Routing & Traffic - Learning Module

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Path-Based Routing

The Routing Problem at Scale

Every request arriving at your API gateway carries a destination encoded in its URL path. The seemingly simple act of parsing /api/v2/users/123/orders and routing it to the correct backend service is, at scale, a complex algorithmic and architectural challenge. Path-based routing is the foundational mechanism that transforms a chaotic stream of incoming requests into organized, directed traffic flowing to the appropriate microservices.

In production systems handling millions of requests per second—systems like Netflix, Uber, and Stripe—the path-based routing layer must operate with sub-millisecond latency, support dynamic configuration updates without downtime, handle thousands of routing rules, and gracefully manage path conflicts and ambiguities. This page provides a Principal Engineer's perspective on path-based routing: the theoretical foundations, algorithmic implementations, and battle-tested production patterns.

What You Will Learn

By the end of this page, you will understand path routing semantics deeply—from URL anatomy to trie-based routing algorithms. You'll master prefix vs. exact matching, path parameter extraction, wildcard patterns, routing priority resolution, and performance optimization techniques used in production gateways.

URL Anatomy and Routing Fundamentals

Before diving into routing strategies, we must establish a precise understanding of URL structure. Every HTTP request contains a Request-URI that follows a well-defined format specified in RFC 3986. The routing layer operates primarily on the path component, though sophisticated gateways may also consider query parameters, fragments, and even complete URIs.

The Anatomy of a URL:

https://api.example.com:443/v2/users/123/orders?status=pending&limit=10#results
└─┬──┘ └───────┬───────┘└┬┘ └─────────┬─────────┘ └──────────┬─────────┘ └──┬───┘
scheme      authority   port        path              query string       fragment

For routing purposes, the path (/v2/users/123/orders) is the primary decision factor. It consists of:

Path segments separated by forward slashes
Static segments (v2, users, orders) that match literally
Dynamic segments (123) representing resource identifiers or parameters
Optional trailing slashes that may or may not be semantically significant

Path Normalization Matters

Before routing, gateways typically normalize paths: removing duplicate slashes (//), resolving dot segments (/. and /..), handling URL encoding (%20 → space), and optionally adding or stripping trailing slashes. Inconsistent normalization is a common source of routing bugs and security vulnerabilities (path traversal attacks).

The Routing Decision Model:

The routing layer implements a function:

route(path: string, method: string) → Backend | null

This function must:

Parse the incoming path into segments
Match the path against a set of routing rules (patterns)
Resolve conflicts when multiple rules match
Extract path parameters from dynamic segments
Return the target backend and any extracted parameters

The challenge lies in executing this function millions of times per second while supporting complex pattern matching and dynamic rule updates.

Path Segment Types in Routing Rules
Segment Type	Syntax Example	Matches	Use Case
Static/Literal	/users	Exactly `/users`	Fixed API endpoints
Path Parameter	/:id or /{id}	Any single segment	Resource identifiers
Wildcard (*)	/files/*	Single segment (any)	Catch-all within level
Glob (**)	/proxy/**	Zero or more segments	Proxy/passthrough routes
Regex	/users/[0-9]+	Regex pattern match	Constrained parameters
Optional	/users/:id?	Segment may be absent	Optional trailing segments

Prefix Matching vs Exact Matching

The two fundamental matching strategies in path-based routing are prefix matching and exact matching. Understanding the semantics and trade-offs of each is essential for designing correct and maintainable routing configurations.

Exact Matching:

Exact matching requires the request path to match the routing rule precisely—no more, no less.

Rule: /api/users (exact)

/api/users        → MATCH ✓
/api/users/       → NO MATCH ✗ (trailing slash)
/api/users/123    → NO MATCH ✗ (extra segment)
/api/user         → NO MATCH ✗ (different spelling)

Exact matching provides maximum precision but requires explicit rules for every distinct endpoint. It's ideal when you have a finite, well-defined set of endpoints and want to prevent unintended routing.

Prefix Matching:

Prefix matching routes any request whose path starts with the specified prefix.

Rule: /api/v2 (prefix)

/api/v2                → MATCH ✓
/api/v2/               → MATCH ✓
/api/v2/users          → MATCH ✓
/api/v2/users/123/orders → MATCH ✓
/api/v1/users          → NO MATCH ✗ (different prefix)

Prefix matching is powerful for microservices architectures where an entire path subtree should route to a single service.

