Human interaction in Durable Functions - Phone verification sample

This sample demonstrates how to build a Durable Functions orchestration that involves human interaction. Whenever a real person is involved in an automated process, the process must be able to send notifications to the person and receive responses asynchronously. It must also allow for the possibility that the person is unavailable. (This last part is where timeouts become important.)

This sample implements an SMS-based phone verification system. These types of flows are often used when verifying a customer's phone number or for multi-factor authentication (MFA). It is a powerful example because the entire implementation is done using a couple small functions. No external data store, such as a database, is required.


Scenario overview

Phone verification is used to verify that end users of your application are not spammers and that they are who they say they are. Multi-factor authentication is a common use case for protecting user accounts from hackers. The challenge with implementing your own phone verification is that it requires a stateful interaction with a human being. An end user is typically provided some code (for example, a 4-digit number) and must respond in a reasonable amount of time.

Ordinary Azure Functions are stateless (as are many other cloud endpoints on other platforms), so these types of interactions involve explicitly managing state externally in a database or some other persistent store. In addition, the interaction must be broken up into multiple functions that can be coordinated together. For example, you need at least one function for deciding on a code, persisting it somewhere, and sending it to the user's phone. Additionally, you need at least one other function to receive a response from the user and somehow map it back to the original function call in order to do the code validation. A timeout is also an important aspect to ensure security. It can get fairly complex quickly.

The complexity of this scenario is greatly reduced when you use Durable Functions. As you will see in this sample, an orchestrator function can manage the stateful interaction easily and without involving any external data stores. Because orchestrator functions are durable, these interactive flows are also highly reliable.

Configuring Twilio integration

This sample involves using the Twilio service to send SMS messages to a mobile phone. Azure Functions already has support for Twilio via the Twilio binding, and the sample uses that feature.

The first thing you need is a Twilio account. You can create one free at Once you have an account, add the following three app settings to your function app.

App setting name Value description
TwilioAccountSid The SID for your Twilio account
TwilioAuthToken The Auth token for your Twilio account
TwilioPhoneNumber The phone number associated with your Twilio account. This is used to send SMS messages.

The functions

This article walks through the following functions in the sample app:

  • E4_SmsPhoneVerification: An orchestrator function that performs the phone verification process, including managing timeouts and retries.
  • E4_SendSmsChallenge: An activity function that sends a code via text message.


The HttpStart function in the sample app and the quickstart acts as Orchestration client which triggers the orchestrator function.

E4_SmsPhoneVerification orchestrator function

public static async Task<bool> Run(
    [OrchestrationTrigger] IDurableOrchestrationContext context)
    string phoneNumber = context.GetInput<string>();
    if (string.IsNullOrEmpty(phoneNumber))
        throw new ArgumentNullException(
            "A phone number input is required.");

    int challengeCode = await context.CallActivityAsync<int>(

    using (var timeoutCts = new CancellationTokenSource())
        // The user has 90 seconds to respond with the code they received in the SMS message.
        DateTime expiration = context.CurrentUtcDateTime.AddSeconds(90);
        Task timeoutTask = context.CreateTimer(expiration, timeoutCts.Token);

        bool authorized = false;
        for (int retryCount = 0; retryCount <= 3; retryCount++)
            Task<int> challengeResponseTask =

            Task winner = await Task.WhenAny(challengeResponseTask, timeoutTask);
            if (winner == challengeResponseTask)
                // We got back a response! Compare it to the challenge code.
                if (challengeResponseTask.Result == challengeCode)
                    authorized = true;
                // Timeout expired

        if (!timeoutTask.IsCompleted)
            // All pending timers must be complete or canceled before the function exits.

        return authorized;


It may not be obvious at first, but this orchestrator does not violate the deterministic orchestration constraint. It is deterministic because the CurrentUtcDateTime property is used to calculate the timer expiration time, and it returns the same value on every replay at this point in the orchestrator code. This behavior is important to ensure that the same winner results from every repeated call to Task.WhenAny.

