Transform

Protocol v2 Features

As of v0.2.0, the Transform agent supports protocol v2 with:

• Capability negotiation: Reports supported features during handshake
• Health reporting: Exposes health status for monitoring
• Metrics export: Counter metrics for requests processed/transformed
• gRPC transport: High-performance gRPC transport via --grpc-address
• Lifecycle hooks: Graceful shutdown and drain handling

Overview

A configuration-driven transformation agent for Sentinel that provides advanced request and response modifications. Supports URL rewriting with regex capture groups, header manipulation with variable interpolation, and comprehensive JSON body transformations.

Features

• URL Rewriting: Regex-based path rewriting with named capture groups (${resource}, ${id})
• Header Manipulation: Add, set, or remove headers with variable interpolation
• JSON Body Transforms: Set, delete, rename, wrap, merge, copy, and move operations
• Variable Interpolation: Access request data, captures, and body fields in transforms
• Conditional Matching: Match on path patterns, HTTP methods, headers, and JSON body content
• Priority-Based Rules: First-match evaluation with configurable rule priorities
• Debug Headers: Optional X-Transform-Rule and X-Transform-Time response headers

Installation

Using Bundle (Recommended)

The easiest way to install this agent is via the Sentinel bundle command:

# Install just this agent
sentinel bundle install transform

# Or install all available agents
sentinel bundle install --all

The bundle command automatically downloads the correct binary for your platform and places it in ~/.sentinel/agents/.

Using Cargo

cargo install sentinel-agent-transform

From Source

git clone https://github.com/raskell-io/sentinel-agent-transform
cd sentinel-agent-transform
cargo build --release

Configuration

Command Line

sentinel-agent-transform \
    --socket /var/run/sentinel/transform.sock \
    --grpc-address 0.0.0.0:50051 \
    --config /etc/sentinel/transform.yaml

CLI Options

Sentinel Configuration

agent "transform" {
    socket "/var/run/sentinel/transform.sock"
    timeout 50ms
    events ["request_headers" "request_body" "response_headers" "response_body"]
}

route {
    match { path-prefix "/api/v1" }
    agents ["transform"]
    upstream "backend"
}

Agent Configuration (YAML)

version: "1"
settings:
  max_body_size: 10485760   # 10MB
  debug_headers: false
  timeout_ms: 100

rules:
  - name: "api-v1-to-v2-migration"
    enabled: true
    priority: 100
    match:
      path:
        pattern: "^/api/v1/(?P<resource>\\w+)/(?P<id>\\d+)$"
        type: regex
      methods: [GET, POST, PUT, DELETE]
    request:
      url:
        rewrite: "/api/v2/${resource}/${id}"
        preserve_query: true
      headers:
        add:
          - { name: "X-API-Version", value: "2" }
          - { name: "X-Migrated-From", value: "v1" }
    response:
      headers:
        add:
          - { name: "Deprecation", value: "true" }
          - { name: "Sunset", value: "2026-06-01" }

Rule Structure

Match Conditions

Rules are evaluated in priority order (highest first). A rule matches when all specified conditions are true:

Path Matching Examples

# Exact match
path:
  pattern: "/api/health"
  type: exact

# Glob pattern
path:
  pattern: "/api/*/users/*"
  type: glob

# Regex with named captures
path:
  pattern: "^/api/v(?P<version>\\d+)/(?P<resource>\\w+)$"
  type: regex

Header Conditions

headers:
  - name: "Content-Type"
    contains: "json"
  - name: "Authorization"
    present: true
  - name: "X-Debug"
    absent: true
  - name: "X-API-Key"
    equals: "secret123"

Body Matching (JSON)

body:
  json:
    - path: "$.type"
      equals: "user"
    - path: "$.auth_token"
      exists: true
    - path: "$.message"
      contains: "error"

Transformations

URL Rewriting

request:
  url:
    rewrite: "/api/v2/${resource}/${id}"
    preserve_query: true
    add_query:
      version: "2"
      source: "${request.client_ip}"
    remove_query:
      - debug
      - trace

Header Manipulation

request:
  headers:
    add:                    # Add if not present
      - { name: "X-Request-ID", value: "${correlation_id}" }
    set:                    # Always set (overwrite)
      - { name: "X-Forwarded-For", value: "${request.client_ip}" }
    remove:
      - "X-Internal-Token"
      - "X-Debug"

response:
  headers:
    add:
      - { name: "X-Response-Time", value: "${now}" }
    remove:
      - "Server"
      - "X-Powered-By"

JSON Body Transforms

request:
  body:
    json:
      operations:
        # Set a value at a path
        - set:
            path: "$.metadata.version"
            value: "2.0"

        # Delete fields
        - delete:
            - "$.internal_id"
            - "$.debug_info"

        # Rename a field
        - rename:
            from: "$.old_field"
            to: "$.new_field"

        # Wrap the entire body
        - wrap:
            path: "$"
            key: "data"

        # Merge additional fields
        - merge:
            path: "$"
            with:
              api_version: "2"
              processed_at: "${now}"

        # Copy a value
        - copy:
            from: "$.user.id"
            to: "$.metadata.user_id"

        # Move a value
        - move:
            from: "$.legacy.field"
            to: "$.modern.field"

Variable Interpolation

Variables can be used in header values, URL rewrites, and JSON transforms:

Common Use Cases

API Version Migration

rules:
  - name: "v1-to-v2-migration"
    match:
      path:
        pattern: "^/api/v1/(.*)$"
        type: regex
    request:
      url:
        rewrite: "/api/v2/${1}"
    response:
      headers:
        add:
          - { name: "Deprecation", value: "true" }

Add Correlation Headers

rules:
  - name: "correlation-headers"
    match:
      path:
        pattern: "^/api/.*$"
        type: regex
    request:
      headers:
        add:
          - { name: "X-Correlation-ID", value: "${correlation_id}" }
          - { name: "X-Request-Start", value: "${now}" }

Remove Sensitive Response Headers

rules:
  - name: "sanitize-response"
    match:
      path:
        pattern: "*"
        type: glob
    response:
      headers:
        remove:
          - "Server"
          - "X-Powered-By"
          - "X-AspNet-Version"

Wrap API Responses

rules:
  - name: "wrap-response"
    match:
      path:
        pattern: "^/api/.*$"
        type: regex
      response:
        content_types: ["application/json"]
    response:
      body:
        json:
          operations:
            - wrap:
                path: "$"
                key: "data"
            - merge:
                path: "$"
                with:
                  success: true
                  timestamp: "${now}"

Normalize Backend Responses

rules:
  - name: "normalize-user-response"
    match:
      path:
        pattern: "^/api/users/.*$"
        type: regex
    response:
      body:
        json:
          operations:
            - rename:
                from: "$.firstName"
                to: "$.first_name"
            - rename:
                from: "$.lastName"
                to: "$.last_name"
            - delete:
                - "$.internal_id"
                - "$.password_hash"

Debug Mode

Enable debug headers to see which rules are applied:

settings:
  debug_headers: true

Response headers added:

• X-Transform-Rule: Name of the applied rule
• X-Transform-Time: Processing time in milliseconds

Performance

• Latency: <2ms typical transform time
• Memory: ~20MB base + rule complexity
• Throughput: >100k requests/second

Related Agents

Comparison with Alternatives