What's the proper way to add/update data in Firestore when offline?

Question

When my app is offline and I am adding or updating a document the memory increases. Also, showing lists of documents take longer to load. If I run the same code when the device is online the memory stays consistent as well as the speed of the activities that have lists of documents.

I'm currently doing saves like the following:

collectionRef.document(id).set(obj, SetOptions.merge());

Or for batching a couple of records:

batch.set(docRef1, obj1);
batch.set(docRef2, obj2);
batch.commit();

I had listeners for onComplete but in the accepted answer for this question, it seems to indicate that listeners are unnecessary in most situations and that you can't wait for it to complete anyway when you're offline.

In another question they indicate in code that a "Snapshot" is required to properly do online and offline saving: Offline issue with Firestore vs Firebase. But I can't find anywhere else indicating if that will make a difference. I think of Snapshots as being something you attach to a document or query when you want to be notified of changes to it and attaching a listener like that will result in a memory leak if it isn't removed.

Another part of all of this is how this slowness might affect data integrity. When I watch in the profiler in Android Studio I see that the FirestoreWorker can get to a point where it is constantly working even if I don't do anything in the app. I'm not just talking a few seconds, more like a minute. There isn't any ordering guarantee of writes when it is offline that I can find. Trying to stop and restart the app doesn't seem to have any effect on the slowness (although it will reset the memory).

So all of this leads to the question: what is the proper way to add/update data in Firestore when offline so that the app's memory doesn't grow unbounded and slow down?

Answer 1

Cloud Firestore uses SQLite for its persistence mechanism. So for intermittent periods of offline activity, you shouldn't have problems with performance or durability.

However, if you intend to use a Firestore database for very long periods of time, there are some things you should be aware of. Cloud Firestore was not built as an offline database, is an online database that continues to work when you're offline for short or longer periods of time. When offline, pending writes that have not yet been synced to the server are held in a queue. If you do too many write operations without going online to sync them, that queue will grow fast and it will not slow down only the write operations it will also slow down your read operations.

So I suggest use this database for its online capabilities. As one of the Firebase engineers said and I quote, "It is impossible to build a slow query in Firestore". So, the performance comes from the new indexing capabilities on the backend and these optimizations don't exist when you're offline.

One more thing, if you have many offline clients who are trying to write to the same document, only the last one will be actually be written to servers when the state is changed.

So, to answer your question, there is no proper way to add/update data in Firestore when offline, to have a less memory usage. Just go online and that's it!