Exact Matching

•Precision: Routes only explicitly defined paths
•Safety: No unintended routing to services
•Maintenance: Requires rule per endpoint
•Use Case: Public APIs with strict contracts
•Example Config: path: /api/health (exact)

Prefix Matching

•Flexibility: Routes entire path subtrees
•Simplicity: Single rule for many endpoints
•Risk: May route unintended paths
•Use Case: Microservice path ownership
•Example Config: pathPrefix: /api/users

The Trailing Slash Dilemma

Whether /users and /users/ are treated as the same path is configuration-dependent. RESTful conventions often treat them as different resources (collection vs. collection with sub-path). Your gateway must be explicit about trailing slash handling—either normalize them consistently or treat them as distinct. Ambiguity here causes subtle, hard-to-debug routing issues.

nginx-routing-example.conf
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# Nginx location block matching semantics
 
# Exact match (highest priority)
location = /api/health {
    proxy_pass http://health-service;
}
 
# Prefix match (case-sensitive)
location /api/v2/users {
    proxy_pass http://user-service;
}
 
# Prefix match with lower priority
location /api/v2 {
    proxy_pass http://default-v2-service;
}
 
# Regex match (evaluated in order of appearance)
location ~ ^/api/v[0-9]+/orders/[0-9]+$ {
    proxy_pass http://order-service;
}
 
# Precedence order:
# 1. Exact match (=)
# 2. Preferential prefix (^~)
# 3. Regex (~ or ~*)
# 4. Prefix match (longest match wins)

Path Parameters and Dynamic Segments

RESTful APIs encode resource identifiers directly in the path: /users/123, /orders/abc-456, /files/documents/report.pdf. The gateway must extract these dynamic segments and either:

Pass them as request headers to the backend
Rewrite the path before forwarding
Use them for routing decisions (e.g., sharding by user ID)

Parameter Syntax Conventions:

Different gateway technologies use varying syntax for path parameters:

Gateway/Framework	Syntax	Example
Express.js	`:param`	`/users/:userId`
Spring/JAX-RS	`{param}`	`/users/{userId}`
OpenAPI	`{param}`	`/users/{userId}`
Envoy	`:param` or regex	`/users/:id`
AWS ALB	Regex groups	`/users/(.*)`

Parameter Constraints:

Production systems often constrain parameters to specific patterns:

/users/{userId:[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}}

This regex constraint ensures userId matches UUID format, preventing invalid requests from reaching the backend.

path-parameter-extraction.ts
TypeScript
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// Path parameter extraction implementation
interface RouteParams {
  [key: string]: string;
}
 
interface RouteMatch {
  matched: boolean;
  params: RouteParams;
  remainingPath: string;
}
 
/**
 * Parse a route pattern into segments with metadata
 * Supports: static, :param, :param?, *wildcard, **glob
 */
function parseRoutePattern(pattern: string): RouteSegment[] {
  const segments: RouteSegment[] = [];
  const parts = pattern.split('/').filter(Boolean);
 
  for (const part of parts) {
    if (part.startsWith(':')) {
      const isOptional = part.endsWith('?');
      const name = isOptional ? part.slice(1, -1) : part.slice(1);
      // Check for constraint: :id([0-9]+)
      const constraintMatch = name.match(/^(\w+)\((.+)\)$/);
      segments.push({
        type: 'param',
        name: constraintMatch ? constraintMatch[1] : name,
        optional: isOptional,
        constraint: constraintMatch ? new RegExp(`^${constraintMatch[2]}$`) : undefined,
      });
    } else if (part === '*') {
      segments.push({ type: 'wildcard' });
    } else if (part === '**') {
      segments.push({ type: 'glob' });
    } else {
      segments.push({ type: 'static', value: part });
    }
  }
 
  return segments;
}
 
/**
 * Match an incoming path against a parsed route pattern
 */
function matchRoute(pathSegments: string[], routeSegments: RouteSegment[]): RouteMatch {
  const params: RouteParams = {};
  let pathIdx = 0;
  let routeIdx = 0;
 
  while (routeIdx < routeSegments.length) {
    const seg = routeSegments[routeIdx];
 
