Develop Reference

Get notification when my app server port down

I have an app server running on port :4567/. Frequently my server gets down and unable to monitor this. Is there a way to notify me a email are message to show that my server gets down.

You can use an external service such as https://cronitor.io/ to do this. Another one is https://uptimerobot.com/

For a push notification, is a websocket mandatory?

I have PHP on the server side, and HTML and javascript on the client side.
I am making an app where a stakeholder types a message that is broadcasted to multiple recievers of a group in real time.
I did some research on google and I understand I need to use WebSockets or Comet for real time push notifications. Is WebSocket or Comet mandatory for sending mass notifications to users?
Is my understanding correct? Any references to start with?

If the client is a browser, then the ONLY two ways a standard browser can connect to a server is via an Ajax (e.g. http) request or a webSocket connection. So, if you want a client to get notified of something from the outside world it has to use one of those two mechanisms.
HTTP requests are transitory. The client makes a request of a server, the server responds. HTTP requests are perfect for the client requesting information from the server. They are not very good at the server sending information to the client because normally the client is not connected. There are hacks and work-arounds where the client "polls" the server on some interval and maybe even the server uses longer running requests to try to simulate a "push" type system, but they are sub-optimal hacks at best.
webSockets are continuous connections. The client connects and the connection remains in place for as long as both sides want. This allows either side the ability to send a message to the other side whenever they want. That means the server can "push" data to the client whenever it wants. webSockets are efficient for push connections and are recommended (this is one of the main things they were designed for).
Comet is a library that was originally built for using HTTP to try to "hack" or "simulate" push before webSockets were invented and then before they were widely supported. I can think of no reason why one would want to use Comet instead of a webSocket unless you had such an old browser that webSocket was not supported.
So, if you are trying to do "realtime server push" to a browser, then you must have a continuously connected socket from the client which means webSocket (or something built on top of webSocket like socket.io).
For phone apps where you have access to the phone SDK, you can use the "push" system built into the OS to push some messages from server to client. This isn't quite the same as the two way webSocket channel, but since you asked about "push notifications", the OS push services available in both Android and IOS could also be an option for pushing notifications from server to client. Here's info on iOS notifications and Google Cloud Messaging
As of 2016, one can also use Server-sent events in all modern browsers except Microsoft browsers (not supported yet in Edge or IE) to push data from server to client. Here's a browser compatibility table. Server-sent events use a long lasting HTTP connection, a special MIME type and a supporting client in order to be able to send events from server to client at any time. Unlike webSockets, server-sent events are one way only (from server to client). A client would then use a traditional Ajax call in order to be able to send data to a server (whereas with a webSocket data can be sent either way over the same webSocket connection).
Here's a good description of how server-sent events work: How do server-sent events actually work?

Is your client application a SPA? (Single Page application)?
It's very important because if not, you have to consider that everytime a client change page, connection with websocket server will be lost.
In this case you have to manage a queue because if stakeholder send a multicast request when one client is disconnected, client won't receive nothing.
Polling won't solve this situation too and it's an orrible solution because mobile clients (for example) with typical internet plan, will consume megabytes for unuseful "ping" traffic.
A real example of polling is a child in a car asking his dad every minute if they are arrived to a destination!
So, Is there a solution without using spa?
Yes, using a "shared storage" between stakeholder and clients, and using websocket only for "wake up" online clients saying: Hey there is something new, go to check!
Everytime a client open a page it will receive from backend also not-read notifications, taken from the storage.
When a stakeholder want to notify something, it will just store the notification message in the shared storage and send a "pulse" to notification server.
Notification server will forward the "pulse" to online clients (just in case someone is stuck reading a page).
If a "pulse" is lost because a client is changing page there is no problem because the client will bring notifications from the storage.
Every page will contain this logic:
Retrive number or unread notifications (server side)
Connect to the notification server after 5 seconds (javascript side).
Hope it helps.

I would suggest that using webSockets is a more efficient way compared to other options, why is this? Well when a client receives a notification that there's a change in the server there is no need to create an AJAX call to the server to get that change, it can be sent to the client with the same webSocket connection more easily than AJAX. This means efficient code and a faster running App!

Send message from Node.js Server to Client via function call

I want to send messages from my server to my client when a function is called. Using the code from this answer messages can be successfully sent from Server to Client every second.
I am building an application that runs node in the background, ideally I would like to be able to click a button that will call a function in the node server.js file which takes a parameter and sends that message to the client. The function in question would look like this
function sendToClient(message) {
clients[0].emit('foo', msg);
}
This would send the passed in message to the first client. How can I go about this?
In terminal, after you run node server.js is there a way to call a function from the server file using terminal, this could be a possible solution if so.

