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Overhead of WebSockets on Apache Server

I have built a website with PHP + Apache + JS + HTML 5. Now there is a point where I have to tell the user every second if the server is connected and ready to receive data and/or internet connection is lost/available or whatever else which can tell the user to not to send data to server because of unavailability of Server or internet connection.
For this purpose I can either move to Long Polling with Ajax and keep pinging my server every second but surely this will cause alot of network overhead on my Apache Server where my clients are about tens of thousands live at a time so keep pinging the server is not a good option. Therefore, I decided to use WebSockets.
I have been googling for about 2 days but yet could not find enough good article to answer my 3 basic questions regarding WebSocket and Apache + PHP.
1) If once the WebSocket connection is made with server, then does it remain active like Long Polling with Server or what is the mechanism behind it? I mean how does WebSocket maintain its relation with server does it keep polling with server and hence there is always a connection between Client and Server?
2) If your answer to above question is yes then what network/IO overhead would be on server side if I use websockets because there is a continuous connection between Client and Server. And imagine if there are hundreds of thousands of clients online at a time what load will it create on Server Network or IO?
3) What is the Best approach to use WebSockets when using Apache + PHP on server? Any good article on this where I can study how to communicate with Apache Server using WebSockets? I found this question, but it doesn't answer well Using WebSocket on Apache server. In this question, it limits the experts to not to include any answer which has sysadmin tools, while I am asking for it if any required.
I have a VPS Server so tunning and installing some tools isn't a problem.
Any help would be highly appreciated. Thanks with Regards.

1) If once the WebSocket connection is made with server, then does it remain active like Long Polling with Server or what is the mechanism behind it? I mean how does WebSocket maintain its relation with server does it keep polling with server and hence there is always a connection between Client and Server?
Yes, it remains active and there is always a connection between Client and Server. However, the client needs to maintain the connection and on connection exceptions. In this case, it is your javascript code.
2) If your answer to above question is yes then what network/IO overhead would be on server side if I use websockets because there is a continuous connection between Client and Server. And imagine if there are hundreds of thousands of clients online at a time what load will it create on Server Network or IO?
WebSocket connections are handled on TCP/IP level and by definition, they are not resource consuming operations when there is no data going through the TCP tunnel. So rather than worrying about your CPU and memory conception, you need to worry about the limit on the number of connections. Consider using a load balancer for your socket connections and utilize multiple servers if you are expecting more than 10000 concurrent connections.
3) What is the Best approach to use WebSockets when using Apache + PHP on server? Any good article on this where I can study how to communicate with Apache Server using WebSockets? I found this question, but it doesn't answer well Using WebSocket on Apache server. In this question, it limits the experts to not to include any answer which has sysadmin tools, while I am asking for it if any required.
For such a use case of yours - getting the status of the server, I would suggest using a message broker rather than load this simple operation to Apache.
Please consider looking at mosquitto, hivemq or rabbitmq
All of them are supporting WebSockets and all of them have their pros and cons. Do the small proof of concepts over them and choose what is best for you.

setInterval($.get or socket.io? [duplicate]

I am developing a web app and I was wondering which method should be suitable for my project.
Basically what I want to display to the users are some notifications which are taken from requests to other servers. My node.js app gets all the info and then it spread out to the users, saving a copy into my MongoDB.
The idea is quite simple but reading about methods I found these two techniques:
Ajax : Client side would be checking all time if there is new content on the server. This would be done by using a jquery ajax get to my server API (every 30/60 seconds).
Socket.io : The client connects once, and then a permanent TCP connection is maintained (more realtime).
Now I have explained the situation, I have the following questions :
Would I not have too many requests with ajax ? imagine I want a check every minute to the server, if we scale the app to 100 users, it will give me 100 queries per minute. Would it be "cheaper" in system resources to have a socket ?
Would the socket.io be a problem for mobile devices ? bandwith and performance. The response of the server is always info in JSON format.
I read that now.js could be used for this but it seems the project is no longer supported, so not sure if using it would be a good idea.
How is the caching on both methods ? I was considering to create a cache file for each user and this would be updated by the node.js in the server side. I guess this could work really well with ajax but what about socket.io ?
Is it true that socket.io is not compatible at all with many browsers ? My app would be more focused to mobile devices and I think this could make me think about choosing ajax instead.
Any alternative suggested ?
I hope this could clear my mind and others who are in the same situation :)
Thanks

