...
The Service Callout policy allows calling an external service (third party API) from your the API flow. Using a Service Callout policy, you can configure it is possible to configure an API proxy to make multiple API calls to deliver relevant geolocation data, customer reviews, items from a partner’s retail catalog, and so on. The service can be either an HTTP service or a JMS service. The target(s) to be invoked can be configured using using the Targets section.
Configuration
The configuration encapsulates different alternatives for the back end server that is used to perform the required informationoperations. The load balancing strategy allows hosting multiple identical services hosted on different servers or different types of servers. It is possible to load balance between an HTTP service and a JMS service.
Load Balancing Strategy
Two The two types of Load Balancing strategies are Round Robin and Weighted Round Robin.
...
This default algorithm forwards a request to each Target Server in the order in which the servers are listed in the Target Configuration ID. You can select Select the configured target from the Target Configuration ID drop-down list and you may add more targets to the list by clicking the Add 
button.
...
The Weighted Round Robin algorithm enables you to configure configuration of proportional traffic loads for your on Target Servers. It distributes requests to your Target Servers following the weight set for them. The targets which are chosen in the Round Robin strategy reflect under Weighted Round Robin where you can it is possible to set the respective weight values as required. You may add more targets which are existing Existing targets can be added by clicking the Add button; the newly added targets reflect under the Round Robin strategy also.
Figure 2: Weighted Round Robin Load Balancing Strategy
...
Using Authorization Services
Consider a case where If an API needs to invoke a third party service for authorization. In such cases, the authorization service can be invoked using a service call out. the The response from the call out can be parsed using the Assign Variables Policy and the resultant variables can be used to send an error message or carry forward with go ahead invoking the target service.
| Note |
|---|
Before the call out, the data that needs to be sent to the target service needs to be stored in variables using the Assign Variable Policy and request . Requests to the target server needs need to be built using the Build Message Policy. |
...
JMS targets are invoked as a JMS requestorrequest. Each request to the JMS system is sent with a property RequestorID and a Header JMSCorrelationID. These properties must be retained in the response message sent on the response destination.
...