When configuring an HTTP callout, you specify the type of request (HTTP or HTTPS), destination and format of the request, the expected format of the response, and, finally, the portion of the response that you want to analyze.
For the destination, you either specify the IP address and port of the HTTP callout agent or engage a load balancing, content switching, or cache redirection virtual server to manage the HTTP callout requests. In the first case, the HTTP callout requests will be sent directly to the HTTP callout agent. In the second case, the HTTP callout requests will be sent to the virtual IP address (VIP) of the specified virtual server. The virtual server will then process the request in the same way as it processes a client request. For example, if you expect a large number of callouts to be generated, you can configure instances of the HTTP callout agent on multiple servers, bind these instances (as services) to a load balancing virtual server, and then specify the load balancing virtual server in the HTTP callout configuration. The load balancing virtual server then balances the load on those configured instances as determined by the load balancing algorithm.
For the format of the HTTP callout request, you can specify the individual attributes of the HTTP callout request (an attribute-based HTTP callout), or you can specify the entire HTTP callout request as a default syntax expression
(an expression-based HTTP callout).
The appliance does not check for the validity of the request. You must make sure that the request is a valid request. An incorrect or incomplete HTTP callout configuration results in a runtime UNDEF condition that is not associated with an action. The UNDEF condition merely updates the Undefined Hits counter, which enables you to troubleshoot an incorrectly configured HTTP callout. However, the appliance parses the HTTP callout request to enable you to configure certain NetScaler
features for the callout. This can lead to an HTTP callout invoking itself. For information about callout recursion and how you can avoid it, see "Avoiding HTTP Callout Recursion
Finally, regardless of whether you use HTTP request attributes or an expression to define the format of the HTTP callout request, you must specify the format of the response from the HTTP callout agent and the portion of the response that you want to evaluate. The response can be a Boolean value, a number, or text. The portion of the response that you want to evaluate is specified by an expression. For example, if you specify that the response contains text, you can use HTTP.RES.BODY(<unit>) to specify that the appliance must evaluate only the first <unit> bytes of the response from the callout agent.
At the command line, you first create an HTTP callout by using the add
command. When you add a callout, all parameters are set to a default value of NONE, except the HTTP method, which is set to a default value of GET. You then configure the callout’s parameters by using the set command. The set
command is used to configure both types of callouts (attribute-based and expression-based). The difference lies in the parameters that are used for configuring the two types of callouts. Accordingly, the command-line instructions that follow include a set
command for configuring an attribute-based callout and a set
command for configuring an expression-based callout. In the configuration utility, all of these configuration tasks are performed in a single dialog box.
Note: Before you put an HTTP callout into a policy, you can modify all configured parameters except the return type. Once an HTTP callout is in a policy, you cannot completely modify an expression that is configured in the callout. For example, you cannot change HTTP.REQ.HEADER("myval") to CLIENT.IP.SRC. However, you can modify the operators and arguments that are passed to the expression. For example, you can change HTTP.REQ.HEADER("myVal1") to HTTP.REQ.HEADER("myVal2"), or HTTP.REQ.HEADER("myVal") to HTTP.REQ.HEADER("myVal").AFTER_STR(<string>). If the set command fails, create a new HTTP callout.
HTTP callout configuration involves configuring default syntax expressions. For more information about configuring default syntax expressions, see "Configuring Default Syntax Expressions: Getting Started."