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Design

This solution uses AWS components (lambda and SQS) to make the retry mechanism fault-tolerant. Localstack is used to simulate AWS locally.

The solution is made up of:

  • express app to handle requests - the OpenAPI schema can be found by querying http://localhost:8000/spec
  • request handler SQS queue and lambda for collecting the data from the providers. Retries are handled using an exponential backoff algorithm.
  • response dispatcher SQS queue and lambda for sending the data back to the provided callbackUrl. Retries are handled in the same way as above.

Prerequisites

Requires

  1. docker

  2. localstack (installed by docker-compose)

  3. terraform

choco install terraform awscli

Ensure docker is running

Install

npm run installAll

Test

npm run testAll

Run

docker-compose up to start localstack

npm run deployTf to deploy SQS and Lambda terraform to localstack

npm run startMockApi to start the mock providers endpoint app

npm run startApp to start the app endpoint

Example queries

NB: I made use of webhook.site for testing the callback

healthcheck: GET http://localhost:8000

request data: POST http://localhost:8000/?provider=gas&callbackUrl=https://webhook.site/e9f6dbce-5b3e-468a-ba7f-de7e4a6caa68

Potential improvements

  • Extract code for sending messages to queues, and any other shared code, into a shared library.
  • Allow multiple providers to be requested, and aggregate the data. This can be done by having the request handler lambda retrieve data from each provider and aggregate them to pass to the response dispatcher. If any providers are unavailable, the whole message is added back to the retry queue to try again later.
  • Replace the app endpoint with API Gateway