Unpredictable result against predicate in multithreading environment - ios

Question

We are developing an application to collect beacon information and sync new/updated data to the server. We are using core data to collect beacon information, but occasionally we've found that the core data framework gives incorrect results when syncing to the server. Sometimes predicates are failing after the first iteration while fetching data from the core data to sync. So, we are seeking advice about any issues you have had with prolonged use of core data and persistent storage of the database.
I am using following steps to sync core data records to server one by one
-(void)syncDataToServer{
if( !isSycInProgress ){
//predicate fetch records which newly created and which are synced to server yet
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"isSynced == %#",[NSNumber numberWithBool:NO]];
//fetch only one record at one time to utilize memory
NSArray *beaconRecordArray = [[ISCoreDataManager sharedManager] fetchObjectList:#"BeaconRecords" predicate:predicate attributeName:#"endTime" batchSize:1 offset:0 isAscending:YES inContext:mainContext];
BeaconRecords *beacon = nil;
if(beaconRecordArray.count){
isSycInProgress = YES;
beacon = [beaconRecordArray firstObject];
{
//create request parameter dictionary
NSDictionary *dict = #{
#"uid": [beacon.uID description],
#"title":beacon.title.length?beacon.title:#"No title",
#"url": beacon.urlString?beacon.urlString:#"",
};
//call webservice to upload records on server
__block NSManagedObjectID *objId = beacon.objectID;
[[WebServiceHelper sharedInstance] callPostDataWithMethod:beconUrlPostAddBecon withParameters:dict withHud:YES success:^(NSDictionary *response)
{
if([response isKindOfClass:[NSDictionary class]] && [response[#"status"] intValue] == 200){
NSString *responseStr =response[#"response_crypt_data"];
NSError *error;
//Decrypt data to fetch parameters in response
NSData *decryptedData = [RNDecryptor decryptData:[[NSData alloc] initWithBase64EncodedString:responseStr options:0] withPassword:secretKey error:&error];
DLog(#"decrypted data %#", [[NSString alloc] initWithData:decryptedData encoding:NSUTF8StringEncoding]);
//parse data
//convert data into collection format (for parsing)
NSError *parseJsonError = nil;
NSDictionary *decryptedResponse = [NSJSONSerialization JSONObjectWithData:decryptedData options:NSJSONReadingMutableContainers error:&parseJsonError];
//store Server id in local db for future reference to update record
BeaconRecords *beaconRec = [self.privateContext existingObjectWithID:objId error:nil];
[self.privateContext performBlock:^{
if(!error && !parseJsonError){
beaconRec.serverID = [NSString stringWithFormat:#"%#",decryptedResponse[#"id"]];
beaconRec.isSynced = #(YES);
[[ISCoreDataManager sharedManager] dbSaveInContext:self.privateContext];
}
//update isSynced flag in local db
isSycInProgressBeaconRecords = NO;
dispatch_async( dispatch_get_main_queue(), ^{
[self syncDataToServer]; // call this method recursively to upload all records which are pending to sync
});
}];
}
else
{
isSycInProgress = NO;
dispatch_async(dispatch_get_main_queue(), ^{
[self syncDataToServer]; // call this method recursively to upload all records which are pending to sync
});
}
} errorBlock:^(id error) {
isSycInProgress = NO;
dispatch_async(dispatch_get_main_queue(), ^{
[self syncDataToServer]; // call this method recursively to upload all records which are pending to sync
});
}];
}
return;
}
isSycInProgress = NO;
}
}
After successfully syncing 1st record to server, when I call same sync function recursively to upload 2nd record, that predicate is giving me same record which I uploaded before. And strange thing is isSynched property of fetched object is (yes) and my predicate says it should be (_NO).