Work with legacy proxies

Article
11/20/2022

Important

Azure Functions proxies is a legacy feature for versions 1.x through 3.x of the Azure Functions runtime. Support for proxies can be re-enabled in version 4.x for you to successfully upgrade your function apps to the latest runtime version. As soon as possible, you should switch to integrating your function apps with Azure API Management. API Management lets you take advantage of a more complete set of features for defining, securing, managing, and monetizing your Functions-based APIs. For more information, see API Management integration.

To learn how to re-enable proxies support in Functions version 4.x, see Re-enable proxies in Functions v4.x.

To help make it easier to migrate from existing proxy implementations, this article links to equivalent API Management content, when available.

This article explains how to configure and work with Azure Functions Proxies. With this feature, you can specify endpoints on your function app that are implemented by another resource. You can use these proxies to break a large API into multiple function apps (as in a microservice architecture), while still presenting a single API surface for clients.

Standard Functions billing applies to proxy executions. For more information, see Azure Functions pricing.

Re-enable proxies in Functions v4.x

After migrating your function app to version 4.x of the Functions runtime, you'll need to specifically reenable proxies. You should still switch to integrating your function apps with Azure API Management as soon as possible, and not just rely on proxies.

Re-enabling proxies requires you to set a flag in the AzureWebJobsFeatureFlags application setting in one of the following ways:

If the AzureWebJobsFeatureFlags setting doesn't already exists, add this setting to your function app with a value of EnableProxies.
If this setting already exists, add ,EnableProxies to the end of the existing value.

AzureWebJobsFeatureFlags is a comma-delimited array of flags used to enable preview and other temporary features. To learn more about how to create and modify application settings, see Work with application settings.

Note

Even when re-enabled using the EnableProxies flag, you can't work with proxies in the Azure portal. Instead, you must work directly with the proxies.json file for your function app. For more information, see Advanced configuration.

Create a proxy

Important

For equivalent content using API Management, see Expose serverless APIs from HTTP endpoints using Azure API Management.

Proxies are defined in the proxies.json file in the root of your function app. The steps in this section show you how to use the Azure portal to create this file in your function app. Not all languages and operating system combinations support in-portal editing. If you can't modify your function app files in the portal, you can instead create and deploy the equivalent proxies.json file from the root of your local project folder. To learn more about portal editing support, see Language support details.

Open the Azure portal, and then go to your function app.
In the left pane, select Proxies and then select +Add.
Provide a name for your proxy.
Configure the endpoint that's exposed on this function app by specifying the route template and HTTP methods. These parameters behave according to the rules for HTTP triggers.
Set the backend URL to another endpoint. This endpoint could be a function in another function app, or it could be any other API. The value doesn't need to be static, and it can reference application settings and parameters from the original client request.
Select Create.

Your proxy now exists as a new endpoint on your function app. From a client perspective, it's the same as an HttpTrigger in Functions. You can try out your new proxy by copying the Proxy URL and testing it with your favorite HTTP client.

Modify requests and responses

Important

API Management lets you can change API behavior through configuration using policies. Policies are a collection of statements that are run sequentially on the request or response of an API. For more information about API Management policies, see Policies in Azure API Management.

With proxies, you can modify requests to and responses from the back-end. These transformations can use variables as defined in Use variables.

Modify the back-end request

By default, the back-end request is initialized as a copy of the original request. In addition to setting the back-end URL, you can make changes to the HTTP method, headers, and query string parameters. The modified values can reference application settings and parameters from the original client request.

Back-end requests can be modified in the portal by expanding the request override section of the proxy detail page.

Modify the response

By default, the client response is initialized as a copy of the back-end response. You can make changes to the response's status code, reason phrase, headers, and body. The modified values can reference application settings, parameters from the original client request, and parameters from the back-end response.

Back-end responses can be modified in the portal by expanding the response override section of the proxy detail page.

Use variables

The configuration for a proxy doesn't need to be static. You can condition it to use variables from the original client request, the back-end response, or application settings.

Reference local functions

You can use localhost to reference a function inside the same function app directly, without a roundtrip proxy request.

