Azure Event Hubs output binding for Azure Functions

This article explains how to work with Azure Event Hubs bindings for Azure Functions. Azure Functions supports trigger and output bindings for Event Hubs.

For information on setup and configuration details, see the overview.

Use the Event Hubs output binding to write events to an event stream. You must have send permission to an event hub to write events to it.

Make sure the required package references are in place before you try to implement an output binding.

Example

The following example shows a C# function that writes a message to an event hub, using the method return value as the output:

[FunctionName("EventHubOutput")]
[return: EventHub("outputEventHubMessage", Connection = "EventHubConnectionAppSetting")]
public static string Run([TimerTrigger("0 */5 * * * *")] TimerInfo myTimer, ILogger log)
{
    log.LogInformation($"C# Timer trigger function executed at: {DateTime.Now}");
    return $"{DateTime.Now}";
}

The following example shows how to use the IAsyncCollector interface to send a batch of messages. This scenario is common when you are processing messages coming from one Event Hub and sending the result to another Event Hub.

[FunctionName("EH2EH")]
public static async Task Run(
    [EventHubTrigger("source", Connection = "EventHubConnectionAppSetting")] EventData[] events,
    [EventHub("dest", Connection = "EventHubConnectionAppSetting")]IAsyncCollector<string> outputEvents,
    ILogger log)
{
    foreach (EventData eventData in events)
    {
        // do some processing:
        var myProcessedEvent = DoSomething(eventData);

        // then send the message
        await outputEvents.AddAsync(JsonConvert.SerializeObject(myProcessedEvent));
    }
}

The following example shows an event hub trigger binding in a function.json file and a function that uses the binding. The function writes an output message to an event hub.

The following example shows an Event Hubs binding data in the function.json file, which is different for version 1.x of the Functions runtime compared to later versions.

{
    "type": "eventHub",
    "name": "outputEventHubMessage",
    "eventHubName": "myeventhub",
    "connection": "MyEventHubSendAppSetting",
    "direction": "out"
}

Here's JavaScript code that sends a single message:

module.exports = function (context, myTimer) {
    var timeStamp = new Date().toISOString();
    context.log('Message created at: ', timeStamp);   
    context.bindings.outputEventHubMessage = "Message created at: " + timeStamp;
    context.done();
};

Here's JavaScript code that sends multiple messages:

module.exports = function(context) {
    var timeStamp = new Date().toISOString();
    var message = 'Message created at: ' + timeStamp;

    context.bindings.outputEventHubMessage = [];

    context.bindings.outputEventHubMessage.push("1 " + message);
    context.bindings.outputEventHubMessage.push("2 " + message);
    context.done();
};

Complete PowerShell examples are pending.

The following example shows an event hub trigger binding in a function.json file and a Python function that uses the binding. The function writes a message to an event hub.

The following examples show Event Hubs binding data in the function.json file.

{
    "type": "eventHub",
    "name": "$return",
    "eventHubName": "myeventhub",
    "connection": "MyEventHubSendAppSetting",
    "direction": "out"
}

Here's Python code that sends a single message:

import datetime
import logging
import azure.functions as func


def main(timer: func.TimerRequest) -> str:
    timestamp = datetime.datetime.utcnow()
    logging.info('Message created at: %s', timestamp)
    return 'Message created at: {}'.format(timestamp)

The following example shows a Java function that writes a message containing the current time to an Event Hub.

@FunctionName("sendTime")
@EventHubOutput(name = "event", eventHubName = "samples-workitems", connection = "AzureEventHubConnection")
public String sendTime(
   @TimerTrigger(name = "sendTimeTrigger", schedule = "0 */5 * * * *") String timerInfo)  {
     return LocalDateTime.now().toString();
 }

In the Java functions runtime library, use the @EventHubOutput annotation on parameters whose value would be published to Event Hub. The parameter should be of type OutputBinding<T> , where T is a POJO or any native Java type.

Attributes

Both in-process and isolated process C# libraries use attribute to configure the binding. C# script instead uses a function.json configuration file.

Use the EventHubAttribute to define an output binding to an event hub, which supports the following properties.

Parameters Description
EventHubName The name of the event hub. When the event hub name is also present in the connection string, that value overrides this property at runtime.
Connection The name of an app setting or setting collection that specifies how to connect to Event Hubs. To learn more, see Connections.

Annotations

In the Java functions runtime library, use the EventHubOutput annotation on parameters whose value would be published to Event Hub. The following settings are supported on the annotation:

Configuration

The following table explains the binding configuration properties that you set in the function.json file, which differs by runtime version.

function.json property Description
type Must be set to eventHub.
direction Must be set to out. This parameter is set automatically when you create the binding in the Azure portal.
name The variable name used in function code that represents the event.
eventHubName Functions 2.x and higher. The name of the event hub. When the event hub name is also present in the connection string, that value overrides this property at runtime.
connection The name of an app setting or setting collection that specifies how to connect to Event Hubs. To learn more, see Connections.

When you're developing locally, add your application settings in the local.settings.json file in the Values collection.

Usage

The parameter type supported by the Event Hubs output binding depends on the Functions runtime version, the extension package version, and the C# modality used.

In-process C# class library functions supports the following types:

This version of EventData drops support for the legacy Body type in favor of EventBody.

Send messages by using a method parameter such as out string paramName. To write multiple messages, you can use ICollector<string> or IAsyncCollector<string> in place of out string.

There are two options for outputting an Event Hub message from a function by using the EventHubOutput annotation:

  • Return value: By applying the annotation to the function itself, the return value of the function is persisted as an Event Hub message.

  • Imperative: To explicitly set the message value, apply the annotation to a specific parameter of the type OutputBinding<T>, where T is a POJO or any native Java type. With this configuration, passing a value to the setValue method persists the value as an Event Hub message.

Complete PowerShell examples are pending.

Access the output event by using context.bindings.<name> where <name> is the value specified in the name property of function.json.

There are two options for outputting an Event Hub message from a function:

  • Return value: Set the name property in function.json to $return. With this configuration, the function's return value is persisted as an Event Hub message.

  • Imperative: Pass a value to the set method of the parameter declared as an Out type. The value passed to set is persisted as an Event Hub message.

Connections

The connection property is a reference to environment configuration which specifies how the app should connect to Event Hubs. It may specify:

If the configured value is both an exact match for a single setting and a prefix match for other settings, the exact match is used.

Connection string

Obtain this connection string by clicking the Connection Information button for the namespace, not the event hub itself. The connection string must be for an Event Hubs namespace, not the event hub itself.

When used for triggers, the connection string must have at least "read" permissions to activate the function. When used for output bindings, the connection string must have "send" permissions to send messages to the event stream.

This connection string should be stored in an application setting with a name matching the value specified by the connection property of the binding configuration.

Identity-based connections

If you are using version 5.x or higher of the extension, instead of using a connection string with a secret, you can have the app use an Azure Active Directory identity. To do this, you would define settings under a common prefix which maps to the connection property in the trigger and binding configuration.

In this mode, the extension requires the following properties:

Note

The environment variable provided must currently be prefixed by AzureWebJobs to work in the Consumption plan. In Premium plans, this prefix is not required.

Property Environment variable template Description Example value
Fully Qualified Namespace AzureWebJobs<CONNECTION_NAME_PREFIX>__fullyQualifiedNamespace The fully qualified Event Hubs namespace. <event_hubs_namespace>.servicebus.windows.net

Additional properties may be set to customize the connection. See Common properties for identity-based connections.

Note

When using Azure App Configuration or Key Vault to provide settings for Managed Identity connections, setting names should use a valid key separator such as : or / in place of the __ to ensure names are resolved correctly.

For example, <CONNECTION_NAME_PREFIX>:fullyQualifiedNamespace.

When hosted in the Azure Functions service, identity-based connections use a managed identity. The system-assigned identity is used by default, although a user-assigned identity can be specified with the credential and clientID properties. Note that configuring a user-assigned identity with a resource ID is not supported. When run in other contexts, such as local development, your developer identity is used instead, although this can be customized. See Local development with identity-based connections.

Grant permission to the identity

Whatever identity is being used must have permissions to perform the intended actions. You will need to assign a role in Azure RBAC, using either built-in or custom roles which provide those permissions.

Important

Some permissions might be exposed by the target service that are not necessary for all contexts. Where possible, adhere to the principle of least privilege, granting the identity only required privileges. For example, if the app only needs to be able to read from a data source, use a role that only has permission to read. It would be inappropriate to assign a role that also allows writing to that service, as this would be excessive permission for a read operation. Similarly, you would want to ensure the role assignment is scoped only over the resources that need to be read.

You will need to create a role assignment that provides access to your event hub at runtime. The scope of the role assignment can be for an Event Hubs namespace, or the event hub itself. Management roles like Owner are not sufficient. The following table shows built-in roles that are recommended when using the Event Hubs extension in normal operation. Your application may require additional permissions based on the code you write.

Binding type Example built-in roles
Trigger Azure Event Hubs Data Receiver, Azure Event Hubs Data Owner
Output binding Azure Event Hubs Data Sender

Exceptions and return codes

Binding Reference
Event Hub Operations Guide

Next steps