03. Rule-Based Routing

Rule-based routing implements routing decisions through explicit conditional logic. Operators define pattern-action pairs that evaluate incoming requests and select backends accordingly. This approach provides transparency, debuggability, and predictable behavior under all conditions.

Pattern definitions constitute the first dimension of rule specification. Exact match patterns compare request attributes against predetermined values. Wildcard patterns use glob expressions for flexible matching. Regular expressions enable complex pattern definitions but introduce debugging complexity. Prefix matching suits content-type and path-based routing efficiently.

Attribute extraction供给 routing logic with decision-making material. Request headers supply routing context without payload inspection. Query parameters enable explicit user routing preferences. Authentication tokens carry user tier information that influences backend selection. Request body inspection extracts structured fields from JSON or XML payloads.

Action specification completes rule definitions. Backend assignment names the target resource. Weight distribution routes percentages of matching traffic to multiple backends. Header injection modifies requests before forwarding. Response transformation applies to backend replies before return.

# Example routing rules configuration
routing_rules:
  - name: "sensitive_data_policy"
    priority: 100
    conditions:
      - attribute: "headers.x-data-classification"
        operator: "equals"
        value: "phi"
      - attribute: "headers.x-user-jurisdiction"
        operator: "in"
        value: ["EU", "CA", "US-CA"]
    action:
      backend: "local-inference"
      fallback: "cloud-with-encryption"
  
  - name: "simple_query_routing"
    priority: 50
    conditions:
      - attribute: "body.prompt_length"
        operator: "less_than"
        value: 100
      - attribute: "body.task_type"
        operator: "equals"
        value: "summarization"
    action:
      backend: "local-small-model"
      weight: 1.0

Rule ordering determines behavior when multiple rules match a request. Explicit priority fields establish evaluation sequence. Rule negation via negative conditions excludes requests from otherwise broad matches. Default catch-all rules ensure every request receives routing assignment even without specific matches.

Testing rule-based systems requires systematic coverage verification. Unit tests confirm individual rule behavior. Integration tests validate rule interactions. Property-based testing discovers edge cases through randomized input generation. Dry-run modes simulate routing decisions without actual backend calls.

Observability for rule-based routing remains straightforward. Each decision ties directly to the matching rule. Logging rule evaluation inputs and outputs enables complete reconstruction of routing history. This traceable decision path simplifies troubleshooting when users report unexpected behavior.