Slack technical guide

@RestResource(urlMapping='/inbound/records/*')
global with sharing class InboundRecordApi {
  @HttpPost
  global static void upsertRecords() {
    List<Row> items = (List<Row>) JSON.deserialize(
      RestContext.request.requestBody.toString(), List<Row>.class);

    List<MyObject__c> rows = new List<MyObject__c>();
    for (Row r : items) {
      rows.add(new MyObject__c(External_Id__c = r.id, Name = r.name, Amount__c = r.amount));
    }
    upsert rows External_Id__c;         // bulk + idempotent on the external id
    RestContext.response.statusCode = 200;
  }
  global class Row { global String id; global String name; global Decimal amount; }
}

public class SyncToServiceQueueable implements Queueable, Database.AllowsCallouts {
  private List<Id> ids;
  public SyncToServiceQueueable(List<Id> ids) { this.ids = ids; }

  public void execute(QueueableContext ctx) {
    for (MyObject__c rec : [SELECT Id, Name, External_Id__c FROM MyObject__c WHERE Id IN :ids]) {
      HttpRequest req = new HttpRequest();
      req.setEndpoint('callout:Service_NC/v1/records'); // secret lives in the Named Credential
      req.setMethod('POST');
      req.setHeader('Content-Type', 'application/json');
      req.setBody(JSON.serialize(new Map<String,Object>{
        'externalId' => rec.External_Id__c, 'name' => rec.Name }));
      HttpResponse res = new Http().send(req);
      if (res.getStatusCode() != 200) ErrorLog.capture(rec.Id, res);
    }
  }
}

global class NightlySyncBatch implements Database.Batchable<SObject>, Database.AllowsCallouts {
  global Database.QueryLocator start(Database.BatchableContext bc) {
    return Database.getQueryLocator([SELECT Id, External_Id__c FROM MyObject__c WHERE Needs_Sync__c = true]);
  }
  global void execute(Database.BatchableContext bc, List<MyObject__c> scope) {
    ServiceClient.sync(scope);       // one callout per 200-record chunk stays under limits
  }
  global void finish(Database.BatchableContext bc) { /* chain the next job or log the run */ }
}

@IsTest
private class SyncToServiceTest {
  private class Mock implements HttpCalloutMock {
    public HttpResponse respond(HttpRequest req) {
      HttpResponse res = new HttpResponse();
      res.setStatusCode(200); res.setBody('{"ok":true}');
      return res;
    }
  }
  @IsTest static void syncsInBulk() {
    Test.setMock(HttpCalloutMock.class, new Mock());
    List<MyObject__c> recs = new List<MyObject__c>();
    for (Integer i = 0; i < 200; i++)
      recs.add(new MyObject__c(Name = 'Row ' + i, External_Id__c = 'EXT-' + i));
    insert recs;

    Test.startTest();     // proves the callout is bulk-safe under governor limits
    System.enqueueJob(new SyncToServiceQueueable(new List<Id>(new Map<Id,MyObject__c>(recs).keySet())));
    Test.stopTest();
  }
}