Routing

Istio's advanced routing features allow fine-grained control over traffic based on various request attributes.

Table of Contents

1. Routing Overview
2. Match Conditions
3. URI-based Routing
4. Header-based Routing
5. Query Parameter-based Routing
6. HTTP Method-based Routing
7. Source-based Routing
8. Priority and Fallback
9. Practical Examples
10. Troubleshooting

Routing Overview

VirtualService routing rules consist of Match conditions and Route destinations.

Basic Structure

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: routing-example
spec:
  hosts:
  - myapp.example.com
  http:
  - match:
    - uri:
        prefix: "/api/v1"
    route:
    - destination:
        host: api-v1
  - match:
    - headers:
        user-agent:
          regex: ".*Mobile.*"
    route:
    - destination:
        host: mobile-app
  - route:  # Default route (no match)
    - destination:
        host: web-app

Match Conditions

Match Condition Types

Condition	Description	Match Type
uri	Request path	exact, prefix, regex
scheme	HTTP/HTTPS	exact, prefix, regex
method	HTTP method	exact, prefix, regex
authority	Host header	exact, prefix, regex
headers	HTTP headers	exact, prefix, regex
queryParams	Query parameters	exact, prefix, regex
sourceLabels	Source workload labels	Label selector
gateways	Gateway name	List

Match Types

# exact: Exact match
match:
- uri:
    exact: "/login"

# prefix: Prefix match
match:
- uri:
    prefix: "/api/"

# regex: Regular expression match
match:
- uri:
    regex: "^/api/v[0-9]+/.*"

Combining Multiple Conditions

# AND condition: All conditions must match
http:
- match:
  - uri:
      prefix: "/api"
    headers:
      x-api-version:
        exact: "v2"
    queryParams:
      debug:
        exact: "true"
  route:
  - destination:
      host: api-debug

# OR condition: Use multiple match blocks
http:
- match:
  - uri:
      prefix: "/api/v1"
  - uri:
      prefix: "/api/v2"
  route:
  - destination:
      host: api-service

URI-based Routing

Prefix Matching

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: uri-prefix-routing
spec:
  hosts:
  - myapp.example.com
  http:
  # API traffic
  - match:
    - uri:
        prefix: "/api/"
    route:
    - destination:
        host: api-service
        port:
          number: 8080

  # Admin traffic
  - match:
    - uri:
        prefix: "/admin/"
    route:
    - destination:
        host: admin-service
        port:
          number: 9000

  # Static files
  - match:
    - uri:
        prefix: "/static/"
    route:
    - destination:
        host: static-service
        port:
          number: 8000

  # Default traffic
  - route:
    - destination:
        host: frontend-service
        port:
          number: 3000

Exact Matching

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: uri-exact-routing
spec:
  hosts:
  - myapp.example.com
  http:
  # Login page
  - match:
    - uri:
        exact: "/login"
    route:
    - destination:
        host: auth-service

  # Logout
  - match:
    - uri:
        exact: "/logout"
    route:
    - destination:
        host: auth-service

  # Health check
  - match:
    - uri:
        exact: "/health"
    route:
    - destination:
        host: health-service

Regex Matching

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: uri-regex-routing
spec:
  hosts:
  - myapp.example.com
  http:
  # API version routing: /api/v1/, /api/v2/, etc.
  - match:
    - uri:
        regex: "^/api/v[0-9]+/.*"
    route:
    - destination:
        host: api-service

  # Numeric resource ID: /users/123
  - match:
    - uri:
        regex: "^/users/[0-9]+$"
    route:
    - destination:
        host: user-service

  # File extension: .jpg, .png, .gif
  - match:
    - uri:
        regex: ".*\\.(jpg|png|gif)$"
    route:
    - destination:
        host: image-service

Header-based Routing

User-Agent-based Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: header-user-agent-routing
spec:
  hosts:
  - reviews
  http:
  # Mobile devices
  - match:
    - headers:
        user-agent:
          regex: ".*Mobile.*"
    route:
    - destination:
        host: reviews
        subset: mobile

  # Tablet devices
  - match:
    - headers:
        user-agent:
          regex: ".*(iPad|Tablet).*"
    route:
    - destination:
        host: reviews
        subset: tablet

