered "down", and how do I calculate it?
• How should I set the Period, Timeout, Required Successes and Required Failures values?
• What does a Health Check do; what happens if I don't assign one?