openova/platform/k8gb

k8gb

Kubernetes Global Balancer for cross-region DNS-based load balancing.

Status: Accepted | Updated: 2026-02-07

---

Overview

k8gb provides cross-region DNS-based load balancing that routes traffic to healthy endpoints only. It acts as authoritative DNS for the GSLB zone.

flowchart TB
    subgraph External["External"]
        Client[Client]
        DNS[DNS Provider<br/>Parent Zone]
        Witnesses[External DNS Witnesses<br/>8.8.8.8, 1.1.1.1, 9.9.9.9]
    end

    subgraph Region1["Region 1"]
        K8GB1[k8gb CoreDNS<br/>Authoritative]
        GW1[Gateway API]
        FC1[Failover Controller]
    end

    subgraph Region2["Region 2"]
        K8GB2[k8gb CoreDNS<br/>Authoritative]
        GW2[Gateway API]
        FC2[Failover Controller]
    end

    Client -->|"1. Resolve GSLB zone"| K8GB1
    Client -.->|"1. Or"| K8GB2
    K8GB1 -->|"2. Healthy IPs"| Client
    Client -->|"3. Request"| GW1

    K8GB1 <-->|"Health sync"| K8GB2
    FC1 -->|"Witness check"| Witnesses
    FC2 -->|"Witness check"| Witnesses

---

How k8gb Works

Health-Based DNS Routing

sequenceDiagram
    participant K8GB1 as k8gb Region 1
    participant K8GB2 as k8gb Region 2
    participant Client as Client

    loop Every 5 seconds
        K8GB1->>K8GB1: Check local endpoints
        K8GB2->>K8GB2: Check local endpoints
        K8GB1->>K8GB2: Share health status
        K8GB2->>K8GB1: Share health status
    end

    alt Both regions healthy
        Client->>K8GB1: Resolve app.gslb.example.com
        K8GB1->>Client: [R1-IP, R2-IP]
    else Region 2 unhealthy
        Client->>K8GB1: Resolve app.gslb.example.com
        K8GB1->>Client: [R1-IP only]
    end

Mechanism

Aspect	Mechanism
Local health	Direct check of Ingress/Gateway endpoints
Cross-cluster "health"	DNS query to localtargets-* record
Communication	DNS only - no direct health checks

---

k8gb Limitations (Critical)

The Split-Brain Problem

k8gb cannot distinguish between:

• "Region is down" (failover needed)
• "Network partition" (failover NOT wanted)

Both produce the same symptom: DNS query fails or times out.

Scenario	k8gb Behavior	Correct?
Region truly down	Removes from DNS	Yes
Network partition	Also removes from DNS	No
Both healthy	Returns both	Yes

Mitigation: Failover Controller

For stateless services: k8gb's behavior is acceptable.

For stateful services: Use Failover Controller with cloud witness (external DNS) to control Gateway/Service readiness. See ADR-FAILOVER-CONTROLLER.

---

k8gb as Authoritative DNS

k8gb CoreDNS serves as the authoritative DNS server for the GSLB zone:

DNS Hierarchy

example.com                    → DNS Provider (Cloudflare, Hetzner)
  └── gslb.example.com (NS)    → k8gb CoreDNS (authoritative)
        ├── app.gslb.example.com     → R1, R2 IPs (health-based)
        ├── api.gslb.example.com     → R1, R2 IPs (health-based)
        └── db.gslb.example.com      → Primary region only (failover)

NS Record Setup

ExternalDNS creates NS records pointing to k8gb LoadBalancer IPs:

# Created by ExternalDNS in parent zone
gslb.example.com.  NS  ns1.gslb.example.com.
gslb.example.com.  NS  ns2.gslb.example.com.
ns1.gslb.example.com.  A  <k8gb-region1-lb-ip>
ns2.gslb.example.com.  A  <k8gb-region2-lb-ip>

---

Routing Strategies

Strategy	Description	Use Case
roundRobin	Even distribution across healthy endpoints	Active-Active
failover	Primary region preferred, DR on failure	Active-Passive
geoip	Route by client geography	Latency optimization

GeoIP Limitations

DNS queries come from resolver IPs, not client IPs. EDNS Client Subnet (ECS) mitigates this but isn't universally supported.

DNS Resolver	ECS Support	Accuracy
Google (8.8.8.8)	Yes	Good
Cloudflare (1.1.1.1)	No (privacy)	Poor

Recommendation: Use failover or roundRobin for predictable behavior.

---

Configuration

Gslb Custom Resource

apiVersion: k8gb.absa.oss/v1beta1
kind: Gslb
metadata:
  name: <tenant>-app
  namespace: <tenant>-prod
spec:
  ingress:
    ingressClassName: cilium
    rules:
      - host: app.gslb.<domain>
        http:
          paths:
            - path: /
              pathType: Prefix
              backend:
                service:
                  name: app-service
                  port:
                    number: 80
  strategy:
    type: roundRobin  # or failover, geoip
    splitBrainThresholdSeconds: 300
    dnsTtlSeconds: 30

Active-Passive Configuration

apiVersion: k8gb.absa.oss/v1beta1
kind: Gslb
metadata:
  name: app
  annotations:
    k8gb.io/primary-geotag: "region-1"
    k8gb.io/weight-region-1: "100"
    k8gb.io/weight-region-2: "0"
spec:
  strategy:
    type: failover

