The API calls return error data that your client application can use to identify and resolve runtime errors. If an error occurs during the invocation of most API calls, then the API provides the following types of error handling:
When you sign on via the login() call, a new client session begins and a corresponding unique session ID is generated. Sessions automatically expire after the amount of time specified in the Security Controls setup area of the Salesforce application (default two hours). When your session expires, the exception code INVALID_SESSION_ID is returned. If this happens, you must invoke the login() call again.