    if (seg.type === 'static') {
      if (pathSegments[pathIdx] !== seg.value) {
        return { matched: false, params: {}, remainingPath: '' };
      }
      pathIdx++;
    } else if (seg.type === 'param') {
      if (pathIdx >= pathSegments.length) {
        if (!seg.optional) {
          return { matched: false, params: {}, remainingPath: '' };
        }
      } else {
        const value = pathSegments[pathIdx];
        // Validate constraint if present
        if (seg.constraint && !seg.constraint.test(value)) {
          return { matched: false, params: {}, remainingPath: '' };
        }
        params[seg.name] = value;
        pathIdx++;
      }
    } else if (seg.type === 'wildcard') {
      if (pathIdx >= pathSegments.length) {
        return { matched: false, params: {}, remainingPath: '' };
      }
      pathIdx++; // Consume exactly one segment
    } else if (seg.type === 'glob') {
      // Glob captures all remaining segments
      params['**'] = pathSegments.slice(pathIdx).join('/');
      pathIdx = pathSegments.length;
    }
 
    routeIdx++;
  }
 
  // Check if all path segments were consumed
  const matched = pathIdx === pathSegments.length;
  const remainingPath = pathSegments.slice(pathIdx).join('/');
 
  return { matched, params, remainingPath };
}
 
// Example usage:
// Pattern: /users/:userId/orders/:orderId([0-9]+)
// Path: /users/abc-123/orders/456
// Result: { matched: true, params: { userId: 'abc-123', orderId: '456' } }

Header Injection Pattern

A common production pattern is for the gateway to extract path parameters and inject them as headers (e.g., X-User-Id: 123) before forwarding to backends. This decouples backends from URL structure—the backend reads headers instead of parsing URLs, enabling path restructuring without backend changes.

Routing Algorithm Internals

When a gateway has thousands of routing rules, the naive approach of iterating through each rule and checking for a match becomes prohibitively slow. Production gateways employ sophisticated data structures—primarily tries (prefix trees) and radix trees—to achieve O(path_length) routing regardless of the number of rules.

The Trie-Based Router:

A trie (pronounced "try") is a tree data structure where:

Each edge represents a path segment
Each node may contain routing metadata (backend, policies)
Traversal from root to leaf spells out a complete path pattern

                    [root]
                      │
                    [api]
                   /      \
                [v1]      [v2]
               /    \        \
           [users] [orders]  [users]
              │               /    \
           [:id]          [:id]  [search]
              │              │
          [orders]       [profile]

Radix Tree Optimization:

A radix tree (compact prefix tree) compresses chains of single-child nodes into single edges, reducing memory usage and improving cache locality:

Trie:          [a] → [p] → [i] → [/] → [v] → [1]
Radix Tree:    [api/v1]

Envoy proxy uses a route table with a combination of:

Static prefix indexing via radix trees
Linear regex matching for complex patterns
Priority ordering for ambiguity resolution

trie-router.ts
TypeScript
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// Production-grade Trie-based Router
interface TrieNode<T> {
  children: Map<string, TrieNode<T>>;
  paramChild?: TrieNode<T>;  // :param nodes
  paramName?: string;
  wildcardChild?: TrieNode<T>; // * nodes
  globChild?: TrieNode<T>;  // ** nodes
  handler?: T;
  priority: number;
}
 
class TrieRouter<T> {
  private root: TrieNode<T> = {
    children: new Map(),
    priority: 0,
  };
 
  /**
   * Insert a route pattern into the trie
   * Complexity: O(pattern_segments)
   */
  insert(pattern: string, handler: T, priority: number = 0): void {
    const segments = pattern.split('/').filter(Boolean);
    let node = this.root;
 
    for (const segment of segments) {
      if (segment.startsWith(':')) {
        // Parameter segment
        if (!node.paramChild) {
          node.paramChild = { children: new Map(), priority: 0 };
        }
        node.paramChild.paramName = segment.slice(1);
        node = node.paramChild;
      } else if (segment === '*') {
        if (!node.wildcardChild) {
          node.wildcardChild = { children: new Map(), priority: 0 };
        }
        node = node.wildcardChild;
      } else if (segment === '**') {
        if (!node.globChild) {
          node.globChild = { children: new Map(), priority: 0 };
        }
        node.globChild.handler = handler;
        node.globChild.priority = priority;
        return; // Glob terminates the pattern
      } else {
        // Static segment
        if (!node.children.has(segment)) {
          node.children.set(segment, { children: new Map(), priority: 0 });
        }
        node = node.children.get(segment)!;
      }
    }
 