Scenario	Region 1 Weight	Region 2 Weight	Traffic
Both healthy	100	0	100% Region 1
Region 1 fails	-	100 (auto)	100% Region 2
Region 1 recovers	100	0	Back to Region 1

---

Deployment

Helm Release

apiVersion: v1
kind: Namespace
metadata:
  name: k8gb
---
apiVersion: helm.toolkit.fluxcd.io/v2beta1
kind: HelmRelease
metadata:
  name: k8gb
  namespace: k8gb
spec:
  interval: 10m
  chart:
    spec:
      chart: k8gb
      version: "0.12.x"
      sourceRef:
        kind: HelmRepository
        name: k8gb
        namespace: flux-system
  values:
    k8gb:
      dnsZone: "gslb.<domain>"
      edgeDNSZone: "<domain>"
      edgeDNSServers:
        - "8.8.8.8"
        - "1.1.1.1"
      clusterGeoTag: "<region>"
      extGslbClustersGeoTags: "<other-region>"
      reconcileRequeueSeconds: 30

k8gb CoreDNS Configuration

.:5353 {
    k8gb_coredns
    health
    ready
    prometheus :9153
}

---

Split-Brain Protection

Cloud Witness (External DNS)

Failover Controller uses external DNS witnesses to determine who should be active:

Component	Role
External DNS (8.8.8.8, 1.1.1.1, 9.9.9.9)	Witness reachability
Failover Controller	Per-cluster controller managing readiness

How This Prevents Split-Brain

Scenario	R1 FC	R2 FC	Result
Normal	Reaches witnesses, ACTIVE	Reaches witnesses, STANDBY	Traffic to R1
R1 down	Unreachable	Witnesses confirm R1 down	Failover to R2
Network partition	Both reach witnesses	R1 keeps priority	No split-brain

---

TTL Configuration

Setting	Value	Purpose
DNS TTL	30s	Balance caching vs failover speed
Health check interval	5s	Detect failures quickly
Split-brain threshold	300s	Prevent flapping

Failover time: 30-60 seconds (DNS TTL + propagation)

---

Monitoring

Key Metrics

Metric	Query	Threshold
GSLB healthy endpoints	k8gb_gslb_healthy_records	<2 = warning
Witness reachability	splitbrain_witness_reachable	0 = warning
Quorum status	splitbrain_quorum_reached	0 when quorum lost

Grafana Dashboard Queries

# GSLB health by region
sum by (gslb_name) (k8gb_gslb_healthy_records)

# Witness status
splitbrain_witness_reachable

# Recent promotions
increase(splitbrain_promotions_total[1h])

Alert Rules

apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  name: k8gb-alerts
  namespace: monitoring
spec:
  groups:
    - name: k8gb
      rules:
        - alert: GslbEndpointDown
          expr: k8gb_gslb_healthy_records < 2
          for: 1m
          labels:
            severity: warning

        - alert: GslbAllEndpointsDown
          expr: k8gb_gslb_healthy_records == 0
          for: 30s
          labels:
            severity: critical

---

Operations

Health Checks

# k8gb pods
kubectl get pods -n k8gb

# Gslb resources
kubectl get gslb -A

# Gslb status
kubectl describe gslb <tenant>-app -n <tenant>-prod

# Verify DNS resolution
kubectl run -it --rm dns-test --image=busybox --restart=Never -- \
  nslookup app.gslb.<domain>

Troubleshooting

Endpoint Not Being Added to DNS:

# Check Gslb resource status
kubectl describe gslb <name> -n <namespace>

# Check k8gb logs
kubectl logs -l app.kubernetes.io/name=k8gb -n k8gb | grep -i error

# Verify backend service is healthy
kubectl get endpoints <service> -n <namespace>

Force DNS Update:

# Trigger k8gb reconciliation
kubectl annotate gslb <name> -n <namespace> k8gb.io/reconcile=$(date +%s) --overwrite

# Verify DNS records updated
dig app.gslb.<domain> +short

Recovery Procedures

k8gb Down:

# Force restart
kubectl rollout restart deployment/k8gb -n k8gb

Region Recovery After Failover:

# Verify region is healthy
kubectl get pods -A | grep -v Running

# Verify Gslb status shows region
kubectl describe gslb <name> -n <namespace>

---

Component Responsibilities

Component	Responsibility	When Active
k8gb	DNS-based traffic routing (automatic failover)	Always
Failover Controller	Stateful service promotion (CNPG, FerretDB)	Only when region fails
ExternalDNS	NS record delegation (one-time setup)	Initial delegation only

Key Clarifications:

• k8gb handles all DNS failover natively
• Failover Controller is ONLY for data service promotion (e.g., CNPG replica to primary)
• If you only have stateless services, you don't need Failover Controller

---

Consequences

Positive:

• Self-hosted authoritative DNS for GSLB
• Health-based routing (only healthy endpoints)
• External witness verification prevents split-brain
• Multiple routing strategies
• Native Kubernetes integration
• "Poor man's LoadBalancer" option (free, DNS-based)

Negative:

• DNS-based (subject to TTL delays)
• Requires cross-cluster communication
• Requires external DNS witnesses for split-brain protection

---

Part of OpenOva