Android：findInBackground（）的Parse.com并发问题

Question

I am using Parse.com as a backend for my app. The local database from Parse seems to be very easy to use, so I decided to use it.

I want to create a database with Name and PhoneNumber . That is easy, just make a new ParseObject and pinInBackground() . But it is more complicated when I want to remove duplicate numbers. First I need to search if the number already exists in the database and then add the new number if it doesn't exists.

The method to do this is:

 public void putPerson(final String name, final String phoneNumber, final boolean isFav) {

    // Verify if there is any person with the same phone number
    ParseQuery<ParseObject> query = ParseQuery.getQuery(ParseClass.PERSON_CLASS);
    query.whereEqualTo(ParseKey.PERSON_PHONE_NUMBER_KEY, phoneNumber);
    query.fromLocalDatastore();
    query.findInBackground(new FindCallback<ParseObject>() {
                               public void done(List<ParseObject> personList,
                                                ParseException e) {
                                   if (e == null) {
                                       if (personList.isEmpty()) {
                                           // If there is not any person with the same phone number add person
                                           ParseObject person = new ParseObject(ParseClass.PERSON_CLASS);
                                           person.put(ParseKey.PERSON_NAME_KEY, name);
                                           person.put(ParseKey.PERSON_PHONE_NUMBER_KEY, phoneNumber);
                                           person.put(ParseKey.PERSON_FAVORITE_KEY, isFav);

                                           person.pinInBackground();
                                       } else {
                                           Log.d(TAG, "Warning: " + "Person with the number " + phoneNumber + " already exists.");
                                       }
                                   } else {
                                       Log.d(TAG, "Error: " + e.getMessage());
                                   }
                               }
                           }
    );

}

Lets say I want to add 3 persons in the database:

    ParseLocalDataStore.getInstance().putPerson("Jack", "0741234567", false);
    ParseLocalDataStore.getInstance().putPerson("John", "0747654321", false);
    ParseLocalDataStore.getInstance().putPerson("Jack", "0741234567", false);
    ParseLocalDataStore.getInstance().getPerson(); // Get all persons from database

Notice that first and third person have the same number so the third souldn't be added to database, but...

The logcat after this is:

12-26 15:37:55.424 16408-16408/D/MGParseLocalDataStore: Person:0741234567 was added.
12-26 15:37:55.424 16408-16408/D/MGParseLocalDataStore: Person:0747654321 was added.
12-26 15:37:55.484 16408-16408/D/MGParseLocalDataStore: Person:0741234567 was added.
12-26 15:37:55.494 16408-16408/D/MGParseLocalDataStore: Person database is empty

The last line from logcat is from the method that shows me all persons from database:

public void getPerson() {
        ParseQuery<ParseObject> query = ParseQuery.getQuery(ParseClass.PERSON_CLASS);
        query.fromLocalDatastore();
        query.findInBackground(new FindCallback<ParseObject>() {
            public void done(List<ParseObject> personList,
                             ParseException e) {
                if (e == null) {
                    if (personList.isEmpty()) {
                        Log.d(TAG, "Person database is empty");
                    } else {
                    for (ParseObject p : personList) {
                        Log.d(TAG, p.getString(ParseKey.PERSON_PHONE_NUMBER_KEY));
                    }
                }
            } else {
                Log.d(TAG, "Error: " + e.getMessage());
            }

        }
    });
}

So there are 2 problems:

1. The third number is added even if I checked if already exists.

2. The method that shows me all persons tell me I have nothing in my database even if in logcat I can see it added 3 persons.

I think the problem is findInBackground() method that does all the job in another thread. Is there any solution to this problem?

Answer 1

Both of your problems are a result of asynchronous work. If you call the putPerson method twice, they will both run near-simultaneously in separate background threads and both find-queries will most likely return almost at the same time, and definitely before the first call has saved the new person.

In your example, the getPerson call will return before the background threads have been able to save your three people as well.

Your problem is not really related to Parse or localDataStore, but is a typical concurrency issue. You need to rethink how you handle concurrency in your app.

As long as this is only a local issue, you can impose synchronous structure with ie the Bolts Framework (which is already a part of your app since you're using Parse). But if calls to addPerson is done in multiple places, you will always face this problem and you'd have to find other solutions or workarounds to handle concurrency.

Concurrency is a big topic which you should spend some time studying.