asynchronous and synchronous communications

• Can I do work while waiting?

  • yes -> asynchronous

• History

  • everythign was synchronous

synchronous IO

• the caller waits for a response from receiver

  • E.g. Asking someone a question in a meeting

• Caller sends a request and blocks Caller

  • must wait for request to finish processing until it continues

  • Caller cannot execute any code while waiting

  • caller/client and receiver/server are in sync

• When the request is made to fetch the data, the cpu is waiting for the hardware/network to do it's thing and return with the data.

• ie IO - files, http requests

• Example

  • Program asks OS to read from disk

  • Program main thread is taken off of the CPU

  • Read completes, program can resume execution

Asynchronous IO

• the response can come whenever. Caller and receiver can do anything meanwhile

• flow

  • Caller sends a request

  • Caller can work until it gets a response (cpu context switches)

  • Caller either:

    • Checks if the response is ready (epoll)

    • Receiver calls back when it's done (io_uring)

    • Spins up a new thread that blocks, while the main thread carries on processing

      • Thread finish reading and calls back main thread

  • Caller and receiver are not necessary in sync

Examples

• Asynchronous Programming (promises/futures)

• Asynchronous backend processing

  • flush to queue with job id, and some call (poll, push) checks the job id to see if complete

  • client sends work for backend to do, the client can still carry on processing, but it is still waiting for the backend to comeback with the completed work

    • from client perspective it is aysnc, but the backend is sync

• Asynchronous commits in postgres

  • To do with transactions (executing multiple actions) and the commit confirms all of them if they were all validated and no failures

• Asynchronous IO in Linux (epoll, io_uring)

• Asynchronous replication

• Asynchronous OS fsync (fs cache)

Clients

• in past, they were synchronous/blocking

• Now most of them are aysnchronous

Links

• https://www.fridayfeedback.com/p/asynchronous-communication/

• https://dzone.com/articles/microservices-why-asynchronous-communications

• https://dzone.com/articles/communicating-between-microservices

• https://danieltammadge.com/2021/01/event-driven-vs-request-driven-architecture-user-registration/