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0.14.5 • Public • Published

AppSync Emulator (Serverless)

This module provides emulation and testing helpers for use with AWS AppSync. Currently this depends on using and following the conventions.

It is possible to use this emulator without serverless by mirroring the structure defined there. In the future we will provide other methods of configuring the emulator.

AppSync Features

We aim to support the majority of appsync features (as we use all of them except elastic search).

  • Lambda source (only tested with serverless functions, including Node, Python)
  • DynamoDB source (batch operations, all single table operations, etc.)
  • HTTP(S) source
  • NONE source
  • Full VTL support ($util) and compatibility with Java stdlib
  • Support for API_KEY and AMAZON_COGNITO_USER_POOLS authentication
  • Subscriptions

Known Deviations from AppSync

This sections lists known deviations from AppSync behaviour

  • VTL String.split: AppSync Emulator is using JavaScript's regex engine which does not support look-behind. There might be other more subtle differences in the regex implementations.


  • Java*

*If installing DynamoDB Local.


DynamoDB Local is an optional dependency and installed by default. If you would rather provide your own DynamoDB server, you can instruct npm/yarn not to install optional dependencies.

npm i @conduitvc/appsync-emulator-serverless [--no-optional]


yarn add @conduitvc/appsync-emulator-serverless [--ignore-optional]


If using the DynamoDB emulator, data is preserved between emulator runs and is stored in .dynamodb in the same directory that package.json would be in.

As a CLI

# NOTE unless you assign a specific port a random one will be chosen. 
yarn appsync-emulator --port 62222

dynamodb with fixed port

optional start dynamodb at a fixed port - e.g. 8000

# NOTE unless you assign a specific port for dynamodb a random one will be chosen. 
yarn appsync-emulator --port 62222 --dynamodb-port 8000

to access the dynamodb instance using javascript you need to use the following configuration:

const { DynamoDB } = require('aws-sdk');
const dynamodb = new DynamoDB({
  endpoint: 'http://localhost:8000',
  region: 'us-fake-1',
  accessKeyId: 'fake',
  secretAccessKey: 'fake',
const client = new DynamoDB.DocumentClient({ service: dynamodb });

configurable dynamodb

The emulator now supports a configurable dynamodb setup. By default, the emulator uses the local dynamodb emulator. To specify an alternate config, create an appSyncConfig.js file which exports your specific configuration.

You may also specify a different file name or path. Path is relative to the location of the serverless.yml config file path.

yarn appsync-emulator --config myConfigFile
// default config
module.exports = {
  DynamoDB: {
    emulator: true,

// config without dynamodb
module.exports = {
  DynamoDB: false,

// config pointing to third party dynamodb
module.exports = {
  DynamoDB: {
    endpoint: 'http://localhost:61023',
    accessKeyId: 'fake',
    secretAccessKey: 'fake',
    region: 'fake',

Custom build prefix for webpack, typescript

For compatibility with plugins such as Serverless Webpack that allow the usage of webpack you will need to add the following configuration to your project's serverless.yml file.

    buildPrefix: $PREFIX_LOCATION

Where $PREFIX_LOCATION is your specified webpack build path i.e. .webpack

Serverless configuration notes

AppSync emulator does support Serverless configurations with multiple endpoints. Like the sample below.

    - name: 'api 1'
    - name: 'api 2'

However: It does not start multiple endpoints, it will only use the first endpoint.



We extensively use jest so bundle a jest specific helper (which likely will work for mocha as well).

const gql = require('graphql-tag');
const { AWSAppSyncClient } = require('aws-appsync');
// we export a specific module for testing.
const createAppSync = require('@conduitvc/appsync-emulator-serverless/jest');
// required by apollo-client
global.fetch = require('node-fetch');
describe('graphql', () => {
  const appsync = createAppSync();
  it('Type.resolver', async () => {
    await appsync.client.query({
      query: gql`


(Below example is jest but any framework will work)

const gql = require('graphql-tag');
const { AWSAppSyncClient } = require('aws-appsync');
// we export a specific module for testing.
const {
} = require('@conduitvc/appsync-emulator-serverless/tester');
// required by apollo-client
global.fetch = require('node-fetch');
describe('graphql', () => {
  let server, client;
  beforeEach(async () => {
    // by default, ths create method will spin up a dynamodb emulator in memory using java
    // to utilize another dynamodb instance instead, pass in a valid dynamodbConfig to create:
    /* below works with localstack
        dynamodbConfig: {
          endpoint: 'http://localhost:61023',
          accessKeyId: 'fake',
          secretAccessKey: 'fake',
          region: 'fake',
    server = await create();
    client = connect(server, AWSAppSyncClient);
  // important to clear state.
  afterEach(async () => server.close());
  // very important not to leave java processes lying around.
  afterAll(async () => server.terminate());
  it('Type.resolver', async () => {
    await client.query({
      query: gql`

Python Lambda

Add the following description for a Python Lambda (the runtime flag is important). In the following example, it will look for a file named in the current directory with a method called composedJSON.

    handler: handler.composedJSON
    runtime: python3.6

Running a Python Lambda requires sls to be on your environment $PATH.

Lambda <> DynamoDB

If you need your lambda functions to interact with the local dynamoDB emulator:

const { DynamoDB } = require('aws-sdk');
const dynamodb = new DynamoDB({
  endpoint: process.env.DYNAMODB_ENDPOINT,
  region: 'us-fake-1',
  accessKeyId: 'fake',
  secretAccessKey: 'fake',
const client = new DynamoDB.DocumentClient({ service: dynamodb });
module.exports.myFn = async (event, context, callback) => {
  const TableName = process.env[`DYNAMODB_TABLE_${YOURTABLENAME}`];
  const { id } = event.arguments;
  const dynamoResult = await client
      Key: { id },
  return dynamoResult;


If you would like to simulate IAM authentication locally, checkout the testJWT module See the tester module for sample usage.




npm i @conduitvc/appsync-emulator-serverless

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