go - Respond to HTTP request while processing in the background

Question

I have an API that receives a CSV file to process. I'd like to be able to send back an 202 Accepted (or any status really) while processing the file in the background. I have a handler that checks the request, writes the success header, and then continues processing via a producer/consumer pattern. The problem is that, due to the WaitGroup.Wait() calls, the accepted header isn't sending back. The errors on the handler validation are sending back correctly but that's because of the return statements.

Is it possible to send that 202 Accepted back with the wait groups as I'm hoping (and if so, what am I missing)?

func SomeHandler(w http.ResponseWriter, req *http.Request) {
    endAccepted := time.Now()
    err := verifyRequest(req)
    if err != nil {
        w.WriteHeader(http.StatusBadRequest)
        data := JSONErrors{Errors: []string{err.Error()}}
        json.NewEncoder(w).Encode(data)
        return
    }

    // ...FILE RETRIEVAL CLIPPED (not relevant)...
    // e.g. csvFile, openErr := os.Open(tmpFile.Name())

    //////////////////////////////////////////////////////
    // TODO this isn't sending due to the WaitGroup.Wait()s below
    w.WriteHeader(http.StatusAccepted)
    //////////////////////////////////////////////////////

    // START PRODUCER/CONSUMER
    jobs := make(chan *Job, 100)    // buffered channel
    results := make(chan *Job, 100) // buffered channel

    // start consumers
    for i := 0; i < 5; i++ { // 5 consumers
        wg.Add(1)
        go consume(i, jobs, results)
    }
    // start producing
    go produce(jobs, csvFile)

    // start processing
    wg2.Add(1)
    go process(results)

    wg.Wait() // wait for all workers to finish processing jobs

    close(results)

    wg2.Wait() // wait for process to finish

    log.Println("===> Done Processing.")
}

question from:https://stackoverflow.com/questions/65892477/respond-to-http-request-while-processing-in-the-background

Answer 1

You're doing all the processing in the background, but you're still waiting for it to finish. The solution would be to just not wait. The best solution would move all of the handling elsewhere to a function you can just call with go to run it in the background, but the simplest solution leaving it inline would just be

w.WriteHeader(http.StatusAccepted)
go func() {
    // START PRODUCER/CONSUMER
    jobs := make(chan *Job, 100)    // buffered channel
    results := make(chan *Job, 100) // buffered channel

    // start consumers
    for i := 0; i < 5; i++ { // 5 consumers
        wg.Add(1)
        go consume(i, jobs, results)
    }
    // start producing
    go produce(jobs, csvFile)

    // start processing
    wg2.Add(1)
    go process(results)

    wg.Wait() // wait for all workers to finish processing jobs

    close(results)

    wg2.Wait() // wait for process to finish

    log.Println("===> Done Processing.")
}()

Note that you elided the CSV file handling, so you'll need to ensure that it's safe to use this way (i.e. that you haven't defered closing or deleting the file, which would cause that to occur as soon as the handler returns).