Azure API Management Policies

Policies in Azure API Management are a powerful feature that allows you to modify the behavior of your APIs through configuration. They are a collection of statements that execute sequentially on the request or response of an API call. This enables you to implement cross-cutting concerns such as caching, rate limiting, authentication, authorization, content transformation, and more, without writing any code.

Key Concepts

• Policy Expressions: Policies use expressions written in a specific XML-based syntax to define logic and access API context.
• Policy Sections: Policies are organized into sections like inbound, backend, outbound, and onerror.
• Policy Elements: Built-in elements (e.g., set-header, rewrite-uri, validate-jwt, rate-limit-by-key) and custom elements allow for various operations.

Common Use Cases

Rate Limiting and Quotas

Control the number of requests a user or application can make within a specific time frame to prevent abuse and ensure fair usage.

<policies>
    <inbound>
        <rate-limit-by-key calls="100" renewal-period="60" counter-key="@(context.Request.IpAddress)" />
        <quota-by-key calls="10000" renewal-period="3600" counter-key="@(context.UserId)" />
    </inbound>
    <backend />
    <outbound />
</policies>

Authentication and Authorization

Secure your APIs by validating JWT tokens, API keys, or implementing custom authentication logic.

<policies>
    <inbound>
        <validate-jwt header-name="Authorization" failed-validation-error-message="Unauthorized" require-expiration-time="true" require-signed-tokens="true" audience="your-audience" issuer="your-issuer" />
        <check-header name="Ocp-Apim-Subscription-Key" failed-validation-error-message="Missing Subscription Key" />
    </inbound>
    <backend />
    <outbound />
</policies>

Content Transformation

Modify request and response bodies, headers, or query parameters. This includes converting between formats like JSON and XML.

<policies>
    <inbound>
        <set-header name="X-Custom-Header" exists-action="override" value="MyValue" />
        <rewrite-uri template="/api/v2/resource?id=@(context.Request.Path.Segments[2])" />
    </inbound>
    <backend>
        <send-request mode="new" response-variable-name="externalData" timeout="10" ignore-error="true">
            <set-url>https://external.service.com/data</set-url>
            <set-method>GET</set-method>
        </send-request>
        <set-body>@(string.Join(", ", context.Variables["externalData"].As<JObject>()["items"]))</set-body>
    </backend>
    <outbound>
        <set-body>@{
            var response = context.Response.Body.As<JObject>();
            response["processedTimestamp"] = DateTime.UtcNow;
            return response.ToString();
        }</set-body>
    </outbound>
</policies>

Caching

Improve API performance and reduce load on backend services by caching responses.

<policies>
    <inbound>
        <cache-lookup vary-by-query-parameter="id" />
    </inbound>
    <backend>
        <forward-request />
    </backend>
    <outbound>
        <cache-store duration="60" />
    </outbound>
</policies>

Policy Scopes

Policies can be applied at different scopes:

• Global: Applies to all APIs in the API Management instance.
• Product: Applies to all APIs within a specific product.
• API: Applies to a specific API.
• Operation: Applies to a specific operation within an API.

For detailed information on available policy elements and advanced scenarios, please refer to the official Azure API Management policy documentation.