Custom Connectors: Azure, Policies & Code

Advanced connector patterns

Azure Function as connector backend

When an external API does not exist or needs a processing layer, build an Azure Function and wrap it in a custom connector.

The pattern

Canvas App / Flow → Custom Connector → Azure Function → External System(s)

When to use this pattern

• The logic is too complex for a direct API call (aggregation, multi-step processing)
• You need to call multiple external APIs and combine results
• The external API format does not match what Power Platform needs
• You need server-side caching or rate limiting

Creating the connector from an Azure Function

1. Build the Azure Function with HTTP trigger
2. Export the OpenAPI definition from the Function App (Azure portal → API Definition)
3. Import the definition into Power Platform as a custom connector
4. Configure authentication (Function key or OAuth via Entra ID — managed identity for connectors is in preview)

💡 Scenario: Marcus builds a price comparison connector

Marcus needs a connector that compares shipping prices across three carriers. No single API provides this — each carrier has its own API.

Solution: An Azure Function that:

1. Accepts package details (weight, dimensions, destination)
2. Calls all three carrier APIs in parallel
3. Compares prices and returns a sorted list
4. Caches results for 5 minutes to reduce API calls

The custom connector wraps this Function with a single action: CompareShippingPrices. Makers see one clean action instead of three separate API calls.

Policy templates

Policy templates modify connector behavior at runtime WITHOUT changing the OpenAPI definition or writing code. They intercept requests and responses.

Policy	What It Does	Example Use
SetHeader	Add or modify HTTP headers	Add X-API-Version: 2.0 to every request
SetQueryParameter	Add query parameters	Append format=json to all requests
SetProperty	Modify request/response body properties	Rename a field from cust_name to customerName
Convert	Transform data types	Convert Unix timestamp to ISO 8601 date
RouteRequest	Redirect requests to a different URL	Route test traffic to a staging API

Policy template example

To add a required header to every API call:

<set-header name="X-API-Version" existsAction="override">
    <value>2.0</value>
</set-header>

To convert a response field:

<set-body>
@{
    var response = context.Response.Body.As<JObject>();
    response["createdDate"] = DateTimeOffset.FromUnixTimeSeconds(
        (long)response["created_timestamp"]
    ).ToString("o");
    return response.ToString();
}
</set-body>

Custom code transforms

For complex transformations that policy templates cannot handle, you can write C# code directly in the connector definition.

When to use code vs policies

Scenario	Use Policy Templates	Use Custom Code
Add a header	Yes	Overkill
Rename a field	Yes (SetProperty)	Overkill
Convert date format	Yes (Convert)	Overkill
Flatten nested JSON	No	Yes
Filter array elements	No	Yes
Aggregate multiple values	No	Yes
Complex conditional logic	No	Yes

Code transform example

public class Script : ScriptBase
{
    public override async Task<HttpResponseMessage> ExecuteAsync()
    {
        // Read the original response
        var response = await this.Context.SendAsync(
            this.Context.Request, this.CancellationToken);
        
        if (response.IsSuccessStatusCode)
        {
            var content = await response.Content.ReadAsStringAsync();
            var data = JObject.Parse(content);
            
            // Transform: flatten nested address object
            var address = data["address"] as JObject;
            if (address != null)
            {
                data["street"] = address["line1"];
                data["city"] = address["city"];
                data["postcode"] = address["postalCode"];
                data.Remove("address");
            }
            
            response.Content = CreateJsonContent(data.ToString());
        }
        
        return response;
    }
}

Extending the OpenAPI definition

After importing an initial definition, you often need to extend it:

• Add x-ms-summary — friendly display names for actions and parameters
• Add x-ms-visibility — control which fields show by default (important, advanced, internal)
• Add x-ms-trigger — mark an operation as a Power Automate trigger
• Add x-ms-dynamic-values — populate dropdowns dynamically from another API call
• Modify response schemas — match the actual API response (imports are not always perfect)

Question

When should you use an Azure Function as a connector backend?

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Answer

When you need server-side processing that the API alone cannot provide: aggregating data from multiple APIs, complex transformations, caching, rate limiting, or when the target API does not exist yet and you need to build the logic yourself.

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Question

What are policy templates in custom connectors?

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Answer

Policy templates modify connector behavior at runtime without changing the OpenAPI definition or writing code. Examples: SetHeader (add headers), SetQueryParameter (add params), SetProperty (rename fields), Convert (transform data types), RouteRequest (redirect to different URL).

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Question

What is x-ms-visibility in an OpenAPI definition?

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Answer

An extension that controls how parameters and properties appear in the Power Platform designer. Values: 'important' (always shown), 'advanced' (shown under 'Show advanced options'), 'internal' (hidden from the designer, used for system values). Improves the maker experience.

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Knowledge Check

An external API returns dates as Unix timestamps (e.g., 1714521600). Power Automate users need to see these as human-readable dates. What is the simplest approach?

AWrite custom C# code in the connector to transform every responseBUse a Convert policy template to transform the timestamp field to ISO 8601 formatCAsk the API provider to change their date formatDUse an expression in every Power Automate flow that uses this connector

Check Answer

🎬 Video coming soon

Next up: Dataverse APIs: Web API & Organisation Service — performing operations with OData, bulk requests, and OAuth authentication.