Once started, this orchestrator function does the following:

  1. Gets a phone number to which it will send the SMS notification.
  2. Calls E4_SendSmsChallenge to send an SMS message to the user and returns back the expected 4-digit challenge code.
  3. Creates a durable timer that triggers 90 seconds from the current time.
  4. In parallel with the timer, waits for an SmsChallengeResponse event from the user.

The user receives an SMS message with a four-digit code. They have 90 seconds to send that same four-digit code back to the orchestrator function instance to complete the verification process. If they submit the wrong code, they get an additional three tries to get it right (within the same 90-second window).


It's important to cancel timers if you no longer need them to expire, as in the example above when a challenge response is accepted.

E4_SendSmsChallenge activity function

The E4_SendSmsChallenge function uses the Twilio binding to send the SMS message with the four-digit code to the end user.

public static int SendSmsChallenge(
    [ActivityTrigger] string phoneNumber,
    ILogger log,
    [TwilioSms(AccountSidSetting = "TwilioAccountSid", AuthTokenSetting = "TwilioAuthToken", From = "%TwilioPhoneNumber%")]
        out CreateMessageOptions message)
    // Get a random number generator with a random seed (not time-based)
    var rand = new Random(Guid.NewGuid().GetHashCode());
    int challengeCode = rand.Next(10000);

    log.LogInformation($"Sending verification code {challengeCode} to {phoneNumber}.");

    message = new CreateMessageOptions(new PhoneNumber(phoneNumber));
    message.Body = $"Your verification code is {challengeCode:0000}";

    return challengeCode;


You will need to install the Microsoft.Azure.WebJobs.Extensions.Twilio Nuget package to run the sample code.

Run the sample

Using the HTTP-triggered functions included in the sample, you can start the orchestration by sending the following HTTP POST request:

POST http://{host}/orchestrators/E4_SmsPhoneVerification
Content-Length: 14
Content-Type: application/json

HTTP/1.1 202 Accepted
Content-Length: 695
Content-Type: application/json; charset=utf-8
Location: http://{host}/runtime/webhooks/durabletask/instances/741c65651d4c40cea29acdd5bb47baf1?taskHub=DurableFunctionsHub&connection=Storage&code={systemKey}


The orchestrator function receives the supplied phone number and immediately sends it an SMS message with a randomly generated 4-digit verification code — for example, 2168. The function then waits 90 seconds for a response.

To reply with the code, you can use RaiseEventAsync (.NET) or raiseEvent (JavaScript) inside another function or invoke the sendEventUrl HTTP POST webhook referenced in the 202 response above, replacing {eventName} with the name of the event, SmsChallengeResponse:

POST http://{host}/runtime/webhooks/durabletask/instances/741c65651d4c40cea29acdd5bb47baf1/raiseEvent/SmsChallengeResponse?taskHub=DurableFunctionsHub&connection=Storage&code={systemKey}
Content-Length: 4
Content-Type: application/json


If you send this before the timer expires, the orchestration completes and the output field is set to true, indicating a successful verification.

GET http://{host}/runtime/webhooks/durabletask/instances/741c65651d4c40cea29acdd5bb47baf1?taskHub=DurableFunctionsHub&connection=Storage&code={systemKey}
HTTP/1.1 200 OK
Content-Length: 144
Content-Type: application/json; charset=utf-8


If you let the timer expire, or if you enter the wrong code four times, you can query for the status and see a false orchestration function output, indicating that phone verification failed.

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 145


Next steps

This sample has demonstrated some of the advanced capabilities of Durable Functions, notably WaitForExternalEvent and CreateTimer APIs. You've seen how these can be combined with Task.WaitAny to implement a reliable timeout system, which is often useful for interacting with real people. You can learn more about how to use Durable Functions by reading a series of articles that offer in-depth coverage of specific topics.