Develop Reference

One to One Android Chat using Firebase

I need some clarifications.
I'm using Firebase realtime database.
Now I've to implement a realtime chat between two android devices. I need also push notifications.
So, the solution based on realtime database is to drop because if the app isn't running, it will not handle the onChildAdded event.
I read on the documentation that I can achieve my aim, using the upstream message from the device to the server.
Now, it's written also that we need an app server.
I just want to be clarified the need of the app server (XMPP or HTTP), and which is the entire flow of message m1 sent from the device A, and the notification on the device B.
In my opinion the app server should act as a man in the middle, so:
A ---> XMPP SERVER ---> FCM
FCM ---> XMPP SERVER ---> B
Is this the flow?
On the documentation I can't understand if I'm right or not.
If so, how do I send a message from the FCM back to the XMPP Server?
The notification message is sent from FCM to the client o from XMPP Server?
Please someone clarify me.
Other solution thought by me:
Since I find really powerful firebase realtime database, I thought these solution to walk around the problem:
Create an app server the is listening for onChildAdded and when it changes send a notification to client device.
Creating a background service on the client app listening to onChildAdded and create notification when necessary.
Even all, I don't like these solutions.
Just want to understand the standard and correct way to achieve my aim.

Firebaser and author of the article debated in the question comments here
To send messages to a device with Firebase Cloud Messaging, you need to specify your project's FCM Server Key. As its name suggests, this key should only be used in code that runs in a trusted environment: i.e. a server that you control.
To send device-to-device messages (such as in a 1:1 chat application where the receiving user is not necessarily online) you need two steps:
one user send an upstream message
the other user receives the downstream message
Using Firebase Cloud Messaging and your custom app server code, you can handle step 2. But that still leaves step 1: the user needs to send a message that somehow triggers your code on the app server. There are quite a few ways to do this.
For example you could implement an HTTP endpoint on your app server and have the chat application send the messages there too (in addition to sending to the database).
You could also implement an XMPP endpoint and have the chat application send the message there (again in addition to sending it to the database).
My article suggest yet another way, one that doesn't require implementing an endpoint at all. It uses a node.js script that runs on your app server and is essentially just a client to the database. Whenever this script detects a relevant chat message, it calls the FCM API and sends the downstream message.
This pattern of using the Firebase Database as your endpoint and then using server-side scripts is quite common when using Firebase. We documented it in our classic blog post Where does Firebase fit in your app? (as option 2) and in this article in the Google Cloud documentation.

How Messages are Received on WhatsApp or any other messaging service?

Can anyone please tell me how whatsapp messages are received on my mobile? Like, does my phone requests whatsapp serve every second or does the server send my phone a ping when a message for me is received at the server end. I don't want to know the exact procedure, but an overview will do the job.

Well i guess, its just not that easy to answer in this scope however here is how it might work.
For this you may need understanding of technology on mobile side and server side.So let me try to explain it in a very simple manner which may be the implementation.
1) No, your mobile does not ping the server everytime. Polling like this adds lots of overhead on the network, also multiple calls can be stacked up if there is delay in responding thus leading to huge traffic on server side. Instead , its other way round. Whenever the server recieves any msg it pings you instead to say that you recieved a msg.
2) Hope point 1 makes it clear that its other way around. Now would like you to know about push notification. Push notification is something by whichserver can notify mobile of any new msgs or information or update.Here's a link which can give more detail on push notification for android https://parse.com/tutorials/android-push-notifications
3)How the server processes the request. This itself is a big topic. Basically it goes like this.I assume there are two users, A and B
First Mr A send a msg to Mr B
Server recieves this request with information of the user A(i.e the person who sent the msg) and user B(i.e the person to whom the sg is to be sent)
The server does some processing to get to know the user id of Mr B and other infomation about Mr B and then does a push so that user B can recieve this notification.
Of course a lot of technology is involved to do this job in order to handle millions of concurrent connections and responding fast without blocking other connections.
If you want to have a basic view of the technology, i would suggest you to look at socket io and node js. Heres a chat example
http://socket.io/get-started/chat/
Hope this helps