    node.handler = handler;
    node.priority = priority;
  }
 
  /**
   * Find the best matching route for a path
   * Complexity: O(path_segments) average case
   */
  find(path: string): { handler: T; params: Record<string, string> } | null {
    const segments = path.split('/').filter(Boolean);
    const matches: Array<{ handler: T; params: Record<string, string>; priority: number }> = [];
    
    this.findRecursive(this.root, segments, 0, {}, matches);
    
    if (matches.length === 0) return null;
    
    // Return highest priority match (static > param > wildcard > glob)
    matches.sort((a, b) => b.priority - a.priority);
    return { handler: matches[0].handler, params: matches[0].params };
  }
 
  private findRecursive(
    node: TrieNode<T>,
    segments: string[],
    depth: number,
    params: Record<string, string>,
    matches: Array<{ handler: T; params: Record<string, string>; priority: number }>
  ): void {
    // Check for complete match
    if (depth === segments.length) {
      if (node.handler !== undefined) {
        matches.push({ 
          handler: node.handler, 
          params: { ...params }, 
          priority: node.priority + 1000 // Exact match bonus
        });
      }
      return;
    }
 
    const segment = segments[depth];
 
    // Priority 1: Static match (highest priority)
    if (node.children.has(segment)) {
      this.findRecursive(
        node.children.get(segment)!,
        segments,
        depth + 1,
        params,
        matches
      );
    }
 
    // Priority 2: Parameter match
    if (node.paramChild) {
      const newParams = { ...params, [node.paramChild.paramName!]: segment };
      this.findRecursive(node.paramChild, segments, depth + 1, newParams, matches);
    }
 
    // Priority 3: Wildcard match
    if (node.wildcardChild) {
      this.findRecursive(node.wildcardChild, segments, depth + 1, params, matches);
    }
 
    // Priority 4: Glob match (captures remaining path)
    if (node.globChild && node.globChild.handler !== undefined) {
      const globPath = segments.slice(depth).join('/');
      matches.push({
        handler: node.globChild.handler,
        params: { ...params, '**': globPath },
        priority: node.globChild.priority
      });
    }
  }
}
 
// Usage example
const router = new TrieRouter<string>();
router.insert('/api/v1/users', 'user-list', 100);
router.insert('/api/v1/users/:userId', 'user-detail', 90);
router.insert('/api/v1/users/:userId/orders', 'user-orders', 80);
router.insert('/static/**', 'static-files', 10);
 
console.log(router.find('/api/v1/users'));
// { handler: 'user-list', params: {} }
 
console.log(router.find('/api/v1/users/123'));
// { handler: 'user-detail', params: { userId: '123' } }
 
console.log(router.find('/static/images/logo.png'));
// { handler: 'static-files', params: { '**': 'images/logo.png' } }

Routing Algorithm Complexity Comparison
Algorithm	Time Complexity (Match)	Space Complexity	Regex Support	Use in Production
Linear Scan	O(n × path_len)	O(n)	Yes	Small rule sets only
Hash Map (exact)	O(path_len)	O(n)	No	Health checks, static routes
Trie	O(path_len)	O(n × path_len)	Limited	Most gateways
Radix Tree	O(path_len)	O(n)	Limited	Envoy, high-perf systems
Hybrid (Radix + Regex)	O(path_len + m)	O(n)	Yes	Production gateways

Priority and Conflict Resolution

When multiple routing rules match an incoming request, the gateway must deterministically select one. The priority resolution strategy is critical—ambiguous or incorrect resolution causes misdirected traffic, often silently.

The Conflict Scenario:

Consider these two rules:

Rule A: /api/users/:id      → user-service
Rule B: /api/users/search   → search-service

Request /api/users/search matches both rules:

Rule A: :id = "search"
Rule B: Exact match on "search"

Which wins? The answer depends on the gateway's resolution strategy.

