Global load balancing uses DNS decisions to direct new application connections toward an eligible site. It can distribute demand, remove an unhealthy destination, support migration, or activate a recovery location. The technique is powerful because it operates before the connection begins and does not need to proxy all application traffic.
That boundary matters. DNS answers can be cached, client location can be approximate, and established sessions usually remain where they connected. Global load balancing must therefore combine authoritative DNS, credible health checks, scheduling policy, TTL design, and application resilience.
The following six scenarios show where the approach fits and what must be tested.
Scenario 1: Active-Active Data Centers
Two or more sites serve the same application at the same time. The global policy removes unhealthy destinations and then distributes new DNS answers among those that remain.
Round-robin is suitable only when sites have comparable capacity and reachability. Weighted round-robin can reflect unequal capacity or intentionally limit traffic to a newer site. The application must handle shared data, session behavior, and dependencies across locations; DNS distribution alone does not provide that consistency.
ZDNS GSLB supports multiple scheduling algorithms, including round-robin, weighted round-robin, global availability, and static proximity. Operators should document the order in which health, priority, and distribution are evaluated.
Scenario 2: Active-Standby Disaster Recovery
A primary site receives normal traffic while a standby remains ready. When health policy declares the primary unavailable, authoritative DNS begins returning the standby destination for new resolutions.
Recovery time includes probe intervals, failure thresholds, policy processing, DNS TTLs, recursive caching, client caching, and application startup. An aggressive TTL cannot remove all caching behavior, and a shallow probe can send users to a site whose port responds while critical dependencies fail.
Use layered health checks and test the full transaction. ZDNS GSLB supports more than twenty Layer 4 through Layer 7 detection approaches, including HTTP, HTTPS, UDP, TCP, and ICMP, plus custom probes. Select the shallowest check that reliably represents user-critical readiness.
Scenario 3: Planned Data Center Maintenance
Maintenance is controlled traffic movement rather than failure response. Operators can reduce or stop new answers to one site, wait for cached answers and sessions to drain, complete work, validate service, and restore eligibility gradually.
Weighted policy can support a progressive drain or return. Define the minimum remaining capacity before removing a site. Coordinate with application teams because long-lived sessions may continue even after DNS answers change.
Preserve a fast rollback path. If the remaining site approaches capacity or a hidden dependency fails, policy should restore the original destination without rebuilding the configuration under pressure.
Scenario 4: Application Migration
During migration, an old and new environment run in parallel. Global load balancing can direct a small share of new traffic to the new destination, increase the weight after validation, and retain the old site as a fallback.
This approach reduces the risk of an all-at-once DNS record change, but only when application and data compatibility are planned. Confirm session handling, database writes, certificates, logging, monitoring, and rollback behavior.
ZDNS positions GSLB with migration, synchronization, and restoration capabilities. Buyers should validate how configuration and operating state move between environments and how administrators prevent conflicting changes during the transition.
Scenario 5: Hybrid or Multi-Provider Placement
An organization may operate destinations in an enterprise data center and one or more hosted environments. DNS-based global load balancing provides a common decision point across those sites.
Health must be observed from meaningful network locations. A destination reachable from one probe may be impaired for another carrier or region. ZDNS describes multidimensional detection and custom probes across data centers and carrier networks, useful when a single local health view is insufficient.
Avoid treating provider diversity as automatic resilience. Shared identity, DNS, connectivity, certificate, or data dependencies can fail across all destinations. Map these dependencies and include them in exercises.
Scenario 6: Geographic or Network-Aware Steering
Static proximity can direct a query toward a preferred site based on available source context. This can reduce distance or satisfy an operational placement rule, but the DNS service often sees a recursive resolver rather than the exact client.
Location should be combined with health and capacity. The nearest site is not useful when it is degraded. Define fallbacks for unknown or misleading source context, and test through the recursive resolvers actual users rely on.
Authoritative DNS operations remain foundational. Delegation, record ownership, access controls, logging, and DNSSEC policy need to work correctly before traffic policy can influence client outcomes.
Build a Health Model for Each Scenario
Start with the user-critical transaction and work backward. Network reachability may be one layer. TCP acceptance proves a service listens. TLS verifies protocol setup. HTTP checks an endpoint. A controlled transaction can verify selected dependencies.
Set failure and recovery thresholds to avoid oscillation. Recovery may require more consecutive successes than failure requires. Probe intervals, timeouts, and source locations should reflect the application objective without overloading it.
Handle uncertain health explicitly. If probes fail because the monitoring path is broken, removing every destination can make the incident worse. Define whether policy uses last known state, a safe default, or a controlled override.
Account for DNS Caching
A DNS policy controls new answers, not every packet. Recursive resolvers cache according to TTL, and clients or applications may cache further. Established connections generally continue until the application or network ends them.
Choose TTLs by balancing agility and query load. Record actual propagation during exercises. Some clients will move later than the nominal TTL, so capacity and recovery plans need a transition period.
Do not declare success when the authoritative answer changes. Verify results from representative resolvers, networks, and clients, then examine application success at the selected destinations.
Operational Controls Across All Six Scenarios
- Assign owners for DNS zones, GSLB policy, probes, applications, and recovery decisions.
- Version changes and separate planned maintenance from automatic failure policy.
- Monitor probe state, authoritative answers, resolver observations, connections, and application outcomes.
- Protect manual overrides with scope, approval, expiration, and audit history.
- Test failover and restoration; recovery is not complete until stable normal policy returns.
- Document dependencies that DNS steering cannot repair.
Use IPAM context to keep service destinations, addresses, sites, and ownership understandable. The traffic policy should not reference anonymous addresses that nobody can trace to infrastructure.
Acceptance Exercise
Replay each chosen scenario with representative resolvers and clients. Capture the interval from changed health to changed authoritative answers, shifted connections, stable application service, and restored normal policy. Those measurements make assumptions visible and give incident teams a tested sequence instead of an improvised response.
Conclusion
Global load balancing supports active-active distribution, disaster recovery, maintenance, migration, hybrid placement, and network-aware steering. Each scenario depends on the same control chain: accurate health evidence, explicit scheduling policy, authoritative DNS, realistic caching assumptions, and application readiness.
ZDNS GSLB combines multi-layer health checks, custom probes, several algorithms, multidimensional detection, and multi-data-center continuity functions. Success should be measured through user outcomes and repeatable recovery exercises, not an isolated policy status. DNS can guide new connections to the best eligible destination; the surrounding architecture must make that destination genuinely ready.