Emergency method with HaCluster Client

• Back to HaCluster

By default, when a client node is overloaded (typically measured as the sum of CPU load), the HaCluster Agent notifies the load balancer that the node should be temporarily decommissioned, and new connections should be routed to less loaded nodes.

While this technique allows for effective load distribution, it only applies to new connections.
Indeed, existing sessions remain bound to their original node.

This feature is available on Artica v4.50 Service Pack 5 or Artica v4.50 Service Pack 4 Hotfix 20250628-23

This constraint is particularly impactful when using source IP stickiness with SSL-based websites, where the original routing decision will not change as long as the session remains active.

To address this scenario, a "forced failover" mechanism is automatically triggered when the node’s load becomes so high that even existing sessions are severely degraded — to the point that internet access may become impossible.

This failover is initiated by the client node itself, meaning it still retains enough resources to notify the load balancer of its degraded state.
For this reason, it’s important not to set the emergency threshold too high:
If, for example, the limit is set to 40, the node might be so overloaded that it becomes unable to communicate with the load balancer at all.

By default, this emergency switch is set at a load level of 15.

When the node’s system load exceeds 15, it informs the load balancer to enter emergency mode.
This mode decommissions the affected node and terminates all active connections.

Locally, the HaCluster client retains the state of this emergency procedure.
If the system load drops back below the defined threshold, the node informs the load balancer that it is ready to be reintegrated and accept new connections again.

Define the max load threshold

• On the status section, select the “HaCluster Client” link.
• Change the value under the Max Load ( Emergency ) field

Simulate the process

To simulate the process, there is no need to overload the machine or set an artificially low threshold.
When the maximum load is reached, the HaCluster Agent creates the file:

/etc/artica-postfix/hacluster-client.emergency

• With the value 0, and informs the load balancer of the node's degraded state.
  The value 0 indicates to Artica: "I am in a degraded state, but I haven't yet notified the load balancer."
• The Artica service then detects the presence of this file.
  If the value is 0, Artica will notify the load balancer to perform a forced failover.
  If the load balancer has been properly notified, the file's value will be updated to 1.
• When the value is 1, Artica enters monitoring mode, checking system load every minute.
  As long as the system load remains above the defined threshold, the file will remain present with the value 1.
  If the load drops below the threshold, Artica will inform the load balancer that the node can be reintegrated and will then delete the file.

To simulate the HaCluster Client task:

• on SSH or Unix console, do 
  echo 0 >/etc/artica-postfix/hacluster-client.emergency
artica-phpfpm-service -rest-api /hacluster/client/emergency
• You will see the node status on In production / Maintenance mode inside the farm table status

• Under the Artica Web console node you see the status to “Down” and a message “Still got into emergency load”

• After few seconds ( 1 minute interval ), you will see the server returned to green and available.