Develop Reference

How to save a part of firebase realtime database in offline mode

I wanted to store the specific path in Firebase database (JSON) in local phone storage, not all data at the realtime-Firebase database. Say I have a news-feed path for each user in my Firebase-database and I want to save only the news-feed which is specified for the user instead of loading whole useless data on the local storage of user's mobile.
I read this paragraph from the official Firabase site which mentioned that
The Firebase Real-time Database synchronizes and stores a local copy of the data for active listeners. In addition, you can keep specific locations in sync.
DatabaseReference scoresRef = FirebaseDatabase.getInstance().getReference("scores");
scoresRef.keepSynced(true);
If this is the solution to my question, then I can use this line directly without writing that line
FirebaseDatabase.getInstance().setPersistenceEnabled(true);

Those two bits of code are not really related. They don't do the same thing.
keepSynced(true) effectively keeps a listener active on the given reference, from the moment it's called, for as long as the app is running, so that the local version of the data is always in sync with the remote version on the server.
setPersistenceEnabled(true) just activates local caching of the data read by the SDK. When persistence is enabled, the app can still query data previously read. It takes effect for all data read by the SDK. While persistence is enabled, you can't control which data is cached - all read data is cached up to 10MB max. When the max is reached the oldest data will be evicted from the cache.

According to me the best way will be to parse the Firebase JSON data and map it into an object and then save only data that you need in a local storage (News-feed in your case), and then access it later whenever you need it.
For this, you can use Paper DB as a local storage to store specific data and use it whenever you need to. It stores data as a key value pair so you can access your data with the same key you inserted it with in the database. (Just like shared preferences work).
It stores data as cache in your local storage and uses Kryo serialization framework which is pretty fast for I/O operations.
Or you can also use Room (a google library) with SQLite to achieve this task.
Haven't tried Room but i think it will suite your purpose.
Here's the official documentation for Room

Firebase offline capabilities causing memory problems

I am developing a chat app that uses firebase database to store data. The usual approach while developing a chat app is to keep the database nodes synced so that you access the messages offline. So the problem rises when I implement the firebase offline capabilities to keep the data nodes synced. Firebase suggests two required steps for accessing data offline:
Enabling disk persistance
this is enabled according to the documentation by using this line of code (in my case I add it in application class):
FirebaseDatabase.getInstance().setPersistanceEnabled(true);
and
Keeping a node synced
this is enabled by simply adding keepsynced(true) to any databasereference that you wish to keep synced, like this:
ChatNode.keepSynced(true);
What is the difference between the two?
According to the firebase team answers on this site, I deduced that:
1) (Disk persistance) stores the data on the device disk to use them when needed, and data is stored wether you write data or read data.
a) If you write data offline: data is stored on disk and is sent to database when you go online again.
b) If you read data offline: the listener that was read online and was kept in disk and stored, you will be able to read it offline from disk.
2) (keep synced true) will keep a database reference synced in 2 ways:
a) If you are also using (disk persistence) with (keep synced) you will be able to keep data synced on disk ... which seems to be the default behavior of (disk persistence).
b) If you are using (keep synced) alone then you only store to what is known as the app memory.
The problem
I did set both of the methods, but my app is now very laggy and slow and sometimes stops on its own.
The question
If all the things that I said above are true, then would this method of offline capability be a heavy load on my app?
If I kept many listeners synced and set persistence enabled, then would the disk become full of data? Should I clean the data? Is the data on disk cleaned by itself in both methods? Is data cleaned by itself from memory?
I want to avoid the lagging and slow response in my app, thanks for your help.

You are right about your assumptions. If you are using FirebaseDatabase.getInstance().setPersistenceEnabled(true); means that Firebase will create a local copy of your database which also means that every change that is made while you are offline, will be added to a queue. So, as this queue grows, local operations and application startup will slow down. So the speed depends on the dimension of that queue. But rememeber, Firebase is designed as an online database that can work for short to intermediate periods of being disconnected and not as an offline database.
Second, if are using many listeners, don't forget to remove the listener accordingly to the life-cycle of your activity like this:
databaseReference.removeEventListener(valueEventListener);

Can I use Firebase for applications that has just database operations?

