Handling external events in Durable Functions (Azure Functions)

Orchestrator functions have the ability to wait and listen for external events. This feature of Durable Functions is often useful for handling human interaction or other external triggers.

Note

External events are one-way asynchronous operations. They are not suitable for situations where the client sending the event needs a synchronous response from the orchestrator function.

Wait for events

The WaitForExternalEvent method allows an orchestrator function to asynchronously wait and listen for an external event. The listening orchestrator function declares the name of the event and the shape of the data it expects to receive.

C#

[FunctionName("BudgetApproval")]
public static async Task Run(
    [OrchestrationTrigger] DurableOrchestrationContext context)
{
    bool approved = await context.WaitForExternalEvent<bool>("Approval");
    if (approved)
    {
        // approval granted - do the approved action
    }
    else
    {
        // approval denied - send a notification
    }
}

JavaScript (Functions 2.x only)

const df = require("durable-functions");

module.exports = df.orchestrator(function*(context) {
    const approved = yield context.df.waitForExternalEvent("Approval");
    if (approved) {
        // approval granted - do the approved action
    } else {
        // approval denied - send a notification
    }
});

The preceding example listens for a specific single event and takes action when it's received.

You can listen for multiple events concurrently, like in the following example, which waits for one of three possible event notifications.

C#

[FunctionName("Select")]
public static async Task Run(
    [OrchestrationTrigger] DurableOrchestrationContext context)
{
    var event1 = context.WaitForExternalEvent<float>("Event1");
    var event2 = context.WaitForExternalEvent<bool>("Event2");
    var event3 = context.WaitForExternalEvent<int>("Event3");

    var winner = await Task.WhenAny(event1, event2, event3);
    if (winner == event1)
    {
        // ...
    }
    else if (winner == event2)
    {
        // ...
    }
    else if (winner == event3)
    {
        // ...
    }
}

JavaScript (Functions 2.x only)

const df = require("durable-functions");

module.exports = df.orchestrator(function*(context) {
    const event1 = context.df.waitForExternalEvent("Event1");
    const event2 = context.df.waitForExternalEvent("Event2");
    const event3 = context.df.waitForExternalEvent("Event3");

    const winner = yield context.df.Task.any([event1, event2, event3]);
    if (winner === event1) {
        // ...
    } else if (winner === event2) {
        // ...
    } else if (winner === event3) {
        // ...
    }
});

The previous example listens for any of multiple events. It's also possible to wait for all events.

C#

[FunctionName("NewBuildingPermit")]
public static async Task Run(
    [OrchestrationTrigger] DurableOrchestrationContext context)
{
    string applicationId = context.GetInput<string>();

    var gate1 = context.WaitForExternalEvent("CityPlanningApproval");
    var gate2 = context.WaitForExternalEvent("FireDeptApproval");
    var gate3 = context.WaitForExternalEvent("BuildingDeptApproval");

    // all three departments must grant approval before a permit can be issued
    await Task.WhenAll(gate1, gate2, gate3);

    await context.CallActivityAsync("IssueBuildingPermit", applicationId);
}

JavaScript (Functions 2.x only)

const df = require("durable-functions");

module.exports = df.orchestrator(function*(context) {
    const applicationId = context.df.getInput();

    const gate1 = context.df.waitForExternalEvent("CityPlanningApproval");
    const gate2 = context.df.waitForExternalEvent("FireDeptApproval");
    const gate3 = context.df.waitForExternalEvent("BuildingDeptApproval");

    // all three departments must grant approval before a permit can be issued
    yield context.df.Task.all([gate1, gate2, gate3]);

    yield context.df.callActivity("IssueBuildingPermit", applicationId);
});

WaitForExternalEvent waits indefinitely for some input. The function app can be safely unloaded while waiting. If and when an event arrives for this orchestration instance, it is awakened automatically and immediately processes the event.

Note

If your function app uses the Consumption Plan, no billing charges are incurred while an orchestrator function is awaiting a task from WaitForExternalEvent (.NET) or waitForExternalEvent (JavaScript), no matter how long it waits.

In .NET, if the event payload cannot be converted into the expected type T, an exception is thrown.

Send events

The RaiseEventAsync method of the DurableOrchestrationClient class sends the events that WaitForExternalEvent (.NET) or waitForExternalEvent (JavaScript) waits for. The RaiseEventAsync method takes eventName and eventData as parameters. The event data must be JSON-serializable.

Below is an example queue-triggered function that sends an "Approval" event to an orchestrator function instance. The orchestration instance ID comes from the body of the queue message.

C#

[FunctionName("ApprovalQueueProcessor")]
public static async Task Run(
    [QueueTrigger("approval-queue")] string instanceId,
    [OrchestrationClient] DurableOrchestrationClient client)
{
    await client.RaiseEventAsync(instanceId, "Approval", true);
}

JavaScript (Functions 2.x only)

const df = require("durable-functions");

module.exports = async function(context, instanceId) {
    const client = df.getClient(context);
    await client.raiseEvent(instanceId, "Approval", true);
};

Internally, RaiseEventAsync (.NET) or raiseEvent (JavaScript) enqueues a message that gets picked up by the waiting orchestrator function. If the instance is not waiting on the specified event name, the event message is added to an in-memory queue. If the orchestration instance later begins listening for that event name, it will check the queue for event messages.

Note

If there is no orchestration instance with the specified instance ID, the event message is discarded. For more information about this behavior, see the GitHub issue.

Warning

When developing locally in JavaScript, you will need to set the environment variable WEBSITE_HOSTNAME to localhost:<port>, ex. localhost:7071 to use methods on DurableOrchestrationClient. For more information about this requirement, see the GitHub issue.

Next steps