FaaSlang defines FaaS execution semantics and type-safety mechanisms

Usage no npm install needed!

<script type="module">
  import faaslang from 'https://cdn.skypack.dev/faaslang';



FaaSlang Logo

travis-ci build npm version

Function as a Service Language

The following is a working draft of the latest FaaSlang specification, version 0.3.x, dated February 12th, 2018.

FaaSlang is a simple open specification intended to define semantics and implementation details around FaaS ("serverless") functions, gateways and client interfaces (requests from any language / SDK). It has been designed with the goal of decreasing organizational complexity around FaaS microservices by encouraging simple conventions for how we document and interface with them, including type safety mechanisms. In the same way GraphQL is intended to provide opinions and a specification for the way developers interface with nested relational (graph) data, FaaSlang does the same for FaaS resources.

If you use a FaaSlang-compliant deployment and API gateway (for example, as used by https://stdlib.com) you get the following benefits over traditional gateways for serverless functions:

  • Standard Calling Conventions (HTTP)
  • Type Safety
  • Enforced Documentation
  • Background Execution (immediately return response, run logic as a worker)

And that's just the beginning. All of the goodies you're looking for like rate limiting, authentication, etc. are not part of the FaaSlang specification but can easily be added to the example provided in this repository.

Table of Contents

  1. Introduction
  2. Why FaaSlang?
  3. Specification
    1. FaaSlang Resource Definition
    2. Context Definition
    3. Parameters
      1. Constraints
      2. Types
      3. Type Conversion
      4. Nullability
    4. FaaSlang Resource Requests
      1. Context
      2. Errors
        1. ClientError
        2. ParameterError
          1. Details: Required
          2. Details: Invalid
        3. FatalError
        4. RuntimeError
        5. ValueError
  4. FaaSlang Server and Gateway: Implementation
  5. Acknowledgements

What is FaaSlang?

To put it simply, FaaSlang defines semantics and rules for a "serverless" function deployment and execution (API) gateway to turn this:

// hello_world.js

* My hello world function!
module.exports = function (name = 'world', callback) {

  callback(null, `hello ${name}`);


Into an infinitely scalable web API (using "serverless" providers) that can be called over HTTP like this (GET):


Or like this (POST):

  "name": "joe"

And gives a result like this:

"hello joe"

Or, when a type mismatch occurs (like {"name":10}):

  "error": {

Why FaaSlang?

The "serverless" space is growing rapidly, and as it grows, so do the toolchains required to keep up. Each infrastructure provider imposes its own standard and way of doing things around FaaS to the point we're relying on individual developers to pick and choose the best framework for deployment.

FaaSlang takes a different approach, and offers a specification for an API Gateway (and a reasonably robust, non-vendor-specific Node.js implementation of such) that acts as a way to "lock in" the way you and your team members deploy to and execute your "serverless" functions.

Take a current example of an AWS Lambda function (A);

exports.handler = (event, context, callback) => {
  let myVar = event.myVar;
  let requiredVar = event.requiredVar;
  myVar = myVar === undefined ? 1 : myVar;
  callback(null, 'Hello from Lambda!');

Or a Microsoft Azure function (B);

module.exports = function (context, req) {
  let myVar = req.query.myVar || req.body && req.body.myVar;
  let requiredVar = req.query.requiredVar || req.body && req.body.requiredVar;
  myVar = myVar === undefined ? 1 : myVar;
  context.res = {body: 'Hello from Microsoft Azure!'};

FaaSlang instead defines the Node.js function footprint;

* @param {Number} myVar A number
* @param {String} requiredVar must be a string!
* @returns {String}
module.exports = (myVar = 1, requiredVar, context, callback) => {
  callback(null, 'Hello from FaaSlang-compliant service vendor.');

Where comments are used as part of the semantic definition for type-safety (if they can't be inferred from defaults), expected parameters can be specifically defined, and you still have an optional context object for more robust execution (argument overloading, etc.)

Here's what the current FaaS workflow looks like:

Current FaaS Workflow

And this is what a FaaSlang-enabled workflow looks like.

FaaSlang Workflow

FaaSlang is the result of tens of thousands of FaaS deployments, by thousands of developers, spread across a number of cloud service providers and the need to standardize our ability to organize and communicate with these functions.


FaaSlang Resource Definition

A FaaSlang definition is a definition.json file that respects the following format.

