Skip to main content


Fetches S3 stored configuration and parses out JSON.


To install this middleware you can use NPM:

npm install --save @middy/s3
npm install --save-dev @aws-sdk/client-s3


  • AwsClient (object) (default S3Client): S3Client class constructor (i.e. that has been instrumented with AWS XRay). Must be from @aws-sdk/client-appconfig.
  • awsClientOptions (object) (default undefined): Options to pass to S3Client class constructor.
  • awsClientAssumeRole (string) (default undefined): Internal key where secrets are stored. See @middy/sts on to set this.
  • awsClientCapture (function) (default undefined): Enable XRay by passing captureAWSv3Client from aws-xray-sdk in.
  • fetchData (object) (required): Mapping of internal key name to API request parameters.
  • disablePrefetch (boolean) (default false): On cold start requests will trigger early if they can. Setting awsClientAssumeRole disables prefetch.
  • cacheKey (string) (default s3): Cache key for the fetched data responses. Must be unique across all middleware.
  • cacheExpiry (number) (default -1): How long fetch data responses should be cached for. -1: cache forever, 0: never cache, n: cache for n ms.
  • setToContext (boolean) (default false): Store credentials to request.context.


  • Lambda is required to have IAM permission for s3:GetObject
  • If the file is stored without ContentType, you can set it on the response using ResponseContentType as part of the input

Sample usage

import middy from '@middy/core'
import appConfig from '@middy/s3'

const handler = middy((event, context) => {
const response = {
statusCode: 200,
headers: {},
body: JSON.stringify({ message: 'hello world' })

return response

fetchData: {
config: {
Bucket: '...',
Key: '...'


To exclude @aws-sdk add @aws-sdk/client-s3 to the exclude list.