ASP .NET Rest API Repeated Calls to another API

Question

How can I make repetitive calls to a URL by time intervals till I get successful result or timeout?

I upload a file to an API and it sends me a URL to let me check if my file is processed successfully. I want to make it so this url is checked in the server until the pending status is changed. There are successful, failed and pending statuses. I want to keep the user wait until the result is either fail or success.

[HttpGet]
public async Task<ActionResult> Get()
{
    ...
    response = await client.GetAsync(other_api_url);

    if(response.IsSuccessStatusCode)
    {
        string result = response.Content.ReadAsStringAsync().Result;
        dynamic output = JsonConvert.DeserializeObject<dynamic>(result);

        statusUrl = output.url_to_check_status;

        //make a call to statusUrl check if content is ready
        //get result and parse it
        ...

        status = output.status;

        if(status == "SUCCESS")
        {
            //good path
            return Ok();
        }
        else
        {
            //make another call after n seconds to check again
        }
    }

    return NotFound();
}

call the same function recursively until the successful response — Hary
– Hary, Commented Oct 5, 2018 at 13:15
Processing might take up to 10 seconds, so that is not a good idea. — Sarge
– Sarge, Commented Oct 5, 2018 at 13:18
repetitive calls to a URL till I get successful result or timeout?? - But this is what you're asking?? — Hary
– Hary, Commented Oct 5, 2018 at 13:19
One approach would be to handle it outside of the code you have posted here. Put the behaviour to handle the pending status code by retrying periodically into a DelegatingHandler. You can add that to the message handling "stack" at the point where you create the HttpClient instance. — Dave Cluderay
– Dave Cluderay, Commented Oct 5, 2018 at 13:40

John Wu · Accepted Answer · 2018-10-05 18:34:07Z

2

You can use a loop with Task.Delay, like this:

bool done = false;
while (!done)
{
    await Task.Delay(1000);
    done = await GetStatusFromServerAsync();
}

Although you might want a timeout:

async Task<bool> CheckForCompletion(int timeoutms)
{
    var timer = StopWatch.StartNew();
    while (timer.ElapsedMilliseconds < timeoutms)
    {
        var ok = await GetStatusFromServerAsync();
        if (ok) return true;
        await Task.Yield(1000);
    }
    return false;
}

edited Oct 5, 2018 at 18:34

answered Oct 5, 2018 at 18:21

John Wu

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Comments

Anderson Matos · Accepted Answer · 2018-10-05 17:57:02Z

2

Not that I'd agree to doing this (you should never block your request this way), but I think this should solve your problem:

[HttpGet]
public async Task<ActionResult> Get()
{
    response = await client.GetAsync(other_api_url);

    if(response.IsSuccessStatusCode)
    {
        // DO NOT USE `.Result` within async method.
        string result = await response.Content.ReadAsStringAsync();
        dynamic output = JsonConvert.DeserializeObject<dynamic>(result);

        statusUrl = output.url_to_check_status;

        bool? result = null;
        while(result == null) result = await CheckIfSuccessfulAsync(statusUrl);

        if (result) return Ok();       
        return NotFound();
    }
}

// Does not *need* to be a separate method, it's just for better readability...
private async Task<bool?> CheckIfSuccessfulAsync(string statusUrl)
{    
    //make a call to statusUrl check if content is ready
    //get result and parse it
    ...

    status = output.status;

    if (status == "SUCCESS") return true;
    else if (status == "PENDING") return false;

    return null;
}

answered Oct 5, 2018 at 17:57

Anderson Matos

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1 Comment

Sarge Over a year ago

Thank you for the response, this partially does the job but I get authentication error even though I provide the same credentials for the first call. works on postman not in asp.net

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