Given a function like this (filename my_function.js):

* This is my function, it likes the greek alphabet
* @param {String} alpha Some letters, I guess
* @param {Number} beta And a number
* @param {Boolean} gamma True or false?
* @returns {Object} some value
module.exports = async function my_function (alpha, beta = 2, gamma, context) {
  /* your code */

You would provide a function definition that looks like this:

  "name": "my_function",
  "format": {
    "language": "nodejs",
    "async": true
  "description": "This is my function, it likes the greek alphabet",
  "bg": {
    "mode": "info",
    "value": ""
  "charge": 1,
  "context": null,
  "params": [
      "name": "alpha",
      "type": "string",
      "description": "Some letters, I guess"
      "name": "beta",
      "type": "number",
      "defaultValue": 2,
      "description": "And a number"
      "name": "gamma",
      "type": "boolean",
      "description": "True or false?"
  "returns": {
    "type": "object",
    "description": "some value"

This definition is extensible, meaning you can add additional fields to it, but it must obey this schema.

A definition must implement the following fields;

Field Definition
name A user-readable function name (used to execute the function), must match /[A-Z][A-Z0-9_]*/i
format An object requiring a language field, along with any implementation details
description A brief description of what the function does, can be empty ("")
bg An object containing "mode" and "value" parameters specifying the behavior of function responses when executed in the background
charge An integer between 0 and 100 defining the cost (arbitrary units) to run this function, charged to authenticated users
params An array of NamedParameters, representing function arguments
returns A Parameter without a defaultValue representing function return value

Context Definition

If the function does not access execution context details, this should always be null. If it is an object, it indicates that the function does access context details (i.e. remoteAddress, http headers, etc. - see Context).

This object does not have to be empty, it can contain vendor-specific details; for example "context": {"user": ["id", "email"]} may indicate that the execution context specifically accesses authenticated user id and email addresses.


Parameters have the following format;

Field Required Definition
name NamedParameter Only The name of the Parameter, must match /[A-Z][A-Z0-9_]*/i
type yes A string representing a valid FaaSlang type
description yes A short description of the parameter, can be empty string ("")
defaultValue no Must match the specified type, if not provided this parameter is required


As FaaSlang is intended to be polyglot, functions defined with it must have a strongly typed signature. Not all types are guaranteed to be consumable in the same way in every language, and we will continue to define specifications for how each language should interface with FaaSlang types. At present, the types are a limited superset of JSON values.

Type Definition Example Input Values (JSON)
boolean True or False true or false
string Basic text or character strings "hello", "GOODBYE!"
number Any double-precision Floating Point value 2e+100, 1.02, -5
float Alias for number 2e+100, 1.02, -5
integer Subset of number, integers between -2^53 + 1 and +2^53 - 1 (inclusive) 0, -5, 2000
object Any JSON-serializable Object {}, {"a":true}, {"hello":["world"]}
object.http An object representing an HTTP Response. Accepts headers, body and statusCode keys {"body": "Hello World"}, {"statusCode": 404, "body": "not found"}, {"headers": {"Content-Type": "image/png"}, "body": new Buffer(...)}
array Any JSON-serializable Array [], [1, 2, 3], [{"a":true}, null, 5]
buffer Raw binary octet (byte) data representing a file {"_bytes": [8, 255]} or {"_base64": "d2h5IGRpZCB5b3UgcGFyc2UgdGhpcz8/"}
any Any value mentioned above 5, "hello", []

Type Conversion

The buffer type will automatically be converted from any object with a single key-value pair matching the footprints {"_bytes": []} or {"_base64": ""}.

Otherwise, parameters provided to a function are expected to match their defined types. Requests made over HTTP via query parameters or POST data with type application/x-www-form-urlencoded will be automatically converted from strings to their respective expected types, when possible (see FaaSlang Resource Requests below):

Type Conversion Rule
boolean "t" and "true" become true, "f" and "false" become false, otherwise do not convert
string No conversion
number Determine float value, if NaN do not convert, otherwise convert
float Determine float value, if NaN do not convert, otherwise convert
integer Determine float value, if NaN do not convert, may fail integer type check if not in range
object Parse as JSON, if invalid do not convert, object may fail type check (array, buffer)
object.http Parse as JSON, if invalid do not convert, object may fail type check (array, buffer)
array Parse as JSON, if invalid do not convert, object may fail type check (object, buffer)
buffer Parse as JSON, if invalid do not convert, object may fail type check (object, array)
any No conversion


All types are nullable, but nullability can only be specified by setting "defaultValue": null in the NamedParameter definition. That is to say, if a default value is provided, the type is no longer nullable.

Setting HTTP headers

The FaaSlang specification is not intended to be solely used over HTTP, though if used over HTTP with a provided callback method, the third parameter passed to callback should be an Object representing HTTP Header key-value pairs.

For example, to return an image that's of type image/png...

module.exports = (imageName, callback) => {

  // fetch image, returns a buffer
  let png = imageName === 'cat' ?
    fs.readFileSync(`/images/kitty.png`) :

  // Forces image/png over HTTP requests, default
  //  for buffer would otherwise be application/octet-stream
  return callback(null, png, {'Content-Type': 'image/png'});


You can use the third parameter only when a callback ends the function, i.e. not for use with async functions. This can be used to serve any type of content via HTTP, set cache details (E-Tag header), etc.

