Attaching a policy to a ProxyEndpoint or TargetEndpoint Flow

Policies are not executed until they are attached to a Flow. You can create a Policy attachment by naming a Policy in a Step configuration.

The choice of attachment point is critical to the behavior of your API proxy. For example, if you attach the Quota policy to a response Flow, then the Quota would be enforced after the request message was sent to the backend service. That would defeat the purpose of applying a Quota policy! Therefore, you need to attach the Quota policy as a processing Step on the request Flow.

The format of a policy attachment is:

<Step>
    <Name>{policy_name}</Name>
</Step>

For example:

<Step>
    <Name>QuotaPolicy</Name>
</Step>

A policy is attached to a Flow by adding the Step configuration to the appropriate request or response Flow element in a ProxyEndpoint or TargetEndpoint configuration.

You can attach a policy to a request or response Flow. Request and response Flows are further subdivided in to PreFlow and PostFlow.

The following example demonstrates the minimal ProxyEndpoint configuration, with no policy attachments. It simply defines the (inbound) HTTPProxyConnection and a RouteRule.

<ProxyEndpoint name="default">
    <HTTPProxyConnection>
        <BasePath>/weather</BasePath>
        <VirtualHost>default</VirtualHost>
    </HTTPProxyConnection>
    <RouteRule name="default">
        <TargetEndpoint>default</TargetEndpoint>
    </RouteRule>
</ProxyEndpoint>

You must modify this configuration so that the ProxyEndpoint enforces a Quota policy (as a processing Step) before the API proxy performs any other processing. If a developer has exceeded a Quota, you don't want to waste any computational resources on additional requests.

To enforce this configuration, you attach a processing Step to the request PreFlow as follows:

<ProxyEndpoint name="default">
  <PreFlow>
    <Request>
      <Step><Name>QuotaPolicy</Name></Step>
    </Request>
  </PreFlow>
  <HTTPProxyConnection> 
    <BasePath>/weather</BasePath> 
    <VirtualHost>default</VirtualHost> 
  </HTTPProxyConnection> 
  <RouteRule name="default"> 
    <TargetEndpoint>default</TargetEndpoint> 
  </RouteRule> 
</ProxyEndpoint>

Sometimes, you might want a policy to execute after some other initial processing on the ProxyEndpoint. For example, you want to check Quota in the PreFlow, then perform another set of processing after Quota is checked, such as converting the request from JSON to XML. To do so, attach a policy to the PostFlow request path. The following is a sample request PostFlow attachment. This policy would execute on the request message after all of the policies in the PreFlow (and any conditional flows) execute.

<PostFlow>
  <Request>
    <Step><Name>JSONtoXMLPolicy</Name></Step>
  </Request>
</PostFlow>

The following is a sample response PostFlow attachment. This policy would execute on the response message. (The ProxyEndpoint response PostFlow is the final processing phase before the response is returned to the requesting client app.)

<PostFlow>
  <Response>
    <Step><Name>XMLtoJSONPolicy</Name></Step>
  </Response>
</PostFlow>

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