"backendUri": "https://localhost/api/httptriggerC#1" will reference a local HTTP triggered function at the route /api/httptriggerC#1

Note

If your function uses function, admin or sys authorization levels, you will need to provide the code and clientId, as per the original function URL. In this case the reference would look like: "backendUri": "https://localhost/api/httptriggerC#1?code=<keyvalue>&clientId=<keyname>" We recommend storing these keys in application settings and referencing those in your proxies. This avoids storing secrets in your source code.

Reference request parameters

You can use request parameters as inputs to the back-end URL property or as part of modifying requests and responses. Some parameters can be bound from the route template that's specified in the base proxy configuration, and others can come from properties of the incoming request.

Route template parameters

Parameters that are used in the route template are available to be referenced by name. The parameter names are enclosed in braces ({}).

For example, if a proxy has a route template, such as /pets/{petId}, the back-end URL can include the value of {petId}, as in https://<AnotherApp>.azurewebsites.net/api/pets/{petId}. If the route template terminates in a wildcard, such as /api/{*restOfPath}, the value {restOfPath} is a string representation of the remaining path segments from the incoming request.

Additional request parameters

In addition to the route template parameters, the following values can be used in config values:

{request.method}: The HTTP method that's used on the original request.
{request.headers.<HeaderName>}: A header that can be read from the original request. Replace <HeaderName> with the name of the header that you want to read. If the header isn't included on the request, the value will be the empty string.
{request.querystring.<ParameterName>}: A query string parameter that can be read from the original request. Replace <ParameterName> with the name of the parameter that you want to read. If the parameter isn't included on the request, the value will be the empty string.

Reference back-end response parameters

Response parameters can be used as part of modifying the response to the client. The following values can be used in config values:

{backend.response.statusCode}: The HTTP status code that's returned on the back-end response.
{backend.response.statusReason}: The HTTP reason phrase that's returned on the back-end response.
{backend.response.headers.<HeaderName>}: A header that can be read from the back-end response. Replace <HeaderName> with the name of the header you want to read. If the header isn't included on the response, the value will be the empty string.

Reference application settings

You can also reference application settings defined for the function app by surrounding the setting name with percent signs (%).

For example, a back-end URL of https://%ORDER_PROCESSING_HOST%/api/orders would have "%ORDER_PROCESSING_HOST%" replaced with the value of the ORDER_PROCESSING_HOST setting.

Tip

Use application settings for back-end hosts when you have multiple deployments or test environments. That way, you can make sure that you are always talking to the right back-end for that environment.

Troubleshoot Proxies

By adding the flag "debug":true to any proxy in your proxies.json, you'll enable debug logging. Logs are stored in D:\home\LogFiles\Application\Proxies\DetailedTrace and accessible through the advanced tools (kudu). Any HTTP responses will also contain a Proxy-Trace-Location header with a URL to access the log file.

You can debug a proxy from the client side by adding a Proxy-Trace-Enabled header set to true. This will also log a trace to the file system, and return the trace URL as a header in the response.

Block proxy traces

For security reasons you may not want to allow anyone calling your service to generate a trace. They won't be able to access the trace contents without your sign-in credentials, but generating the trace consumes resources and exposes that you're using Function Proxies.

Disable traces altogether by adding "debug":false to any particular proxy in your proxies.json.

Advanced configuration

The proxies that you configure are stored in a proxies.json file, which is located in the root of a function app directory. You can manually edit this file and deploy it as part of your app when you use any of the deployment methods that Functions supports.

Tip

If you have not set up one of the deployment methods, you can also work with the proxies.json file in the portal. Go to your function app, select Platform features, and then select App Service Editor. By doing so, you can view the entire file structure of your function app and then make changes.

Proxies.json is defined by a proxies object, which is composed of named proxies and their definitions. Optionally, if your editor supports it, you can reference a JSON schema for code completion. An example file might look like the following:

{
    "$schema": "http://json.schemastore.org/proxies",
    "proxies": {
        "proxy1": {
            "matchCondition": {
                "methods": [ "GET" ],
                "route": "/api/{test}"
            },
            "backendUri": "https://<AnotherApp>.azurewebsites.net/api/<FunctionName>"
        }
    }
}

Each proxy has a friendly name, such as proxy1 in the preceding example. The corresponding proxy definition object is defined by the following properties:

matchCondition: Required--an object defining the requests that trigger the execution of this proxy. It contains two properties that are shared with HTTP triggers:
- methods: An array of the HTTP methods that the proxy responds to. If it isn't specified, the proxy responds to all HTTP methods on the route.
- route: Required--defines the route template, controlling which request URLs your proxy responds to. Unlike in HTTP triggers, there's no default value.
backendUri: The URL of the back-end resource to which the request should be proxied. This value can reference application settings and parameters from the original client request. If this property isn't included, Azure Functions responds with an HTTP 200 OK.
requestOverrides: An object that defines transformations to the back-end request. See Define a requestOverrides object.
responseOverrides: An object that defines transformations to the client response. See Define a responseOverrides object.

