What is a Backup Service; when does it take over, and when does it step back?

Short answer

A Backup Service is a standby Backend Service that normally does not receive traffic; it is applied on HTTP and L7 TCP vServices. It takes over when all the main Backend Services in the same list become unavailable (when they fall into the Error status in the health check or while they are in Maintenance Mode). When one of the main Backend Services becomes OK again, new connections return to it; connections that remain open on the backup are not cut off and continue until they end on their own. If there is more than one backup, only the first available backup in the list order receives the traffic.

How Does It Work?

• When Backup Service is turned on in the Backend Service, TR7 writes this server into the vService configuration as a backup; while the main servers are up, the algorithm does not send traffic to the backup.
• Taking over: When all the main Backend Services in the list are in the Error status (the health check failed Required Failures times in a row, as set in the profile) or in Maintenance Mode, new connections go to the backup.
• Stepping back: When one of the main Backend Services succeeds Required Successes times in a row and becomes OK again, new connections go to it. Connections open on the backup are not cut off; they continue until they end on their own.
• More than one backup: Traffic is not distributed among the backups; the first available backup in the list order receives all the traffic, and if it also becomes unavailable, the next one takes over.
• If none is available: If the main Backend Services and the backups are unavailable, clients receive 503.

Scope: Backup of Which List?

A backup replaces only the main Backend Services in the list it is in:

Where the backup is	When it takes over
The vService's Default Backend Services list	When all the main Backend Services in the Default list are unavailable
The list of a Conditional Backend Services set	When all the main Backend Services in that set's list are unavailable

Each conditional set is evaluated separately; the backup in the Default list does not take over when the servers of a conditional set go down. For conditional sets, a backup is added separately to that set's list.

Relationship with Health Check

• If no health check is assigned to the main Backend Services, TR7 never considers them Error; in this case, the backup takes over only when all the main servers are put into Maintenance Mode.
• If no health check is assigned to the backup, traffic may be sent to the backup server even while it is down.
• Therefore, assigning a Health Checks profile to both the main and the backup Backend Services is recommended.

In TR7

• Individual Backend Service: Load Balancer → Backend Services → Add or Edit → Backup Service is turned on and saved with the form's button; its default value is off. In the list, an amber backup server icon appears in the name column.
• Backend Group: Backup Service is at group level; all the servers in the group become backups, but at any one time only the first available server in the row order receives traffic (Backend Groups · Backup Service).
• Network vService: Backup Service is not applied to the distribution of Network vServices; the backup behavior is for HTTP and L7 TCP vServices.
• Where to monitor: Whether the backup is receiving traffic is seen from the current session and request counts of the Backend Service row on the Live Stats screen; the status of the main servers is monitored from the Health column of the Backend Services list.
• Cluster: While Clustering is on and Manual Sync Mode is off (default), the setting is transferred to the peer device automatically.

Common Misconceptions

• "When the main servers are full, the backup takes over." Reaching the Connection Limit does not make the server unavailable; new requests wait in the queue, and the backup does not take over.
• "If there is more than one backup, traffic is distributed among them." Only the first available backup receives traffic.
• "The backup in the Default list protects the whole vService." The lists of conditional sets are evaluated separately.
• "When the main server comes back, the connections on the backup are cut off." Open connections continue; only new connections return to the main server.
• "A backup also stays on standby while it is in maintenance." A backup in Maintenance Mode is never used.

How it is done in the documentation

• Backend Services · Backup Service
• Backend Services · Backup Service
• Backend Groups · Backup Service
• HTTP (L7) vService · Conditional Backend Services

Related questions

• If I define more than one Backup Service, is traffic distributed across all of them?
• What does Maintenance Mode do?
• What do users see if all servers are down; can I show a custom page instead?