global class ServiveNowBatch implements Database.Batchable<sObject>, Database.AllowsCallouts{

    global Database.QueryLocator start(Database.BatchableContext BC){

       String query = 'SELECT Id, Priority, Status, Subject, ServiceNowId__c, Origin FROM Case WHERE Status=1 LIMIT 50';

       return Database.getQueryLocator(query);
    }

    global void execute(Database.BatchableContext BC, List<Case> scope){

    }

    global void finish(Database.BatchableContext BC){

    }
}

@future (callout=true)
global static void getIncident(String subject){

    Http http = new Http();
    HttpRequest req =  new HttpRequest();
    HttpResponse res = new HttpResponse();

    string text = subject;
    req.setEndpoint('https://myinstance.service-now.com/api/now/table/incident?sysparm_fields=impact%2Cincident_state%2Cshort_description%2Csys_id%2Ccontact_type&sysparm_limit=2&u_sftype=true');

    req.setMethod('GET');
    req.setHeader('Content-Type', 'application/json');

    String username = 'admin';
    String password = 'abcd';
    Blob headerValue = Blob.valueOf(username + ':' + password);
    String authorizationHeader = 'BASIC ' +
    EncodingUtil.base64Encode(headerValue);
    req.setHeader('Authorization', authorizationHeader);
    res = http.send(req);
    System.debug('jsonrResult :' + res.getBody());
    Deserialization.ResponseResult res1= (Deserialization.ResponseResult)JSON.deserialize(res.getBody(),  Deserialization.ResponseResult.class);
    System.debug('Results == :' + res1 );

    List<Case> casesToUpsert = new List<Case>();
    for(Deserialization d : res1.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()>0){
        Database.upsert(casesToUpsert,false) ;
    }

global Database.QueryLocator start(Database.BatchableContext BC){
     String query = 'SELECT Id, Priority, Status, Subject, ServiceNowId__c, Origin '+
                    'FROM Case '+
                    'WHERE **condition**'+//your condition to get records
                    'LIMIT 50';//Add if you need to limit the records
     return Database.getQueryLocator(query);
}

String query = 'SELECT Id, Priority, Status, Subject, ServiceNowId__c, Origin '+
                        'FROM Case '+
                        'WHERE **condition**'+//your condition to get records
                        'LIMIT 50';//Add if you need to limit the records
MyBatchClass batch = new MyBatchClass(query);
database.executeBatch(batch);

global class MyBatchClass implements
    Database.Batchable<sObject>, Database.Stateful,Database.AllowsCallout{

    global final String query;
    global integer summary;

    global MyBatchClass(String q){
        query = q;
        summary = 0;
    }

    global Database.QueryLocator start(Database.BatchableContext BC){
        return Database.getQueryLocator(query);
    }

    global void execute( Database.BatchableContext BC, List<sObject> scope){
        for(sObject s : scope){
            summary = Integer.valueOf(s.get('total__c'))+summary;
        }
    }

    global void finish(Database.BatchableContext BC){
    }
}