  # Desktop (default)
  - route:
    - destination:
        host: reviews
        subset: desktop

Custom Header-based Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: header-custom-routing
spec:
  hosts:
  - myapp
  http:
  # Developer/Tester routing
  - match:
    - headers:
        x-dev-user:
          exact: "true"
    route:
    - destination:
        host: myapp
        subset: dev

  # Beta testers
  - match:
    - headers:
        x-beta-tester:
          exact: "true"
    route:
    - destination:
        host: myapp
        subset: beta

  # VIP users
  - match:
    - headers:
        x-user-tier:
          exact: "vip"
    route:
    - destination:
        host: myapp
        subset: vip

  # Regular users
  - route:
    - destination:
        host: myapp
        subset: stable

API Version-based Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: header-api-version-routing
spec:
  hosts:
  - api.example.com
  http:
  # API v3
  - match:
    - headers:
        x-api-version:
          exact: "v3"
    route:
    - destination:
        host: api-service
        subset: v3

  # API v2
  - match:
    - headers:
        x-api-version:
          exact: "v2"
    route:
    - destination:
        host: api-service
        subset: v2

  # API v1 (default)
  - route:
    - destination:
        host: api-service
        subset: v1

Query Parameter-based Routing

Basic Query Parameter Matching

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: query-param-routing
spec:
  hosts:
  - search.example.com
  http:
  # Debug mode: ?debug=true
  - match:
    - queryParams:
        debug:
          exact: "true"
    route:
    - destination:
        host: search-service
        subset: debug

  # Premium search: ?premium=1
  - match:
    - queryParams:
        premium:
          exact: "1"
    route:
    - destination:
        host: search-service
        subset: premium

  # A/B testing: ?variant=b
  - match:
    - queryParams:
        variant:
          exact: "b"
    route:
    - destination:
        host: search-service
        subset: variant-b

  # Default search
  - route:
    - destination:
        host: search-service
        subset: standard

Combining Multiple Query Parameters

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: query-param-combined-routing
spec:
  hosts:
  - api.example.com
  http:
  # Debug mode + verbose logging
  - match:
    - queryParams:
        debug:
          exact: "true"
        verbose:
          exact: "true"
    route:
    - destination:
        host: api-service
        subset: debug-verbose

  # Debug mode only
  - match:
    - queryParams:
        debug:
          exact: "true"
    route:
    - destination:
        host: api-service
        subset: debug

  # Normal mode
  - route:
    - destination:
        host: api-service
        subset: production

HTTP Method-based Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: method-based-routing
spec:
  hosts:
  - api.example.com
  http:
  # POST requests → Write-only service
  - match:
    - method:
        exact: "POST"
      uri:
        prefix: "/api/"
    route:
    - destination:
        host: api-write-service

  # PUT/PATCH requests → Update service
  - match:
    - method:
        regex: "PUT|PATCH"
      uri:
        prefix: "/api/"
    route:
    - destination:
        host: api-update-service

  # DELETE requests → Delete service
  - match:
    - method:
        exact: "DELETE"
      uri:
        prefix: "/api/"
    route:
    - destination:
        host: api-delete-service

  # GET requests → Read-only service
  - match:
    - method:
        exact: "GET"
      uri:
        prefix: "/api/"
    route:
    - destination:
        host: api-read-service

Source-based Routing

Namespace-based Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: source-namespace-routing
  namespace: production
spec:
  hosts:
  - database.production.svc.cluster.local
  http:
  # Requests from production namespace
  - match:
    - sourceNamespace: production
    route:
    - destination:
        host: database
        subset: production

  # Requests from staging namespace
  - match:
    - sourceNamespace: staging
    route:
    - destination:
        host: database
        subset: staging

  # Block other namespaces
  - route:
    - destination:
        host: access-denied

Service Account-based Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: source-sa-routing
spec:
  hosts:
  - payment-service
  http:
  # Allow access only from specific Service Account
  - match:
    - sourceLabels:
        app: frontend
        version: v2
    route:
    - destination:
        host: payment-service
        subset: v2

  # Legacy service
  - match:
    - sourceLabels:
        app: frontend
        version: v1
    route:
    - destination:
        host: payment-service
        subset: v1

Priority and Fallback

Match Rule Priority

Istio evaluates VirtualService HTTP routing rules from top to bottom.