Common Resolution Strategies:

Priority Resolution Strategies

•Specificity-Based (Most Common): More specific patterns win. Static segments beat parameters beat wildcards beat globs. /users/search beats /users/:id beats /users/* beats /**.
•Longest Match: The pattern that consumes the most path segments wins. Ties broken by specificity.
•Explicit Priority: Each rule has a numeric priority. Highest priority wins regardless of pattern specificity.
•First Match: Rules are evaluated in definition order; first match wins. Requires careful rule ordering.
•Most Recently Defined: Last defined rule wins (rarely used, error-prone).

Production Best Practice: Explicit Priority with Logging

Combine specificity-based defaults with explicit priority overrides and conflict logging:

routes:
  # Explicit high priority for specific endpoints
  - path: /api/users/search
    priority: 100
    backend: search-service
    
  # Lower priority for parameterized routes  
  - path: /api/users/:id
    priority: 50
    backend: user-service
    
  # Default catch-all with lowest priority
  - path: /api/**
    priority: 1
    backend: legacy-api

Log all ambiguous matches (where multiple rules have equal priority) to detect configuration issues before they cause incidents.

Silent Routing Errors

One of the most insidious production issues is silent misrouting—where requests go to the wrong service but still receive a "valid" response. For example, /users/status hitting a user detail endpoint with id="status" and returning a 404. Monitor routing decisions with request tracing and validate routing tables in CI/CD.

priority-resolution-demo.ts
TypeScript
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// Priority calculation based on pattern specificity
interface RouteRule {
  pattern: string;
  backend: string;
  explicitPriority?: number;
}
 
function calculatePriority(pattern: string): number {
  const segments = pattern.split('/').filter(Boolean);
  let priority = 0;
  
  for (let i = 0; i < segments.length; i++) {
    const segment = segments[i];
    const positionWeight = (segments.length - i) * 10; // Earlier segments matter more
    
    if (segment === '**') {
      // Glob: lowest priority
      priority += positionWeight * 1;
    } else if (segment === '*') {
      // Wildcard: low priority
      priority += positionWeight * 2;
    } else if (segment.startsWith(':')) {
      // Parameter: medium priority
      priority += positionWeight * 5;
    } else {
      // Static: highest priority
      priority += positionWeight * 10;
    }
  }
  
  // Longer patterns generally more specific
  priority += segments.length * 5;
  
  return priority;
}
 
function resolveConflict(rules: RouteRule[], path: string): RouteRule | null {
  const matches = rules.filter(rule => matchPattern(rule.pattern, path));
  
  if (matches.length === 0) return null;
  if (matches.length === 1) return matches[0];
  
  // Sort by explicit priority first, then calculated priority
  matches.sort((a, b) => {
    const aPriority = a.explicitPriority ?? calculatePriority(a.pattern);
    const bPriority = b.explicitPriority ?? calculatePriority(b.pattern);
    return bPriority - aPriority;
  });
  
  // Log if ambiguous (top two have same priority)
  const top = matches[0];
  const second = matches[1];
  const topPriority = top.explicitPriority ?? calculatePriority(top.pattern);
  const secondPriority = second.explicitPriority ?? calculatePriority(second.pattern);
  
  if (topPriority === secondPriority) {
    console.warn(`Ambiguous routing for ${path}:`, 
      `${top.pattern} and ${second.pattern} have equal priority (${topPriority})`);
  }
  
  return top;
}
 
// Example:
const rules: RouteRule[] = [
  { pattern: '/api/users/:id', backend: 'user-service' },
  { pattern: '/api/users/search', backend: 'search-service' },
  { pattern: '/api/**', backend: 'legacy-api' },
];
 
const match = resolveConflict(rules, '/api/users/search');
// Returns: { pattern: '/api/users/search', backend: 'search-service' }
// Because static 'search' beats parameter ':id'

Path Rewriting and Stripping

Routing is only half the story. After selecting a backend, the gateway often needs to transform the request path before forwarding. Common transformations include:

Path Stripping: Remove the matched prefix
Path Prepending: Add a prefix to the path
Path Replacement: Completely replace the path
Regex-Based Rewriting: Complex transformations

Path Stripping Example:

Incoming:  /api/v2/users/123
Rule:      /api/v2/users → user-service (strip prefix)
Forwarded: /123          (prefix stripped)

This pattern enables the gateway to "own" the API namespace (/api/v2/users) while backends receive root-relative paths.

