Average Number Service

Description

Lightweight express app which calls a configured endpoint periodically and calculates a cumulative average of random numbers.

Behaviours and assumptions

• The configured handler assumes a response type as detailed here https://csrng.net/documentation/csrng-lite/.

• If any call to the endpoint returns an error, the application exits

  • This is to ensure any application runner/container can restart if required

• The endpoint is subject to rate limiting, but in this use case it is not predicted to occur, as such any error of this nature is subject to the above.

  • If requirements favoured uptime instead of calling every second, then a retry mechanism with backoff would be introduced to adhere to the rate limit.

• The latest number retrieved from the service is stored in memory, but instead of storing all collected numbers in memory the application calculates a cumulative average to avoid memory bloat over time.

• Using a recursive timeout to call the endpoint every second is a trade off:

  • It is simple to implement and understand
  • Can be tested with jest's fake timers
  • It is cancellable (i.e on exit); but
  • It relies on the event loop so may not actually be run every second
  • It may still call endpoint after it's been cancelled.

Future Improvements

• This application will handle a single average counter per process, if it were to scale (e.g consume many average numbers per second) then either:

  • Use of worker threads to parallelise, with limited scaling.
  • Caching with a persistence layer would be required

• Indeed, more expensive/latent processing would need to be asynchronous in nature, perhaps using an event-driven architecture

  • Care would need to be taken to keep the main thread free to repsond to requests

• Although the application would be suitable for a container based deployment model, any further configuration may need to be accompanied by something more dynamic than environment variables

Run

To start the service, run

npm run start

Testing

There are unit tests to cover the functionality and an integration test to verify calling the express server.

To run

npm run test