Ndtyjky

I am developing messenger IOS app based on Firebase Realtime Database.
I want that all messages to be ordered based on timestamp.

There is a scenario as like below.

There are 3 clients. A, B and C.

1)

All clients register 'figure-1' listener to receive messages from others.

<figure-1>



ref.queryOrdered(byChild: "timestamp").queryStarting(atValue: startTime).observe(.childAdded, with:

{

    ....

    // do work for the messages, print, save to storage, etc.

    ....



    // save startTime to storage for next open.     

    startTime = max(timeOfSnapshot, startTime)

    saveToStorage(startTime)

}

2)

Client A write  message 1 to server with ServerValue.timestamp().

Client B write  message 2 to server with ServerValue.timestamp().

Client C write  message 3 to server with ServerValue.timestamp().

They sent messages extremely the same moment.

All clients have good speed wifi.

So, finally. Server data saved like 'figure-2'

<figure-2>



text : "Message 1",  timestamp : 100000001

text : "Message 2",  timestamp : 100000002

text : "Message 3",  timestamp : 100000003

As my listener's code, i keep messages on storage and next listening timestamp for preventing downloading duplicated messages.

In this case.

Does Firebase always guarantee to trigger callback in order as like below?

Message 1

Message 2

Message 3

If it is not guaranteed, my strategy is absolutely wrong.

For example, some client received messages as like below.

Message 3  // the highest timestamp.



// app crash or out of storage



Message 1

Message 2

The client do not have chance to get message 1, 2 anymore.

I think if there are some nodes already, Firebase might trigger in order for those. Because, that is role of 'queryOrdered' functionality.

However, there are no node before register the listener and added new nodes additionally after then. What is will happen?

I suppose Firebase might send 3 packets to clients. (No matter how quickly the message arrives, Firebase has to send it out as soon as it arrives.)

Packet1 for message1

Packet2 for message2

Packet3 for message3



ClientA fail to receive for packet 1,2 

ClientA success to receive for packet 3

Firebase re-send packet 1,2 again. 

ClientA success to receive for packet 1,2 

Eventually, all datas are consistent. But ordering is corrupted.

Does Firebase guarantee to occur events in order?

I have searched stack overflow and google and read official documents many times. However, i could not find the clear answer.

I have almost spent one week for this. Please give me piece of advice.

The order in which the data for a query is returns is consistent, and determined by the server. So all clients are guaranteed to get the results in the same order.

For new data that is sent to the database after the listeners are attached, all remote clients will receive it in the same order. The local client will see events for it's write operation right away though, before the data even reaches the database server.

In figure 2, it is actually quite simple: since each node has a unique timestamp, and they will be returned in the order of that timestamp. But even if they'd have the same timestamp, they'd be returned in the same order (timestamp first, then key) for each client.