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: priority-example
spec:
  hosts:
  - myapp.example.com
  http:
  # Priority 1: Most specific condition
  - match:
    - uri:
        exact: "/api/v2/users/admin"
      headers:
        x-admin:
          exact: "true"
    route:
    - destination:
        host: admin-api-v2

  # Priority 2: Medium specificity
  - match:
    - uri:
        prefix: "/api/v2/"
    route:
    - destination:
        host: api-v2

  # Priority 3: Less specific
  - match:
    - uri:
        prefix: "/api/"
    route:
    - destination:
        host: api-v1

  # Priority 4 (last): Default fallback
  - route:
    - destination:
        host: frontend

Fallback Strategy

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: fallback-strategy
spec:
  hosts:
  - myapp
  http:
  # Requests matching specific conditions
  - match:
    - headers:
        x-canary:
          exact: "true"
    route:
    - destination:
        host: myapp
        subset: canary
    fault:
      abort:
        percentage:
          value: 0
        httpStatus: 503
    # No fallback on canary failure - return error

  # Default requests - with fallback
  - route:
    - destination:
        host: myapp
        subset: stable
      weight: 100

Practical Examples

Example 1: Multi-tenant Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: multi-tenant-routing
spec:
  hosts:
  - api.example.com
  http:
  # Tenant A (identified by header)
  - match:
    - headers:
        x-tenant-id:
          exact: "tenant-a"
    route:
    - destination:
        host: api-service
        subset: tenant-a

  # Tenant B (identified by subdomain)
  - match:
    - authority:
        exact: "tenant-b.api.example.com"
    route:
    - destination:
        host: api-service
        subset: tenant-b

  # Tenant C (identified by path)
  - match:
    - uri:
        prefix: "/tenant-c/"
    rewrite:
      uri: "/"
    route:
    - destination:
        host: api-service
        subset: tenant-c

Example 2: Feature Flag-based Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: feature-flag-routing
spec:
  hosts:
  - myapp
  http:
  # Users with new feature enabled
  - match:
    - headers:
        x-feature-new-ui:
          exact: "enabled"
    route:
    - destination:
        host: myapp
        subset: new-ui

  # Beta feature testers
  - match:
    - headers:
        x-feature-beta:
          exact: "enabled"
    route:
    - destination:
        host: myapp
        subset: beta

  # Experimental features (employees only)
  - match:
    - headers:
        x-feature-experimental:
          exact: "enabled"
      sourceLabels:
        role: employee
    route:
    - destination:
        host: myapp
        subset: experimental

  # Default (stable version)
  - route:
    - destination:
        host: myapp
        subset: stable

Example 3: Geographic Routing

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: geo-routing
spec:
  hosts:
  - content.example.com
  http:
  # Korean users
  - match:
    - headers:
        x-country-code:
          exact: "KR"
    route:
    - destination:
        host: content-service
        subset: korea

  # Japanese users
  - match:
    - headers:
        x-country-code:
          exact: "JP"
    route:
    - destination:
        host: content-service
        subset: japan

  # US users
  - match:
    - headers:
        x-country-code:
          exact: "US"
    route:
    - destination:
        host: content-service
        subset: us

  # European users
  - match:
    - headers:
        x-country-code:
          regex: "DE|FR|UK|IT|ES"
    route:
    - destination:
        host: content-service
        subset: europe

  # Other regions (global)
  - route:
    - destination:
        host: content-service
        subset: global

Example 4: API Gateway Pattern

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: api-gateway-routing
spec:
  hosts:
  - api.example.com
  gateways:
  - api-gateway
  http:
  # APIs requiring authentication
  - match:
    - uri:
        prefix: "/api/v1/protected/"
      headers:
        authorization:
          regex: "Bearer .*"
    route:
    - destination:
        host: protected-api-service

  # Public APIs
  - match:
    - uri:
        prefix: "/api/v1/public/"
    route:
    - destination:
        host: public-api-service

