Policies in Azure API Management

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APPLIES TO: All API Management tiers

In Azure API Management, API publishers 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. API Management provides more than 50 policies out of the box that you can configure to address common API scenarios such as authentication, rate limiting, caching, and transformation of requests or responses. For a complete list, see API Management policy reference.

Popular policies include:

  • Format conversion from XML to JSON
  • Call rate limiting to restrict the number of incoming calls from a developer
  • Filtering requests that come from certain IP addresses

Policies are applied inside the gateway between the API consumer and the managed API. While the gateway receives requests and forwards them, unaltered, to the underlying API, a policy can apply changes to both the inbound request and outbound response.

Understanding policy configuration

Policy definitions are simple XML documents that describe a sequence of statements to apply to requests and responses. To help you configure policy definitions, the portal provides these options:

  • A guided, form-based editor to simplify configuring popular policies without coding XML
  • A code editor where you can insert XML snippets or edit XML directly

For more information about configuring policies, see Set or edit policies.

The policy XML configuration is divided into inbound, backend, outbound, and on-error sections. This series of specified policy statements is executed in order for a request and a response.

<policies>
  <inbound>
    <!-- statements to be applied to the request go here -->
  </inbound>
  <backend>
    <!-- statements to be applied before the request is forwarded to 
         the backend service go here -->
  </backend>
  <outbound>
    <!-- statements to be applied to the response go here -->
  </outbound>
  <on-error>
    <!-- statements to be applied if there is an error condition go here -->
  </on-error>
</policies>

For policy XML examples, see API Management policy snippets repo.

Error handling

If an error occurs during the processing of a request:

  • Any remaining steps in the inbound, backend, or outbound sections are skipped.
  • Execution jumps to the statements in the on-error section.

By placing policy statements in the on-error section, you can:

  • Review the error using the context.LastError property.
  • Inspect and customize the error response using the set-body policy.
  • Configure what happens if an error occurs.

For more information, see Error handling in API Management policies.

Policy expressions

Unless the policy specifies otherwise, policy expressions can be used as attribute values or text values in any of the API Management policies. A policy expression is either:

  • a single C# statement enclosed in @(expression), or
  • a multi-statement C# code block, enclosed in @{expression}, that returns a value

Each expression has access to the implicitly provided context variable and an allowed subset of .NET Framework types.

Policy expressions provide a sophisticated means to control traffic and modify API behavior without requiring you to write specialized code or modify backend services. Some policies are based on policy expressions, such as Control flow and Set variable.

Scopes

API Management allows you to define policies at the following scopes, from most broad to most narrow:

  • Global (all APIs)
  • Workspace (all APIs associated with a selected workspace)
  • Product (all APIs associated with a selected product)
  • API (all operations in an API)
  • Operation (single operation in an API)

When configuring a policy, you must first select the scope at which the policy applies.

Policy scopes

Things to know

  • For fine-grained control for different API consumers, you can configure policy definitions at more than one scope

  • Not all policies are supported at each scope and policy section

  • When configuring policy definitions at more than one scope, you control policy inheritance and the policy evaluation order in each policy section by placement of the base element

  • Policies applied to API requests are also affected by the request context, including the presence or absence of a subscription key used in the request, the API or product scope of the subscription key, and whether the API or product requires a subscription.

    Note

    If you're using an API-scoped subscription, an all-APIs subscription, or the built-in all-access subscription, policies configured at the product scope aren't applied to requests from that subscription.

For more information, see:

GraphQL resolver policies

In API Management, a GraphQL resolver is configured using policies scoped to a specific operation type and field in a GraphQL schema.

  • Currently, API Management supports GraphQL resolvers that specify either HTTP API, Cosmos DB, or Azure SQL data sources. For example, configure a single http-data-source policy with elements to specify a request to (and optionally response from) an HTTP data source.
  • You can't include a resolver policy in policy definitions at other scopes such as API, product, or all APIs. It also doesn't inherit policies configured at other scopes.
  • The gateway evaluates a resolver-scoped policy after any configured inbound and backend policies in the policy execution pipeline.

For more information, see Configure a GraphQL resolver.

Examples

Apply policies specified at different scopes

If you have a policy at the global level and a policy configured for an API, both policies can be applied whenever that particular API is used. API Management allows for deterministic ordering of combined policy statements via the base element.

Example policy definition at API scope:

<policies>
    <inbound>
        <cross-domain />
        <base />
        <find-and-replace from="xyz" to="abc" />
    </inbound>
</policies>

In the example policy definition above:

  • The cross-domain statement would execute first.
  • The find-and-replace policy would execute after any policies at a broader scope.

Note

If you remove the base element at the API scope, only policies configured at the API scope will be applied. Neither product nor global scope policies would be applied.

Use policy expressions to modify requests

The following example uses policy expressions and the set-header policy to add user data to the incoming request. The added header includes the user ID associated with the subscription key in the request, and the region where the gateway processing the request is hosted.

<policies>
    <inbound>
        <base />
        <set-header name="x-request-context-data" exists-action="override">
            <value>@(context.User.Id)</value>
            <value>@(context.Deployment.Region)</value>
      </set-header>
    </inbound>
</policies>

Next steps

For more information about working with policies, see: