when i execute this schedule apex class from developer console  the class is scheduled at 4 different timings as queue. when "schedule Job2" running i got this exception in debug log shows      "|EXCEPTION_THROWN|[21]|System.AsyncException: The Apex job named   "Scheduled Job0" is already scheduled for execution"   and in debug log the future call shows  : "Number of future calls: 1 out of 50"  but  i'm not getting the any records pulling from external source   this is scheduled class  global class SchedulingService implements Schedulable {      global void execute(SchedulableContext SC){            ServiceNowUpsert.GetIncident('abcd','1234','https://abcd.service-now.com');                    String day = string.valueOf(system.now().day());           String month = string.valueOf(system.now().month());           String hour = string.valueOf(system.now().hour());           String minute = string.valueOf(system.now().minute() + 15);           String second = string.valueOf(system.now().second());           String year = string.valueOf(system.now().year());            String cronStr = '0 0,15 * * * *';//changes required           String strJobName = 'Job-' + second + '_' + minute + '_' + hour + '_' + day + '_' + month + '_' + year;           // String strSchedule = '0 ' + minute + ' ' + hour + ' ' + day + ' ' + month + ' ?' + ' ' + year;           //System.schedule(strJobName, cronStr, new SchedulingServiceNow()); //changes required           System.schedule('Scheduled Job0', '0 0 * * * ?',  new SchedulingService ());              System.schedule('Scheduled Job1', '0 15 * * * ?', new SchedulingService ());           System.schedule('Scheduled Job2', '0 30 * * * ?', new SchedulingService ());           System.schedule('Scheduled Job3', '0 45 * * * ?', new SchedulingService ());            for( CronTrigger c:[Select State,Id,EndTime,CronExpression From CronTrigger where                                               NextFireTime= null  AND State='DELETED' Limit 100]){                          System.abortJob(c.id);                 }             }        }     This is my apex class   global with sharing class ServiceNowUpsert {      @TestVisible static HttpResponse res;      @future(callout=true)      global static void GetIncident(String username1, String password1, String endpointUrl1){           Http http = new Http();         HttpRequest req =  new HttpRequest();         Blob headerValue = Blob.valueOf(username1 + ':' + password1);         String authorizationHeader = 'BASIC ' +         EncodingUtil.base64Encode(headerValue);         req.setHeader('Authorization', authorizationHeader);         req.setHeader('Content-Type', 'application/json');  //displays data in JSON format           req.setEndpoint(endpointUrl1+'/api/now/table/incident?sysparm_query=sys_updated_onONLast+hour%40javascript%3Ags.hoursAgoStart%282%29%40javascript%3Ags.hoursAgoEnd%280%29&amp;sysparm_fields=impact%2Cincident_state%2Cshort_description%2Csys_id%2Ccontact_type&amp;u_sftype=true');                req.setMethod('GET');           res = http.send(req);         System.debug('MyResult == :'+res.getBody());            Deserialization.ResponseResult theresult1 = (Deserialization.ResponseResult)JSON.deserialize(res.getBody(),  Deserialization.ResponseResult.class);         System.debug('MyResult == :' + theresult1 );           List&lt;Case&gt; casesToUpsert = new List&lt;Case&gt;();                 for(Deserialization d : theresult1.result ){                  Case c = new Case();                  c.Priority = d.impact;                 c.Status = d.incident_state;                 c.Subject = d.short_description;                 c.ServiceNowId__c = d.sys_id;                 c.Origin = d.contact_type;                  casesToUpsert.add(c);          }         system.debug('Cases to UPsert ::: ' +casesToUpsert);          if(casesToUpsert.size()&gt;0){             Database.upsert(casesToUpsert,false) ;         }              }  }   How to work my scheduled class perfectly running for every 15 mins and pulling the records from external source?
Using the salesforce GUI it seams impossible to schedule a job every 15 minutes. Or more than once a day.
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