Note

The route property in Azure Functions Proxies does not honor the routePrefix property of the Function App host configuration. If you want to include a prefix such as /api, it must be included in the route property.

Disable individual proxies

You can disable individual proxies by adding "disabled": true to the proxy in the proxies.json file. This will cause any requests meeting the matchCondition to return 404.

{
    "$schema": "http://json.schemastore.org/proxies",
    "proxies": {
        "Root": {
            "disabled":true,
            "matchCondition": {
                "route": "/example"
            },
            "backendUri": "https://<AnotherApp>.azurewebsites.net/api/<FunctionName>"
        }
    }
}

Application Settings

The proxy behavior can be controlled by several app settings. They're all outlined in the Functions App Settings reference

Reserved Characters (string formatting)

Proxies read all strings out of a JSON file, using \ as an escape symbol. Proxies also interpret curly braces. See a full set of examples below.

Character	Escaped Character	Example
{ or }	{{ or }}	`{{ example }}` --> `{ example }`
\	\\	`example.com\\text.html` --> `example.com\text.html`
"	\"	`\"example\"` --> `"example"`

Define a requestOverrides object

The requestOverrides object defines changes made to the request when the back-end resource is called. The object is defined by the following properties:

backend.request.method: The HTTP method that's used to call the back-end.
backend.request.querystring.<ParameterName>: A query string parameter that can be set for the call to the back-end. Replace <ParameterName> with the name of the parameter that you want to set. If an empty string is provided, the parameter is still included on the back-end request.
backend.request.headers.<HeaderName>: A header that can be set for the call to the back-end. Replace <HeaderName> with the name of the header that you want to set. If an empty string is provided, the parameter is still included on the back-end request.

Values can reference application settings and parameters from the original client request.

An example configuration might look like the following:

{
    "$schema": "http://json.schemastore.org/proxies",
    "proxies": {
        "proxy1": {
            "matchCondition": {
                "methods": [ "GET" ],
                "route": "/api/{test}"
            },
            "backendUri": "https://<AnotherApp>.azurewebsites.net/api/<FunctionName>",
            "requestOverrides": {
                "backend.request.headers.Accept": "application/xml",
                "backend.request.headers.x-functions-key": "%ANOTHERAPP_API_KEY%"
            }
        }
    }
}

Define a responseOverrides object

The requestOverrides object defines changes that are made to the response that's passed back to the client. The object is defined by the following properties:

response.statusCode: The HTTP status code to be returned to the client.
response.statusReason: The HTTP reason phrase to be returned to the client.
response.body: The string representation of the body to be returned to the client.
response.headers.<HeaderName>: A header that can be set for the response to the client. Replace <HeaderName> with the name of the header that you want to set. If you provide the empty string, the header isn't included on the response.

Values can reference application settings, parameters from the original client request, and parameters from the back-end response.

An example configuration might look like the following:

{
    "$schema": "http://json.schemastore.org/proxies",
    "proxies": {
        "proxy1": {
            "matchCondition": {
                "methods": [ "GET" ],
                "route": "/api/{test}"
            },
            "responseOverrides": {
                "response.body": "Hello, {test}",
                "response.headers.Content-Type": "text/plain"
            }
        }
    }
}

Note

In this example, the response body is set directly, so no backendUri property is needed. The example shows how you might use Azure Functions Proxies for mocking APIs.

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Work with legacy proxies

Re-enable proxies in Functions v4.x

Create a proxy

Modify requests and responses

Modify the back-end request

Modify the response

Use variables

Reference local functions

Reference request parameters

Route template parameters

Additional request parameters

Reference back-end response parameters

Reference application settings

Troubleshoot Proxies

Block proxy traces

Advanced configuration

Disable individual proxies

Application Settings

Reserved Characters (string formatting)

Define a requestOverrides object

Define a responseOverrides object

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