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1.5.2 • Public • Published

Middy ssm middleware

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SSM (AWS System Manager Parameter) middleware for the middy framework, the stylish Node.js middleware engine for AWS Lambda

This middleware fetches parameters from AWS Systems Manager Parameter Store.

Parameters to fetch can be defined by path and by name (not mutually exclusive). See AWS docs here.

By default parameters are assigned to the Node.js process.env object. They can instead be assigned to the function handler's context object by setting the setToContext flag to true. By default all parameters are added with uppercase names.

The Middleware makes a single API request to fetch all the parameters defined by name, but must make an additional request per specified path. This is because the AWS SDK currently doesn't expose a method to retrieve parameters from multiple paths.

For each parameter defined by name, you also provide the name under which its value should be added to process.env or context. For each path, you instead provide a prefix, and by default the value from each parameter returned from that path will be added to process.env or context with a name equal to what's left of the parameter's full name after the defined path, with the prefix prepended. If the prefix is an empty string, nothing is prepended. You can override this behaviour by providing your own mapping function with the getParamNameFromPath config option.


To install this middleware you can use NPM:

npm install --save @middy/ssm


  • cache (boolean) (optional): Defaults to false. Set it to true to skip further calls to AWS SSM
  • paths (object) (optional*): Map of SSM paths to fetch parameters from, where the key is the prefix for the destination name, and value is the SSM path. Example: {paths: {DB_: '/dev/service/db'}}
  • cacheExpiryInMillis (int) (optional): Defaults to undefined. Use this option to invalidate cached parameter values from SSM
  • names (object) (optional*): Map of parameters to fetch from SSM, where the key is the destination, and value is param name in SSM. Example: {names: {DB_URL: '/dev/service/db_url'}}
  • onChange (function) (optional): Callback triggered when call was made to SSM. Useful when you need to regenerate something with different data. Example: { onChange: () => { console.log('New data available')} }
  • awsSdkOptions (object) (optional): Options to pass to AWS.SSM class constructor.
  • awsSdkInstance (object) (optional): an alternative instance of AWS.SSM to use (e.g. that has been instrumented with AWS XRay) Defaults to { maxRetries: 6, retryDelayOptions: {base: 200} }
  • setToContext (boolean) (optional): This will assign parameters to the context object of the function handler rather than to process.env. Defaults to false
  • throwOnFailedCall (boolean) (optional): Defaults to false. Set it to true if you want your lambda to fail in case call to AWS SSM fails (parameters don't exist or internal error). It will only print error if parameters are not already cached.


  • While you don't need both paths and names, you do need at least one of them!
  • Lambda is required to have IAM permissions for ssm:GetParameters* actions
  • aws-sdk version of 2.176.0 or greater is required. If your project doesn't currently use aws-sdk, you may need to install it as a devDependency in order to run tests

Sample usage

const middy = require('@middy/core')
const ssm = require('@middy/ssm')

const handler = middy((event, context, cb) => {
  cb(null, {})

  cache: true,
  paths: {
    SOME_PREFIX_: '/dev/db'
  names: {
    SOME_ACCESS_TOKEN: '/dev/service_name/access_token'

// Before running the function handler, the middleware will fetch SSM params
handler(event, context, (_, response) => {
  expect(process.env.SOME_PREFIX_CONNECTION_STRING).toEqual('some-connection-string') // The '/dev/db' path contains the CONNECTION_STRING parameter

Export parameters to context object, override AWS region.

const middy = require('middy')
const { ssm } = require('middy/middlewares')

const handler = middy((event, context, cb) => {
  cb(null, {})

    cache: true,
    names: {
      SOME_ACCESS_TOKEN: '/dev/service_name/access_token'
    awsSdkOptions: { region: 'us-west-1' },
    setToContext: true

handler(event, context, (_, response) => {

Middy documentation and examples

For more documentation and examples, refers to the main Middy monorepo on GitHub or Middy official website.


Everyone is very welcome to contribute to this repository. Feel free to raise issues or to submit Pull Requests.


Licensed under MIT License. Copyright (c) 2017-2018 Luciano Mammino and the Middy team.

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