(iOS) Offline Sync DB - Server

Question

Trying to implement an app which sends offline data stored on local db to web server when connected to internet. I use the code shown below. As far I have tested it works fine, not sure it will work fine for huge number of records. I would like to know whether any tweaking on this code may increase the performance???

NOTE

• I know this would be a worst code for offline sync purpose, so trying to tweak it better.

• Its a single way synchronization, from app to server.

  -(void)FormatAnswersInJSON {
  
    DMInternetReachability *checkInternet = [[DMInternetReachability alloc] init];
    if ([checkInternet isInternetReachable]) {
       if ([checkInternet isHostReachable:@"www.apple.com"]) {//Change to domain
          responseArray = [[NSMutableArray alloc] init];
  
          dispatch_async(backgroundQueue, ^(void) {
  
              NSArray *auditIDArray = [[NSArray alloc] initWithArray: [self getUnuploadedIDs]];
              for (int temp = 0; temp < [auditIDArray count]; temp ++) {
  
                  // Code to post JSON to server
  
                  NSURLResponse *response;
                  NSData *urlData=[NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
                  if (!error) {
                      NSString *responseID = [[NSString alloc]initWithData:urlData encoding:NSUTF8StringEncoding];
                      if ([responseID isEqualToString:@"ERROR"]) {
                          //Error uploading records
                      } else {
                         [responseArray addObject:responseID];
                      }
                  } else {
                     //Error
                     return;
                  }
              }
              dispatch_async( backgroundQueue, ^{
  
                  /* Based on return code update local DB */
                  for (int temp = 0; temp < [responseArray count]; temp ++) {
                     [self updateRecordsForID:[auditIDArray objectAtIndex:temp] withID:[responseArray objectAtIndex:temp]];
                  }
              });
          });
       }
    }
  }
  
  - (void)upload { //Called when internet connection available
  
      if(backgroundQueue){
          dispatch_suspend(backgroundQueue);
          dispatch_release(backgroundQueue);
          backgroundQueue = nil;
      }
      backgroundQueue = dispatch_queue_create("com.XXXX.TestApp.bgqueue", NULL);
      dispatch_async(backgroundQueue, ^(void) {
          [self FormatAnswersInJSON];
      });    
  }
  

Answer 1

If this code were sitting in front of me, my approach would be:

• Look at the use cases and define 'huge number of records': Will 50 record updates at a time occur regularly? Or will it be in 1s and 2s? Do my users have wifi connections or is it over the paid network?, etc.
• If possible, test in the wild. If my user base was small enough, gather real data and let that guide my decisions, or only release the feature to a subset of users/beta tests and measure.
• If the data tells you to, then optimize this code to be more efficient.

My avenue of optimization would be doing group processing. The rough algorithm would be something like:

for records in groups of X
  collect
  post to server {
    on return:
      gather records that updated successfully
      update locally
  }

This assumes you can modify the server code. You could do groups of 10, 20, 50, etc. all depends on the type of data being sent, and the size.

A group algorithm means a bit more pre-processing client side, but has the pro of reducing HTTP requests. If you're only ever going to get a small number of updates, this is YAGNI and pre-mature optimization.

Don't let this decision keep you from shipping!