Database.Callouts

Database.Allowscallouts allows us to use a callout in batch Apex.Callouts include HTTP
requests as well as methods defined with the webservice keyword.

How to make webservice callout from scheduler

scenario-1 : if scheduler doesn't depends on Batch or Queueable(depends on future methods)
scenario-2 : If Scheduler depends on Batch
scenario-3 : If Scheduler depends on Queueable

Note :
if we try to make a call from scheduler,we will get the below exception:

system.calloutException: callout form scheduled Apex not supported

Reason : All we know that we cannot schedule a class which contains callout.
The only solution for this is to use @future annotation to enforce(for Normal apex classes)
but we cannot process future methods inside batch.

Scenario -1 :
if scheduler doesn't depends on Batch or queueable

ex :

//Scheduled Apex
public class DemoScheduler1 implements Schedulable{
    public void execute(SchedulableContext sc){
        system.debug('*******Going to call future method ');
        DemoAsynchronousTest.futureMethodCallFromScheduler();
    }
}
//apex class containing future method
public class DemoAsynchronousTest{
    @future
    public static void futureMethodCallFromScheduler(){
        system.debug('******futureMethodCallFromScheduler get called');
    }
   
}

Scenario -2 :
if scheduler depends on Batch

ex :

public class ExampleScheduler implements Schedulable, Database.AllowsCallouts, Database.Batchable<sObject> {
 
    public void execute(SchedulableContext SC) {
      Database.executebatch(new ExampleScheduler());
   }
 
 public Iterable<sObject> start(Database.Batchablecontext BC){
        ExampleHelper.makeWebserviceCallout();
     return null;   
 }

    public void execute(Database.BatchableContext BC, List<sObject> scope){     
    }

  public void finish(Database.BatchableContext info){
       
    }
}

public class ExampleHelper{

  public static void makeWebserviceCallout(){
  HttpRequest req = new HttpRequest();
  req.setEndpoint('http://www.yahoo.com');
  req.setMethod('GET');
  String username = 'myname';
  String password = 'mypwd';
 
  Blob headerValue = Blob.valueOf(username + ':' + password);
  String authorizationHeader = 'BASIC ' +
  EncodingUtil.base64Encode(headerValue);
  req.setHeader('Authorization', authorizationHeader);
   
  // Create a new http object to send the request object
  // A response object is generated as a result of the request 
 
  Http http = new Http();
  HTTPResponse res = http.send(req);
  System.debug(res.getBody());
 }
}

Scenario-3 :
if scheduler depends on queueable

public class ExampleScheduler implements Schedulable{
 
    public void execute(SchedulableContext SC) {
      System.enqueueJob(new ExampleQueueable());
   }
}

public class ExampleQueueable implements Queueable, Database.AllowsCallouts {

    public void execute(QueueableContext context) {
        ExampleHelper.makeWebserviceCallout();
    }
}

or

global class SampleCalloutCls implements Queueable, Schedulable, Database.AllowsCallouts{
    //variable declaration
    private List<Account> accountList ;

//Queueable interface method 
    public void execute(QueueableContext QC){
        // Perform callout here to get the accounts from external system.       
        //Calling batch process if list contains records.
        if(!accountList.isEmpty()){
           Database.executeBatch(new SampleBatchCls(accountList));
        }
    }

//Method to Schedule the current class
   global void execute(SchedulableContext sc) {
      system.enqueueJob(new SampleCalloutCls()); 
   }

 }
//End of class