  # GraphQL endpoint
  - match:
    - uri:
        exact: "/graphql"
      method:
        exact: "POST"
    route:
    - destination:
        host: graphql-service

  # REST API
  - match:
    - uri:
        prefix: "/api/v1/"
    route:
    - destination:
        host: rest-api-service

  # Health check
  - match:
    - uri:
        exact: "/health"
    route:
    - destination:
        host: health-service

  # 404 handling
  - route:
    - destination:
        host: error-service

Troubleshooting

Routing Not Working

# 1. Check VirtualService status
kubectl get virtualservice -A
kubectl describe virtualservice <name> -n <namespace>

# 2. Check routing rules
istioctl proxy-config routes <pod-name> -n <namespace>

# 3. Check routing to specific service
istioctl proxy-config routes <pod-name> -n <namespace> --name <route-name> -o json

# 4. Validate configuration
istioctl analyze -n <namespace>

Debugging Match Conditions

# Increase Envoy log level
istioctl proxy-config log <pod-name> -n <namespace> --level debug

# Trace requests
kubectl logs -n <namespace> <pod-name> -c istio-proxy -f

# Debug Pilot
kubectl logs -n istio-system -l app=istiod --tail=100

Common Issues

1. Match Order Issues

# ❌ Wrong order - default route first ignores other rules
http:
- route:  # All traffic goes here
  - destination:
      host: myapp
- match:  # Never executed
  - uri:
      prefix: "/api"
  route:
  - destination:
      host: api-service

# ✅ Correct order
http:
- match:  # Specific rules first
  - uri:
      prefix: "/api"
  route:
  - destination:
      host: api-service
- route:  # Default route last
  - destination:
      host: myapp

2. Regex Syntax Errors

# ❌ Incorrect regex
match:
- uri:
    regex: "/api/v[1-3]"  # Missing anchor

# ✅ Correct regex
match:
- uri:
    regex: "^/api/v[1-3].*"

3. Header Case Sensitivity Issues

# HTTP headers are case-insensitive
# Istio automatically converts to lowercase
match:
- headers:
    X-Custom-Header:  # Automatically converted to x-custom-header
      exact: "value"

Best Practices

1. Place Specific Rules First

# ✅ Good example
http:
- match:
  - uri:
      exact: "/api/v2/admin"  # Most specific
  route:
  - destination:
      host: admin-v2
- match:
  - uri:
      prefix: "/api/v2/"  # Medium
  route:
  - destination:
      host: api-v2
- match:
  - uri:
      prefix: "/api/"  # General
  route:
  - destination:
      host: api-v1
- route:  # Default
  - destination:
      host: frontend

2. Minimize Regular Expressions

# ❌ Avoid - complex regex degrades performance
match:
- uri:
    regex: "^/(api|admin|public)/v[0-9]+/(users|products|orders)/[a-zA-Z0-9_-]+$"

# ✅ Recommended - use prefix or exact
match:
- uri:
    prefix: "/api/v1/"

3. Clear Naming

# ✅ Use clear names
apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: product-api-routing  # Clear name
  labels:
    app: product-service
    purpose: routing
spec:
  hosts:
  - product-api.example.com

4. Documentation

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: myapp-routing
  annotations:
    description: "Routes traffic based on API version and user type"
    owner: "platform-team"
spec:
  hosts:
  - myapp.example.com
  http:
  # Route VIP users to dedicated instance
  - match:
    - headers:
        x-user-tier:
          exact: "vip"
    route:
    - destination:
        host: myapp
        subset: vip

5. Testing Strategy

# Routing rule test script
#!/bin/bash

# API v1 test
curl -H "Host: api.example.com" http://$GATEWAY_URL/api/v1/users

# API v2 test
curl -H "Host: api.example.com" http://$GATEWAY_URL/api/v2/users

# Mobile user test
curl -H "User-Agent: Mobile" http://$GATEWAY_URL/

# Header-based test
curl -H "x-canary: true" http://$GATEWAY_URL/

References

• Istio Traffic Management
• VirtualService Reference
• HTTP Route Matching
• Traffic Routing