Schedule Batch Class for Every 20 Seconds.

To Schedule the batch class for every 20 seconds need to write 2 class (1 Batch, 1 Schedule).

Once batch is Executed those will have next run this will make conflicts, so immediately it need to be deleted/abort, for those we need to write abort job in the same schedule class.

Batch Class:

global class MyBatchClass implements Database.Batchable<sObject> {
    global (Database.QueryLocator | Iterable<sObject>) start(Database.BatchableContext bc) {
        // collect the batches of records or objects to be passed to execute
    }
    global void execute(Database.BatchableContext bc, List<P> records){
        // process each batch of records
    }   
    global void finish(Database.BatchableContext bc){
        // execute any post-processing operations
    }   
}

Schedule Class:

global  class MyBatchClass implements Database.Batchable<sObject>,schedulable {
                                       
    global void execute(SchedulableContext SC)
    {
         doScheduling();
    }
   
    public void doScheduling(){
       try{
           
            dateTime dt=System.now().addSeconds(20);                                                                         
            String Csec,Cmin,Chr,Cday,Cmonth,CYear;
            Csec=String.valueof(dt.second());
            Cmin=String.valueof(dt.minute());
            Chr=String.valueof(dt.hour());
            Cday=String.valueof(dt.day());
            Cmonth=String.valueof(dt.month());
            CYear=String.valueof(dt.Year());
            String SchTimer=Csec+' '+Cmin+' '+Chr+' '+Cday+' '+Cmonth+' ? '+CYear;
            system.debug('*************SchTimer:'+SchTimer);
           
            MyBatchClass batchclass = new MyBatchClass();
            database.executebatch(batchclass,100);

            //we will delete completed apex scheduled jobs for which state is DELETED
            for( CronTrigger c:[Select State,Id,EndTime,CronExpression From CronTrigger where 
                                        NextFireTime= null  AND State='DELETED' Limit 100]){
                    System.abortJob(c.id);
            }
        }
        catch(exception e){
                 ....your code....
        }
   }     
}