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Faithful simulation of AWS Cloud Services using node.js and leveldb, useful for testing interactions with AWS

Build Status


  • State Management - capture, save, and restore state for all APIs
  • API Call Hooks - callbacks before and after any API call
  • 100% node.js - no need for external dependencies
  • Performance - all services can be run in-process for better performance, debugging, and flexibility
  • Multi-API flows - test interaction across multiple APIs using multiple components


  • APIGateway (still under construction)
  • CognitoIdentity
  • CognitoIdentityServiceProvider
  • Kinesis
  • DynamoDB
  • Lambda (coming soon)
  • S3 (coming soon)
  • IAM (roles only)

Why SimDown?

Testing applciations that work with AWS API's can be difficult because there's no way to simulate them locally. Existing solutions like localstack, s3rver, and many others are awesome (!) but they have a few important shortcomings:

  • No ability to save/restore API state
  • No hooks before/after method calls
  • Missing important services
  • Services missing important features
  • Inconsistent implementation and testing
  • Multiple languages and binary dependencies

SimDown launches a series of http servers that simulate AWS services. The services can interact with each other to properly simulate Amazon's real system. In the background SimDown keeps track of AWS state using leveldb, which allows a lot of flexible storage options.


Command Line

SimDown has a simple CLI that runs simulations and manages API states. To learn more, run simdown help

Start simulaiton services

simdown start

This will startup all AWS simulators locally and print out their addresses eg:

Save current state

simdown save [yourStateName]

SimDown will stash all API states into a compressed binary file for efficient storage. If no name is provided a timestamp will be used.

Load a saved state

simdown load [yourStateName]

The current API state will be cleared and a previously saved state will be loaded.

List saved states

simdown list

Displays a list of all archived states.

Remove a saved state

simdown remove [yourStateName]

Deletes an archived state. Does not affect the current API state.

Programmatic Usage


Setup a SimDown server

  const SimDown = require('simdown')
  const simdown = new SimDown()
  simdown.setup((err, endpoints) => {
    console.log(endpoints['CognitoIdentity'])   // http://localhost:14353

Close down a running server

  simdown.stop(() => {
    console.log("Finished cleaning up")


To use SimDown, simply use the endpoint option of the AWS SDK (works from browser, node, python, Java, or any AWS SDK)

  const AWS = require('aws-sdk')
  const cognito = new AWS.CognitoIdentity({
    region: 'us-east-1',
    endpoint: 'http://localhost:14353'


Hooks allow you to specify functions to be called before/after a given API call. They'll be passed a copy of the incoming request for inspection/modification.

  function beforeTableCreate(callPath, data, done) {
    console.log("Before", callPath.join(':')) // outputs "Before DynamoDB:CreateTable"
  const simdown = new SimDown({
    hooks: {
      'DynamoDB:CreateTable:before': beforeTableCreate


Coverage for DynamoDB and Kinesis come from mhart's excellent simulators: