RudderStack Transformations with Rudder CLI

Overview

RudderStack transformations allow real-time event manipulation using JavaScript or Python. Libraries provide reusable code shared across transformations. Both are managed as code using YAML specs and the Rudder CLI.

Recommended Workflow

Follow this loop for every change — authoring a new transformation or library, or editing existing code. Testing locally with fixtures is the key step that distinguishes transformations work from other CLI workflows.

digraph transformations_workflow {
    rankdir=TB;
    "rudder-cli workspace info" [shape=box];
    "Authenticated?" [shape=diamond];
    "rudder-cli auth login" [shape=box];
    "Edit YAML / JS / fixtures" [shape=box];
    "rudder-cli validate -l ./" [shape=box];
    "Validation errors?" [shape=diamond];
    "Fix errors" [shape=box];
    "rudder-cli transformations test --all -l ./" [shape=box];
    "Tests pass?" [shape=diamond];
    "Fix code or expected output" [shape=box];
    "rudder-cli apply --dry-run -l ./" [shape=box];
    "Diff matches intent?" [shape=diamond];
    "Adjust specs" [shape=box];
    "rudder-cli apply -l ./" [shape=box];
    "Done" [shape=doublecircle];

    "rudder-cli workspace info" -> "Authenticated?";
    "Authenticated?" -> "rudder-cli auth login" [label="no"];
    "rudder-cli auth login" -> "rudder-cli workspace info";
    "Authenticated?" -> "Edit YAML / JS / fixtures" [label="yes"];
    "Edit YAML / JS / fixtures" -> "rudder-cli validate -l ./";
    "rudder-cli validate -l ./" -> "Validation errors?";
    "Validation errors?" -> "Fix errors" [label="yes"];
    "Fix errors" -> "rudder-cli validate -l ./";
    "Validation errors?" -> "rudder-cli transformations test --all -l ./" [label="no"];
    "rudder-cli transformations test --all -l ./" -> "Tests pass?";
    "Tests pass?" -> "Fix code or expected output" [label="no"];
    "Fix code or expected output" -> "rudder-cli validate -l ./";
    "Tests pass?" -> "rudder-cli apply --dry-run -l ./" [label="yes"];
    "rudder-cli apply --dry-run -l ./" -> "Diff matches intent?";
    "Diff matches intent?" -> "Adjust specs" [label="no"];
    "Adjust specs" -> "rudder-cli validate -l ./";
    "Diff matches intent?" -> "rudder-cli apply -l ./" [label="yes"];
    "rudder-cli apply -l ./" -> "Done";
}

Steps:

1. Verify auth — rudder-cli workspace info; re-auth with rudder-cli auth login if needed.
2. Validate — rudder-cli validate -l ./ catches YAML schema errors, missing files, camelCase import_name violations.
3. Test locally — rudder-cli transformations test --all -l ./ runs each transformation against its tests/input/*.json fixtures and diffs against tests/output/*.json. Use --modified for faster iteration or pass a specific transformation id.
4. Dry run — rudder-cli apply --dry-run -l ./ shows the diff that would be applied. Check for unexpected deletions.
5. Apply — rudder-cli apply -l ./ publishes libraries and transformations atomically (correct order handled by the CLI).

For the broader validate → apply cycle that applies to all CLI-managed resources, see the rudder-cli-workflow skill. This skill specializes it with the local-test step.

Worked example

A complete end-to-end project — library, transformation, fixtures, and README — lives at examples/transformations-workflow/ in this repo. It includes a ported Base64 library (base64-lib), a sample transformation that uses it, and input/output test fixtures. Use it as a scaffold for new transformations or as a reference for tests/ structure.

Directory Structure

transformations/
  my-transformation.yaml         # Transformation spec
  my-library.yaml                # Library spec
  javascript/
    my-transformation.js         # JavaScript code
    my-library.js               # Library code
  python/
    my-transformation.py         # Python code
    my-library.py               # Library code
  tests/
    input/
      event1.json               # Test input events
    output/
      event1.json               # Expected outputs

YAML Schemas

Transformation Spec

version: "rudder/v1"
kind: "transformation"
metadata:
  name: "transformations"
  # Optional: import section for linking to existing workspace resources
  import:
    workspaces:
      - workspace_id: "your-workspace-id"
        resources:
          - remote_id: "existing-transformation-id"
            urn: "transformation:my-transformation"
spec:
  id: "my-transformation"           # Unique identifier (used as external ID)
  name: "My Transformation"         # Human-readable name
  description: "Description"        # Optional
  language: "javascript"            # "javascript" or "python"
  file: "javascript/my-transformation.js"  # Path to code (relative to YAML)
  # OR inline:
  # code: |
  #   export function transformEvent(event) { return event; }
  tests:                            # Optional - local testing only
    - name: "basic-test"
      input: "./tests/input"        # Directory with JSON events
      output: "./tests/output"      # Directory with expected results

Note: The metadata.import section is auto-generated when importing existing resources from a workspace. It links local files to remote resources.