Path Prepending Example:

Incoming:  /users/123
Rule:      /users → legacy-api (prepend /v1/accounts)
Forwarded: /v1/accounts/users/123

This enables API facade patterns where the public API differs from internal service paths.

path-rewrite-examples.yaml
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# Envoy route configuration with path rewriting
routes:
  - match:
      prefix: "/api/v2/users"
    route:
      cluster: user-service
      # Strip /api/v2/users prefix, keep the rest
      prefix_rewrite: "/"  
      
  - match:
      prefix: "/public/users"
    route:
      cluster: user-service
      # Rewrite /public/users/123 → /internal/accounts/123
      regex_rewrite:
        pattern:
          google_re2: {}
          regex: "^/public/users/(.*)"
        substitution: "/internal/accounts/\1"
        
  - match:
      safe_regex:
        google_re2: {}
        regex: "^/api/v([0-9]+)/(.*)$"
    route:
      cluster: versioned-api
      # Capture version and path, restructure
      regex_rewrite:
        pattern:
          google_re2: {}
          regex: "^/api/v([0-9]+)/(.*)$"
        substitution: "/\2?version=\1"  # /api/v2/users → /users?version=2

Debug Header for Path Transformations

In non-production environments, inject a debug header showing the original and rewritten paths: X-Original-Path: /api/v2/users/123 and X-Rewritten-Path: /123. This dramatically simplifies debugging routing issues.

Production Considerations

Path-based routing in production requires addressing several operational concerns that don't surface in development:

1. Configuration Hot-Reloading

Routing rules change frequently. The gateway must support live configuration updates without dropping connections or causing traffic disruption. Production gateways use:

Graceful configuration draining (finish in-flight requests with old config)
Atomic configuration swaps
Configuration versioning with rollback capability

2. Routing Table Size Limits

Large routing tables impact:

Memory consumption (~100 bytes per rule for simple patterns, more for regex)
Startup time (parsing and compiling patterns)
Match latency (though well-optimized tries mitigate this)

Most gateways support 10,000+ routes, but regex-heavy configurations may hit performance cliffs.

3. Security Implications

Path Traversal: Ensure /../ and encoded variants are normalized before routing
Case Sensitivity: Inconsistent case handling can bypass security rules
Path Injection: Validate that path parameters don't inject control characters
Regex DoS: Complex regex patterns can cause catastrophic backtracking

Production Routing Checklist

•Validate Routes in CI/CD: Parse and compile routing configuration in pipeline, catch syntax errors early
•Test Route Conflicts: Automated tests for expected routing behavior, especially edge cases
•Monitor Route Latency: Track P99 routing decision time; alert if trie traversal slows
•Log Unmatched Requests: 404s should log the exact path for debugging
•Implement Rate Limiting per Route: Different routes may have different limits
•Version Your Configuration: Enable quick rollback of routing changes
•Document Route Ownership: Which team owns which path prefix?
•Plan for Path Migration: Strategy for changing path structure without breaking clients

The 'Works on My Machine' Trap

Local gateway configurations often differ from production. Path matching behavior, case sensitivity, and trailing slash handling can vary between gateway versions and configurations. Always validate routing changes in a staging environment that mirrors production exactly.

Summary: Path-Based Routing Mastery

Path-based routing is the foundational mechanism that directs API traffic to appropriate backend services. Mastering it requires understanding URL anatomy, matching semantics, algorithmic implementations, and production operational concerns.

Key Takeaways

•URL Structure: Paths consist of static segments, parameters, wildcards, and globs—each with distinct matching semantics
•Exact vs Prefix: Exact matching provides precision; prefix matching provides flexibility. Choose based on use case.
•Path Parameters: Dynamic segments extracted by gateways enable resource-based routing and can be injected as headers
•Trie-Based Routing: Production gateways use tries/radix trees for O(path_length) matching regardless of rule count
•Priority Resolution: Conflicts are resolved by specificity, explicit priority, or rule order—know your gateway's strategy
•Path Rewriting: Gateways transform paths (strip, prepend, regex rewrite) before forwarding to backends
•Production Concerns: Hot-reload support, security normalization, and monitoring are essential for reliable routing

Page Complete

You now have a deep understanding of path-based routing—from URL fundamentals to production-grade trie implementations. The next page covers header-based routing, which adds another dimension to routing decisions by considering HTTP headers, enabling sophisticated traffic management patterns.