public class LogOutService extends Service {

    private String sessionId;
    private String userId;

    public LogOutService() {
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        sessionId = intent.getStringExtra("sessionId");
        userId = intent.getStringExtra("userId");
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onTaskRemoved(Intent rootIntent) {
        super.onTaskRemoved(rootIntent);
        final DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference();
        mDatabase.child("sessions").child(sessionId).child("users").child(userId).removeValue()
                .addOnSuccessListener(new OnSuccessListener<Void>() {
                    @Override
                    public void onSuccess(Void aVoid) {
                        mDatabase.child("sessions").child(sessionId).child("users").addValueEventListener(new ValueEventListener() {
                            @Override
                            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                                if (dataSnapshot.getChildrenCount()==0){
                                    mDatabase.child("sessions").child(sessionId).removeValue();
                                }
                            }

                            @Override
                            public void onCancelled(@NonNull DatabaseError databaseError) {

                            }
                        });
                    }
                });
        this.stopSelf();
    }
}