Library Spec

version: "rudder/v1"
kind: "transformation-library"
metadata:
  name: "transformation-libraries"
spec:
  id: "my-library"                  # Unique identifier
  name: "My Library"                # Human-readable name
  description: "Reusable utilities" # Optional
  language: "javascript"            # "javascript" or "python"
  import_name: "myLibrary"          # MUST be camelCase of name field!
  file: "javascript/my-library.js"  # Path to code

IMPORTANT: import_name must be the exact camelCase conversion of name:

• "My Library" → "myLibrary"
• "Base64 Library" → "base64Library"
• "URL Parser Utils" → "urlParserUtils"

Code Patterns

Transformation Code (JavaScript)

// transformEvent is the entry point - REQUIRED
// metadata is a FUNCTION that takes the event and returns metadata
export function transformEvent(event, metadata) {
    // Modify event
    event.context = event.context || {};
    event.context.transformed = true;

    // Access metadata - call as function with event
    const meta = metadata(event);
    event.metadata = meta;

    // Return event (or null/undefined to drop the event)
    return event;
}

Key points:

• metadata is a function, not an object - call it as metadata(event)
• Return null or undefined to drop/filter out an event
• The function must be exported with export

Library Code (JavaScript)

// Export functions for use in transformations
export function encode(str) {
    // Implementation
    return encodedStr;
}

export function decode(str) {
    // Implementation
    return decodedStr;
}

Importing Libraries in Transformations

// Import using the library's import_name (camelCase of name)
// If library name is "My Library", import_name is "myLibrary"
import { encode, decode } from "myLibrary";

export function transformEvent(event, metadata) {
    event.properties.encoded = encode(event.properties.data);
    return event;
}

Remember: The import string must exactly match the library's import_name field.

CLI Commands

Workflow: Adding a New Library

1. Create library YAML spec:

   version: "rudder/v1"
   kind: "transformation-library"
   metadata:
     name: "transformation-libraries"
   spec:
     id: "base64-lib"
     name: "Base64 Library"
     description: "Base64 encoding/decoding"
     language: "javascript"
     import_name: "base64Library"  # camelCase of "Base64 Library"
     file: "javascript/base64-lib.js"

1. Create library code file at javascript/base64-lib.js

1. Validate: rudder-cli validate -l ./

1. Dry-run: rudder-cli apply --dry-run -l ./

1. Apply: rudder-cli apply -l ./

See `rudder-cli-workflow` skill for detailed iteration workflow.

Workflow: Using Library in Transformation

1. Import the library using its import_name:

   import { encode } from "base64Library";

1. Dependencies auto-detected: The CLI parses imports and creates dependency graph

1. Batch publish: Libraries and transformations publish atomically

Porting External Libraries

When porting npm/external libraries to RudderStack:

1. Remove UMD/CommonJS wrappers - Use ES modules only
2. Remove Node.js-specific code - No Buffer, require(), etc.
3. Export functions directly - Not as object methods
4. Keep dependencies minimal - RudderStack sandbox is limited
5. Use polyfills for browser APIs - btoa/atob may need polyfills
6. Convert internal object methods to standalone functions

Example: Porting a Library

Original (UMD with Buffer):

(function(global, factory) {
    typeof exports === 'object' ? module.exports = factory() : ...
})(this, function() {
    var _hasBuffer = typeof Buffer === 'function';
    var encode = _hasBuffer
        ? (s) => Buffer.from(s).toString('base64')
        : (s) => btoa(s);
    return { encode: encode };
});

Ported (ES Module with polyfill):

// Polyfill for environments without btoa
const btoaPolyfill = (bin) => {
    const b64chs = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
    let asc = '';
    // ... pure JS implementation
    return asc;
};

const _btoa = typeof btoa === 'function' ? (s) => btoa(s) : btoaPolyfill;

// Export as standalone function
export function encode(s) {
    return _btoa(s);
}

Porting Checklist

• [ ] Remove IIFE/UMD wrapper
• [ ] Remove module.exports, exports, define() calls
• [ ] Replace require() with import
• [ ] Replace Buffer usage with pure JS (Uint8Array, manual encoding)
• [ ] Add polyfills for btoa/atob if used
• [ ] Convert class methods to exported functions if needed
• [ ] Test with various input types (UTF-8, special chars, edge cases)

Common Mistakes

Critical: The import_name field is validated to be the exact camelCase conversion of the name field. This is enforced by rudder-cli validate.

RudderStack Built-in Libraries

Access built-in libraries with @rs/ prefix:

import { get } from "@rs/lodash/v1";
import { v4 as uuidv4 } from "@rs/uuid/v1";

Testing

Test files use JSON format:

Input (`tests/input/event1.json`):

{
  "type": "track",
  "event": "Test Event",
  "properties": {
    "data": "hello"
  }
}

Expected Output (`tests/output/event1.json`):

{
  "type": "track",
  "event": "Test Event",
  "properties": {
    "data": "hello",
    "encoded": "aGVsbG8="
  }
}

Quick Reference

Publish Order

The CLI handles publish order automatically:

1. Workspace credentials (secrets)
2. Libraries (dependencies)
3. Transformations (depend on libraries)

All published atomically - no partial updates.

Credential Security

• Never hardcode API keys in transformation code - use workspace credentials/secrets instead
• Store workspace tokens in environment variables - never commit RUDDER_ACCESS_TOKEN to git
• Add `.env` to `.gitignore` - if using dotenv files for local development
• Use CI/CD secrets - for automated deployments, use repository secrets
• Reference secrets via workspace credentials - transformation code can access configured secrets securely

Handling External Content

When processing events in transformations:

• Validate event structure - check for expected fields before processing
• Sanitize string inputs - escape or validate user-generated content in event properties
• Don't eval dynamic content - never use eval() or Function() on event data
• Extract only expected fields - access known properties, don't iterate over unknown keys
• Log safely - avoid logging full event payloads that may contain PII