Recently, I reviewed how Akamai Global Traffic Management (GTM) handles load distribution and failover between multiple data centers. Two property types that often cause confusion are Mirror Failover and Weighted Random Load Balancing. At first glance, they seem similar because both can return two IPs and appear to distribute traffic evenly. However, their internal logic works very differently.
Mirror Failover: Prioritizing High Availability
In a Mirror Failover configuration, GTM continuously monitors the health of the primary data center. When the primary is healthy, all traffic is directed there. If the primary goes down, GTM immediately redirects users to the backup data center.
When combined with the All-live-IPs setting, both IPs can be returned under normal conditions. But once a health check fails, the failing IP is automatically removed from the DNS response.
This approach focuses on high availability and immediate recovery, making it ideal for services where failover speed and stability are more important than precise load balancing accuracy.
Weighted Random Load Balancing: Prioritizing Distribution Accuracy
Weighted Random Load Balancing focuses on distributing traffic as evenly as possible based on predefined weights. GTM assigns each DNS query to a data center randomly, but the probability is determined by the configured ratio.
For example, if two data centers have a 50:50 ratio, roughly half of the traffic will go to each. This happens regardless of where the users are located.
When a data center fails, GTM removes it from the rotation after detecting the failure through its health check. However, this change does not occur instantly, so some users may still receive the unhealthy IP for a short time until GTM updates its status.
Are They the Same?
Not exactly. Both configurations can look similar in normal operation since they may return two IPs and distribute traffic relatively evenly. The key difference lies in their behavior during failover.
Mirror Failover prioritizes instant redirection to a healthy data center, while Weighted Random prioritizes consistent load balancing. Mirror Failover reacts immediately to failures, while Weighted Random relies on probabilistic assignment and may take slightly longer to fully switch away from an unhealthy node.
Choosing Between Them
If your goal is disaster recovery (DR) testing, where one data center is intentionally taken offline and you want traffic to reroute automatically, Mirror Failover with All-live-IPs is more reliable. It ensures that failover happens immediately when a failure is detected.
If your focus is on continuous and balanced load distribution across multiple live data centers, and immediate failover is not critical, Weighted Random Load Balancing can be a suitable option.
Final Takeaway
Both configurations distribute traffic fairly evenly under normal conditions, but they serve different purposes. Mirror Failover is designed for high availability and immediate failover, while Weighted Random is designed for precise load balancing and traffic optimization.
Understanding these subtle differences helps ensure that your GTM configuration aligns with your operational goals, whether that means prioritizing stability or optimizing distribution.
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