Invoking Callouts Using Apex* An Apex callout enables you to tightly integrate your Apex with an external service by making a call to an external Web service or sending a HTTP request from an Apex script and then receiving the response.
Tightly Couple Architecture
* Apex provides integration with Web services that utilize SOAP and WSDL, or HTTP services (RESTful services).
* Before any Apex callout can call an external site, that site must be registered in the Remote Site Settings page, or the callout fails.
* Salesforce prevents calls to unauthorized network addresses.
* Callouts enable Apex to invoke external web or HTTP services.
*Apex Web services allow an external application to invoke Apex methods through Web services.
Adding Remote Site SettingsTo add a remote site setting:
1. Click Your Name ➤ Setup ➤ Security Controls ➤ Remote Site Settings.
2. Click New Remote Site.
3. Enter a descriptive term for the Remote Site Name.
4. Enter the URL for the remote site.
5. Optionally, enter a description of the site.
6. Click Save.
Types of WSDL's
1. Enterprise WSDL* It is a big WSDL which contains information about schema of organization.
* Strictly Typed: For Integer if we give 3.19 like that then it will not accept and throw error.
* When schema is modified then we have to recreate new enterprise WSDL.
2. Partner WSDL* Loosly Typed: For Integer if we give 3.19 like that then it will accept and not throw error.
* When schema is modified then it will automatically updated.