Apex callout from one Salesforce Org to another Salesforce Org

Setting Up the Web Service in Org B

1. In Org B, create an Apex class with a web service method. For example, create a class named AccountPlan containing a method createAccountPlan() that performs the desired operation and returns a String result. Mark the method with the webservice keyword so it can be exposed as a WSDL-based web service.

2. Once the class is created, generate the WSDL for the class from Setup | Apex Classes, and download it.

Creating the REST Service in Org A to Invoke Org B

1. In Org A, use the downloaded WSDL to auto-generate the stub classes: navigate to Setup | Apex Classes | Generate from WSDL. This creates a stub class (for example, soapSforceComSchemasClassAccplan and AsyncSoapSforceComSchemasClassAccplan )

2. Create an Apex class as follows: 
 

@RestResource(urlMapping='/createAccountPlan/*')
global with sharing class CallAccountPlan{
    
    @HttpGet
    global static String doGet(){
        soapSforceComSchemasClassAccplan.AccountPlan stub = new soapSforceComSchemasClassAccplan.AccountPlan();
        stub.SessionHeader = new soapSforceComSchemasClassAccplan.SessionHeader_element();
        stub.SessionHeader.sessionId = Session Id;
        return stub.createAccountPlan();
        }
}

Testing the Integration

1. Add Org B's endpoint in Setup | Security | Remote Site Settings in Org A.
2. Open Workbench in Org A and use the REST Explorer. Make a GET request to the REST endpoint path defined in Org A's @RestResource annotation (for example, /services/apexrest/createAccountPlan/).
3. Verify the response returns the expected output from Org B's createAccountPlan() method.

Conclusion:
When a REST web service in Org A is called by an external system, that REST service can in turn make callouts to other Salesforce orgs, provided Org B does not make further callouts within the same transaction.