The best way to send messages from server to client right now is using webSockets. The basic concept is this:
Client A loads web page from server B.
Client A runs some javascript that creates a webSocket connection to server B.
Server B accepts that webSocket connection and the socket stays open for the duration of the life of the web page.
Server B registers event handlers to handle incoming messages from the web page.
Client A registers event handlers to handle incoming messages from the server.
At any point in time, the server can proactively send data to the client page and it will receive that data.
At any point in time, the client may sent data to the server and it will receive that data.
A popular node.js library that makes webSocket support pretty easy is socket.io. It has both client and server support so you can use the same library for both ends of the connection. The socket.io library supports the .emit() method mentioned in your question for sending a message over an active webSocket connection.
You don't directly call functions from client to server. Instead, you send a message that triggers the server to run some particular code or vice versa. This is cooperative programming where the remote end has to be coded to support what you're asking it to do so you can send it a message and some optional data to go with the message and then it can receive that message and data and execute some code with that.
So, suppose you wanted the server to tell the client anytime a temperature changed so that the client could display in their web page the updated temperature (I actually have a Raspberry Pi node.js server that does exactly this). In this case, the client web page establishes a webSocket connection to the server when the page loads. Meanwhile, the server has its own process that is monitoring temperature changes. When it sees that the temperature has changed some meaningful amount, it sends a temperature change message to each connected client with the new temperature data. The client receives that message and data and then uses that to update it's UI to show the new temperature value.
The transaction could go the other way too. The client could have a matrix of information that it wants the server to carry out some complicated calculation on. It would send a message to the server with the type of calculation indicated in the message type and then send the matrix as the data for the message. The server would receive that message, see that this is a request to do a particular type of calculation on some data, it would then call the appropriate server-side function and pass it the client data. When the result was finished on the server, it would send a message back to the client with the result. The client would receive that result and then do whatever it needed to with the calculated result.
Note, if the transactions are only from client to server with a response then coming back from the server, a webSocket is not needed for that type of transaction. That can be done with just an Ajax call. Client makes ajax call to the server, server formulates a response and returns the response. Where webSockets are most uniquely useful is if you want to initiate the communication from the server and send unsolicited data to the client at a time that the server decides. For that, you need some continuous connection between client and server which is what a webSocket is designed to be.
It appears there may be more to your question about how to communicate from a C# server to your node.js server so it can then notify the client. If this is the case, then since the node.js server is already a web server, I'd just add a route to the node.js server so you can simply do an http request from the C# server to the node.js server to pass some data to the node.js server which it can then use to notify the appropriate client via the above-described webSocket connection. Depending upon your security needs, you may want to implement some level of security so that the http request can only be sent locally from your C# server, not from the outside world to your node.js server.

In order to send a command to a client via the console there are two options, single process or multiprocess:
Single Process
When the command is run from console, temporary socket.io server starts listening on a port.
Once the client connects, send the message to the client.
Disconnect and stop the console app.
The reason this works is that socket.io clients are always trying to connect to the server. As long as the browser is open, they will try to connect. So even if the server only comes on for a few seconds, it should connect and receive messages. If the client is not running then simply create a timeout that will stop the console app and inform the user that it failed to broadcast the command.
While this approach is very easy, it's not robust nor efficient. For small projects this would work, but you'll have better luck with the next approach:
Multi-Process
This approach is much more reliable, expandable, and just better looking when you are talking about architecture. Here's the basic summary:
Spin up a stand-alone server that connects with clients.
Create a very similar console node app that will send a message to the server to forward on to clients.
Console app completes but the main server stays up and running.
This technique is just interprocess communication. Luckily you already have Socket.IO on the primary server, so your console app just needs to be another socket.io client. Check out this answer on how to implement that.
The downside to this is that you must secure that socket communication. Maybe you can enforce it to just allow localhost connections, that way you need access to the server to send the run command message (you absolutely don't want web clients executing code on other web clients).
Overall it comes down to the needs of your project. If this is a quick little experiment you want to try out, then just do it single process. But if will be hosting an express server (other webservers are available) and need to be running anyways, then multi-process is the way to go!
Example
I've created a simple example of this process using only Socket.io. Instructions to run it all are in the readme.
Implementations
In order to have C# (app) -> Node.js (server) -> Browser (client) communication then I would do one of the following:
Use redis as a message queue (add items to the queue with the app, consume with the server, which sends commands to client).
Live on the wild side and merge your NodeJS and C# runtimes with Edge.js. If you can run NodeJS from C# you will be able to send messages from your app to the server (messages are then handled by the server, just like any other socket.io client-server model).
Easier, but still kinda hacky, use System.Diagnostics.Process to start a console tool explained in the Multi-Process section. This would simply run the process with arbitrary parameters. Not very robust but worth considering, simple means harder to break (And again, messages are then handled by the server, just like any other socket.io client-server model).

I would create a route for sending the message and send message from post parameter. From CLI you can use curl or from anywhere really:
app.get('/create', function(req, res) {
if( data.type && data.content && data.listeners){
notify( data );
}
});
var notify = function( notification ){
ns_mynamespace.in(notification.listeners.users)
.emit("notification", {
id: notification.id,
title: 'hello', text: notification.content });
}
}

socket.io - encrytpting comunication between server and client

Hello everytone,
I have created a simple socket.io server that sends messages to a web application, once the application receives anything, it responds back to the server with another message..
And I suppose that all the communication between them is unencrypted. Now the question is, what's the best way to make the communication encrypted?
Is there any module for this?
Thanks in advance,
Alex

Just use wss:// as protocol instead of ws:// in the JavaScript. Then (if your server supports it) you connection will be secure.
Sadly i can't add any details, cause i dont know what server you are using..

callback javascript

I write a browser game (php, javascript) and I do not know how to make a callback. necessary that the server itself
found a client and call the function (only had one)

Don't write a browsergame if you don't know the basics! Browsergames are way too complex to learn programming.
If you want to make the server notify a client about something you will need to keep a connection open (search keywords: COMET, long polling) as you cannot initiate connections from the server to clients.

For this I can suggest you using Firebase. It is a API that let you to add cloud Data management that your user clients do. You can use that communication to search for client.

If I understand your question, what you need is a socket. Since you're using PHP and Javascript, a WebSocket might be just what you're looking for. With WebSockets, the connection between the client and the server is persisted, so the server can just push data/messages to any or all of the clients connected to it at any point in time. Likewise, the any client connected to the server can push messages/data up to the server.
Here's a video that describes how it works a bit https://www.youtube.com/watch?v=oJxWhmt5m-o