In a recent article, I showed how to create a Durable Azure Function. If you are unfamiliar with Durable Functions, I recommend you read that article first.

In that article, the Durable Function called 3 Activity Functions in sequence. No Function executed until the Function before it completed. Sometimes, it is important that Functions execute in a certain order. But sometimes it does not matter in which order a Function executes - only that they each complete successfully before another Activity Function is called. In these cases, executing sequentially is a waste of time. It is more efficient to execute these Azure Functions in parallel.

In this article, I will show how to create a durable function that executes three Activity Functions in parallel; then waits for all 3 to complete before executing a fourth function.
 
Fig. 1 illustrates this pattern.

Fig. 1
 
As we noted in the earlier article, a Durable function is triggered by a starter function, which is in turn triggered by an HTTP request, database change, timer, or any of the many triggers supported by Azure Functions, as shown in Fig. 2.

Fig. 2

I created 4 Activity Functions that do nothing more than write a couple messages to the log (I use LogWarning, because it causes the text to display in yellow, making it easier to find); delay a few seconds (to simulate a long-running task); and return a string consisting of the input string, concatenated with the name of the current function. The functions are nearly identical: Only the Function Name, the message, and the length of delay are different.

The 4 functions are shown below:

    public static class Function1
     {
         [FunctionName("Function1")]
         public static async Task<string> Run(
             [ActivityTrigger] string msg,
             ILogger log)
         {
             log.LogWarning("This is Function 1");
             await Task.Delay(15000);
             log.LogWarning("Function 1 completed");
             msg += "Function 1";
            return msg;
        }
    }
  

Listing 1

    public static class Function2 
    { 
        [FunctionName("Function2")] 
        public static async Task<string> Run( 
            [ActivityTrigger] string msg, 
            ILogger log) 
        { 
             log.LogWarning("This is Function 2"); 
            await Task.Delay(10000); 
            log.LogWarning("Function 2 completed"); 
            msg += "Function 2"; 
            return msg; 
        } 
    }
  

Listing 2

    public static class Function3 
     { 
        [FunctionName("Function3")] 
        public static async Task<string> Run( 
            [ActivityTrigger] string msg, 
            ILogger log) 
        { 
            log.LogWarning("This is Function 3"); 
            await Task.Delay(5000); 
             log.LogWarning("Function 3 completed"); 
            msg += "Function 3"; 
            return msg; 
        } 
    }
  

Listing 3

    public static class Function4 
    { 
        [FunctionName("Function4")] 
        public static async Task<string> Run( 
             [ActivityTrigger] string msg, 
            ILogger log) 
         { 
            log.LogWarning("This is Function 4"); 
             int secondsDelay = new Random().Next(8, 12); 
            await Task.Delay(1000); 
            log.LogInformation("Function 4 completed"); 
            msg += "\n\rFunction 4"; 
            return msg; 
        } 
    }
  

Listing 4

We use the Parallel Task library to launch the first 3 functions and have them run in parallel; then, wait until each of the first 3 complete before executing the 4th Activity Function.

Listing 5 shows this code in our Durable Orchestration function.

    public static class DurableFunction1 
    { 
        [FunctionName("DurableFunction1")] 
         public static async Task Run( 
            [OrchestrationTrigger] DurableOrchestrationContext ctx, 
            ILogger log) 
        { 
            var msg = "Durable Function: "; 
            var parallelTasks = new Liststring>>(); 
             Task<string> task1 = ctx.CallActivityAsync<string>("Function1", msg); 
            parallelTasks.Add(task1); 
            Task<string> task2 = ctx.CallActivityAsync<string>("Function2", msg); 
            parallelTasks.Add(task2); 
            Task<string> task3 = ctx.CallActivityAsync<string>("Function3", msg); 
             parallelTasks.Add(task3);

            await Task.WhenAll(parallelTasks);

            // All 3 Activity functions finished 
            msg = task1.Result + "\n\r" + task2.Result + "\n\r" + task3.Result;

            // Use LogWarning, so it shows up in Yellow, making it easier to spot 
            log.LogWarning($"All 3 Activity functions completed for orchestration {ctx.InstanceId}!");

            msg = await ctx.CallActivityAsync<string>("Function4", msg); 
            log.LogWarning(msg);

            return new OkObjectResult(msg); 
        } 
    }
  

Listing 5

We create a new List of Tasks and add each activity to that list:

var msg = "Durable Function: ";
var parallelTasks = new Liststring>>();
Task<string> task1 = ctx.CallActivityAsync<string>("Function1", msg);
parallelTasks.Add(task1);
Task<string> task2 = ctx.CallActivityAsync<string>("Function2", msg);
parallelTasks.Add(task2);
Task<string> task3 = ctx.CallActivityAsync<string>("Function3", msg);
parallelTasks.Add(task3);

The following line tells the system to wait until all 3 tasks in that list are completed.

await Task.WhenAll(parallelTasks);

When all 3 tasks complete, we resume the program flow, calling the 4th Activity and logging the output:

log.LogWarning($"All 3 Activity functions completed for orchestration {ctx.InstanceId}!");
msg = await ctx.CallActivityAsync<string>("Function4", msg);
log.LogWarning(msg);

As in the previous article, we launch this Durable Orchestration Function with a starter function (in this case a function with an HTTP trigger), as shown in Listing 6 below.

    public static class StarterFunction1 
    { 
        [FunctionName("StarterFunction1")] 
        public static async Task Run( 
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] 
            HttpRequestMessage req, 
            [OrchestrationClient] DurableOrchestrationClient starter, 
            TraceWriter log) 
        { 
             log.Info("About to start orchestration");

            var orchestrationId = await starter.StartNewAsync("DurableFunction1", log); 
            return starter.CreateCheckStatusResponse(req, orchestrationId); 
        } 
    }
  

Testing the Orchestration

We can test this orchestration by running the solution, which displays the HTTP Trigger URL, as shown in Fig. 3

Fig. 3

We can then open a browser, type the HTTP Trigger URL in the address bar, and press [ENTER] to trigger the function, as shown in Fig. 4

Fig. 4

Switch back to the function output to view the messages as they scroll past. You should see output from each of the first 3 functions (although not necessarily in the order called), followed by a message indicating the first 3 are complete; then output from Function 4. This is shown in Fig. 5.

Fig. 5

You can view this project under “Durable Functions” in this GitHub repository.

In this article, I showed how to create a Durable Orchestration Function that launches activity functions that run in parallel.