186 Citrix NetScaler Policy Configuration and Reference Guide
About Calling Out to an External Application
A callout to an external application consists of an HTTP request and a set of
parameters that parse the response to the request. You configure the entire
request, or significant parameters in the request, in the HTTP callout policy. The
HTTP callout policy also contains information about the recipient of the request
and advanced expressions for parsing the HTTP response when it is received.
A callout can send only an HTTP request. The service can only be an HTTP or
HTTPS service.
After configuring an HTTP callout policy, you do not bind it or add it to a policy
bank, as you would another policy. Instead, you invoke it using an advanced
expression in one of the following:
• Content Switching: For content switching of HTTP and TCP data
•Rewrite
• Responder: For content filtering functions
• Load Balancing: For token-based load balancing
Task overview: Configuring a callout to an external application
1. Create a callout policy in the Rewrite feature, as described in “Configuring
an HTTP Callout Policy,” on page 188.
2. In the feature where the callout policy is needed, invoke the callout policy
from an advanced expression, as described in “Invoking an HTTP Callout
Policy,” on page 193.
3. Configure an HTTP callout agent to format the callout to the receiving
application, as described in the “HTTP Service Callout” chapter in the
Citrix NetScaler AppExpert Guide.
About HTTP Callout Policies
You configure HTTP callout policies in the Rewrite feature. After you define the
callout policy, it can be invoked from a policy expression in other NetScaler
features. For information on configuring the callout policy, see “Configuring an
HTTP Callout Policy,” on page 188. For information on invoking the callout
policy, see “Invoking an HTTP Callout Policy,” on page 193.
An HTTP callout policy contains the following components:
• Parameters that identify the application to be queried.