WhatsApp uses XMPP (eXtensible Messaging and Presence Protocol) to handle the message delivery system.
XMPP is mostly like HTTP where the client opens the socket with the XMPP server and keeps it open as long as the client is logged in. It's not like the regular REST API where the client opens the socket send/receive the data and close the socket. The socket is opened as long as you are signed in. In case of WhatsApp that's eternity (not really, WhatsApp reconnects automatically if the connection terminates)
XMPP protocol has been used in various chat applications such as Google Talk, Facebook messenger etc.
As far as actual technology goes, WhatsApp uses heavily customized version of Smack library on Android to build their client and uses customized eJabberd Server to handle the XMPP traffic. They might have different backend solution for handling the data though which might be on one of the cloud storage/computing network (I think it's heroku, no real idea though).
On iOS and other platforms, I suppose they might have developed their own libraries. Developing own libraries is not a lot of work, especially when you have customized needs and have a team of developers. I have used one of the libraries available for Windows Phone and heavily customized them to work for us. I made some improvement on the library, but due to time shortage I couldn't submit them to the original repo (Documentation is really tough).
Anyways, if you are interested in learning the tech, you can read Oriely's "XMPP: The Definitive Guide" and can visit The XMPP Standards Foundation.

Basically to say that when there is a message, Server will sends a notification to Client via Push Service. In before Client must register to use Service with Server, Server will provide an Id for Client and then uses it to specific which Client will receive the message.
Read more in Here and Here

GCM between User to User from same Android App

I would like to implement GCM in my Android app:
One Android app
Registered- U_user and another M_user.
I have Google Sign In in my app thats works fine with JSON and GSON and local MySQL database server with tomcat-server restutil to keep storing Gmail login users.
So, I want to send a notification from U_user to M_user.
U_user does a registration that has been pushed to M_user.
How can I do this?

I am assuming you mean the same Android app installed on different phones (U_user's phone and a separate phone for M_user).
To quote a section in the docs:
To make sure that messages go to the intended user:
Your app server can maintain a mapping between the current user and the registration ID.
The app can then check to ensure that messages it receives match the logged in user.
What this means is that you can send messages to another user through GCM. To let the app server know what to send to M_user, let U_user send an upstream message to your GCM app server (only possible in app servers using XMPP - see here). Alternatively, you have the option to do POST requests to your app server via HttpUrlConnection (can work on both XMPP- and HTTP-based GCM app servers).
Regardless of which method you use, specify in these messages to the server that you would like to send a message to M_user.

Implementing a chat application server for clients on Android/Windows Phone

I need to implement a group chat server which can support more than 50 users at a time.
The users will be only on mobile clients which include Android/Windows Phone.
Sending push notifs to the apps will not be difficult, as the server needs to do a basic curl request. So "polling" is not an issue.
My current skill set includes.
Flask
Google App Engine( I intent to use it or a web hosting
server.)
Basic php
So from what I know, I can make http requests to the server with user specific information+chat msg.
The server processes this and sends a broadcast using push notifs to all the devices stored in database.The server identifies the device from the data in the http request.
How efficient will this be?
If this is not the correct approach, what all do I need learn(sockets programming?) and any framework which can make my work easier (it will help if it is also supported on GAE).

I think you need XMPP messaging. It's support by App Engine but not in PHP : https://developers.google.com/appengine/features/
XMPP permit to send and receive chat messages.

Using XMPP for push user notifications on Android - any way to target these so they don't go to Google Talk etc

I want to use XMPP to push data to my client which would translate to a notification for the user, and plan on using their google-id for login to my application. The way I understand XMPP is that its a simple message that goes directly to the adress, but I want the client to get it as instructions and turn it into gamestate information.
It would be very frustrating for a user to get a bunch of command tuples or hex garbage in their google chat! Am I misunderstanding the way this all works?

JIDs (Jabber/XMPP IDs and therefore Google Talk IDs) have several forms.
First is a "bare JID", e.g.: user#example.com
Secondly is a "full JID", e.g.: user#example.com/xyz123, where xyz123 is called the resource.
One user can be logged into their XMPP account from multiple clients simultaneously, so long as each client uses a unique resource. With Google Talk, usually the server allocates the resource.
When you talk to someone on XMPP normally you address messages using their bare JID — you don't care about which client the message gets delivered to; that person's server directs it to the appropriate client(s) based on their availability etc.
But you can also address a message directly to a full JID. This is what you want to do, so that only user#example.com/my-game-some-unique-ID receives your game-specific messages.
Whether the Google App Engine API gives you this flexibility, I have no idea :)
An alternative — again, if GAE allows this — is to send a different message type.
XMPP is eXtensible, meaning that you don't have to send a plain old <message/> stanza to the end user; you can stick a specialised payload inside there like a <game/> child stanza and, because regular IM clients don't know how to parse your <game/> stanzas, they show nothing to the user. Only your game client will parse and understand this info.

“Xtify has an out of the box solution that runs a variant of XMPP. You can then use their webservice to push notifications and custom data and intents to the device and your app. I've been using there more standard solution (platform for push and managment), but the push API works and is free.