I am working on a budget management application, which needs to store users incomes and expenses then show back to user. I am using SQLite in my project which takes lots of code to save and retrieve data. I wonder whether I can use Firebase just to save and retrieve data. Any answer is appreciated.

Yes, sure you can! Just just use the Firebase Realtime Database service. Think of it like a JSON-ish storage in the cloud :-)
As a bonus, you can enable offline persistence and query your data even offline:
Firebase applications work even if your app temporarily loses its network connection. In addition, Firebase provides tools for persisting data locally, managing presence, and handling latency.
The Firebase Realtime Database stores data returned from a query for use when offline. For queries constructed while offline, the Firebase Realtime Database continues to work for previously loaded data. If the requested data hasn't loaded, the Firebase Realtime Database loads data from the local cache. When network connectivity is available again, the data loads and will reflect the query.
You can enable disk persistence with just one line of code:
FirebaseDatabase.getInstance().setPersistenceEnabled(true);
Firebase database writes, while offline, would be added to a queue which would sync once network is back and online.

Firebase how to detect if cached data is available

I have an application that pulls some simple data from Firebase Realtime Database, and to make things simple here is the flow that I want to have.
If phone is connected to the internet:
Get the data from the firebase database
If phone is offline
If there is data in firebase cache -> get data from cache
If there is nothing in firebase cache -> show some default data stored locally in app
The problem is that I don't see possible way to detect if data is available in firebase cache. So when phone is offline I cannot see if I should display the locally stored data or cache from firebase database.

Agree above answer, Firebase will do it for you, if you do it in the right way. In order to solve your problem, you should understand the difference between the two approaches of firebase offline cache - reading-from-memory and reading-from-disk.
Read this article -> https://pamartinezandres.com/lessons-learnt-the-hard-way-using-firebase-realtime-database-c609b52b9afb
in your case, just add, FirebaseDatabase.getInstance().setPersistenceEnabled(true); to enable reading from disk
and add yourReferenceForFirebase.keepSynced(true); to keep the data synched
then your app will work as you expected.
But, only this part: "If there is nothing in firebase cache -> show some default data stored locally in app", seems impossible.

Firebase does it for you.
Firebase applications work even if your app temporarily loses its network connection.
Also firebase apps automatically handle temporary network interruptions.
Cached data is available while offline and Firebase resends any writes when network connectivity is restored.
If it is not enough and you need to work offline you can also enable the disk persistence just adding:
FirebaseDatabase.getInstance().setPersistenceEnabled(true);
More info here.
EDIT:
There is no way to know whether a value comes from the local cache or from the remote server.
There is only a way to know if your app is currently connected to the Firebase Database server.
To achieve it you can detect the connection state by listening for the .info/connected value.

How Secure is Firebase Local Database in Android?

I am planning to switch to Firebase as my local and online database for my Android app. As per the docs, Firebase stores changes to the local database first and then pushed it to the online DB when network is available.
In my app, I would be putting some really sensitive data about the user in the database. So here are my questions,
How secure is the local Firebase database?
How difficult is it for a well-intentioned hacker with the right tools to hack it?
Is it just a simple JSON file like the online database, which anyone with root access can open?
Thanks.

In a general sense, Firebase Realtime Database can be used while offline. However, the expectation is that the app is supposed to be connected most of the time, and changes to the database that happen while offline will be synchronized when it has connectivity. 100% offline use is not really a supported use case, because the canonical data store is on the server.
The local copy of the database is limited to (10MB, at least on Android this is the case). If you intend to write to the database beyond this limit while offline, it will evict part of your cached data to make room for whatever you’re adding. Then, you will no longer be able to read those evicted values until the app goes back online. Worse, managing a growing list of writes to apply when back online is taxing on the app, so you don’t want to plan a lot of writes while offline.
Also, if you have permissions or validations defined for your database, these can only be checked on the server. So, if you’re doing offline writing to your local cache and you no longer have an active listener, you may never know if those writes fail.
Because of these caveats, it’s better not to think of Firebase Realtime Database as an “offline” database. It’s better to think of it as a “synchronized” database that actively syncs to the server while connectivity is present.