FaaSlang Resource Requests

FaaSlang-compliant requests must complete the following steps;

  1. Ensure the Resource Definition is valid and compliant, either on storage or accession.
  2. Performs a handshake (i.e. HTTP) with initial request details
  3. Accept an Array, Object or a string of URLencoded variables
  4. If over HTTP and query parameters present, query parameters used as URL encoded variables
  5. If over HTTP POST and query parameters present, reject requests that try to specify a POST body as well with a ClientError
  6. If over HTTP POST, requests must include a Content-Type header or a ClientError is immediately returned
  7. If over HTTP POST, Content-Type must be application/json for Array or Object data, or application/x-www-form-urlencoded for string data or a ClientError is immediately returned
  8. If application/x-www-form-urlencoded values are provided (either via POST body or query parameters), convert types based on Type Conversion and knowledge of the function definition and create an Object
  9. If Array: Parameters will be checked for type consistency in the order of the definition params
  10. If Object: Parameters will be checked for type consistency based on names of the definition params
  11. If any inconsistencies are found, cease execution and immediately return a ParameterError
  12. If a parameter has no defaultValue specified and is not provided, immediately return a ParameterError
  13. Try to execute the function, if the function fails to parse or is not valid, immediately return a FatalError
  14. If a function hits a specified timeout (execution time limit), immediately return a FatalError
  15. If a function returns an error (via callback) or one is thrown and not caught, immediately return a RuntimeError
  16. If function returns inconsistent response (does not match returns type), immediately return a ValueError
  17. If no errors are encountered, return the value to the client
  18. If over HTTP and content-type is not being overloaded (i.e. developer specified through a vendor-specific mechanism), return buffer type data as application/octet-stream and any other values as application/json.


Every function intended to be consumed via FaaSlang has the option to specify an optional magic context parameter that receives vendor-specific information about the function execution context - for example, if consumed over HTTP, header details. FaaSlang definitions must specify whether or not they consume a context object. Context objects are extensible but MUST contain the following fields;

Field Definition
params An object mapping called parameter names to their values
http null if not accessed via http, otherwise an object
http.headers If accessed via HTTP, an object containing header values


Errors returned by FaaSlang-compliant services must follow the following JSON format:

  "error": {
    "type": "ClientError",
    "message": "You know nothing, Jon Snow",
    "details": {}

details is an optional object that can provide additional Parameter details. Valid Error types are:

  • ClientError
  • ParameterError
  • FatalError
  • RuntimeError
  • ValueError


ClientErrors are returned as a result of bad or malformed client data, including lack of authorization or a missing function (not found). If over HTTP, they must returns status codes in the range of 4xx.


ParameterErrors are a result of Parameters not passing type-safety checks, and must return status code 400 if over HTTP.

Parameter Errors must have the following format;

  "error": {
    "type": "ParameterError",
    "message": "ParameterError",
    "details": {...}

"details" should be an object mapping parameter names to their respective validation (type-checking) errors. Currently, this specification defines two classifications of a ParameterError for a parameter; required and invalid. The format of "details": {} should follow this format;

Details: Required
  "param_name": {
    "message": "((descriptive message stating parameter is required))",
    "required": true
Details: Invalid
  "param_name": {
    "message": "((descriptive message stating parameter is invalid))",
    "invalid": true,
    "expected": {
      "type": "number"
    "actual": {
      "type": "string",
      "value": "hello world"


FatalErrors are a result of function mismanagement - either your function could not be loaded, executed, or it timed out. These must return status code 500 if over HTTP.


RuntimeErrors are a result of uncaught exceptions in your code as it runs, including errors you explicitly choose to throw (or send to clients via a callback, for example). These must return status code 403 if over HTTP.


ValueErrors are a result of your function returning an unexpected value based on FaaSlang type-safety mechanisms. These must return status code 502 if over HTTP.

ValueError looks like an invalid ParameterError, where the details Object only ever contains a single key called "returns". These are encountered due to implementation issues on the part of the function developer.

  "error": {
    "type": "ValueError",
    "message": "ValueError",
    "details": {
      "returns": {
        "message": "((descriptive message stating return value is invalid))",
        "invalid": true,
        "expected": {
          "type": "boolean"
        "actual": {
          "type": "number",
          "value": 2017

FaaSlang Server and Gateway: Implementation

A fully-compliant FaaSlang gateway (that just uses local function resources) is available with this package, simply clone it and run npm test or look at the /tests folder for more information.

The current FaaSlang specification is used in production by the FaaS provider StdLib, and is available for local use with the StdLib CLI Package which relies on this repository as a dependency.


The software contained within this repository has been developed and is copyrighted by the StdLib Team at Polybit Inc. and is MIT licensed. The specification itself is not intended to be owned by a specific corporate entity, and has been developed in conjunction with other developers and organizations.