Many of the generic tradeoffs between webSocket and Ajax are discussed here:
websocket vs rest API for real time data?
Some tradeoff issues for mobile devices are discussed here:
Cordova: Sockets, PushNotifications, or repeatedly polling server?
In a nutshell, if your data is primarily server-driven and then needs to be sent out to clients and you desire fairly good latency for when the clients see the new data, then that is the exact problem that webSockets are good for. webSockets work best in this situation because the client does not need to poll frequently, the server does not need to handle regular polling requests from lots of clients. Instead, each client just sets up the one persistent webSocket communication channel which the server can then send data down upon demand at any time.
Would I not have too many requests with ajax ? imagine I want a check
every minute to the server, if we scale the app to 100 users, it will
give me 100 queries per minute. Would it be "cheaper" in system
resources to have a socket ?
Sockets require very little resources when they are not active so yes, a peristent webSocket is more efficient than lots of clients polling endlessly. This is why webSockets were invented because they are better at solving this particular problem.
Would the socket.io be a problem for mobile devices ? bandwith and
performance. The response of the server is always info in JSON format.
socket.io is not a problem for bandwidth or performance. There are some mobile issues with trying to use webSockets in the background because mobile devices are also trying to do active power management, though a similar issue exists with client polling too.
How is the caching on both methods ? I was considering to create a
cache file for each user and this would be updated by the node.js in
the server side. I guess this could work really well with ajax but
what about socket.io ?
It is unclear what you are asking about caching? In a webSocket implementation, the server gets the data and then just sends it to each user. No server-side caching would generally be needed. In a client Ajax polling implementation, the server would have to store the data somewhere and "wait" for each client to then request the data. There is no "built-in" caching mechanism for either webSocket or Ajax.
Is it true that socket.io is not compatible at all with many browsers
? My app would be more focused to mobile devices and I think this
could make me think about choosing ajax instead.
socket.io is fully compatible with all browsers that have webSockets which is pretty much everything in use today except for IE9 and older. If you use the socket.io library, it will automatically fall back to long polling if webSockets don't exist. Your mobile issues are likely going to be similar whether you're doing regular polling or webSocket because the mobile device wants to power manage long running things, but you don't want to stop polling. I don't think this is a reason to avoid webSockets/socket.io. socket.io has some really nice auto-reconnect logic whenever it loses the connection which can be really useful.
In the mobile world, I think you're just going to find that you can't reliably do real-time notifications in the background without using some sort of native app component that can plug into the native "push" system on the device because that's the only system that is both battery efficient and fully compatible with power management. A web page is going to be power managed as soon as it is not the foreground task or when the device has been idle.

Advice on which technology to use for real time notifications

I have X amount of activity sensors connected to a server that inserts data to a database everytime a sensor is triggered. What I'm trying to do is create a web interface with a blue print of the facility (svg) and whenever a sensor is triggered, besides the db insert, I want it to show some sort of alert in my blue print. For that I need to keep an open connection to the server I think.
I was thinking of using web sockets, but it might be overkill since I only need to retrieve data from the server. But running an ajax call every second doesn't sound very efficient either. Are there any other alternatives?
Thank you

Some potential choices include:
WebSocket
Adobe® Flash® Socket
AJAX long polling
AJAX multipart streaming
Forever Iframe
JSONP Polling
Which actual transport you end up using will depend on the your requirements for browser support and what technology you are using on the server to handle these requests. The transport choice may also depend on your network topology - what types of load balancers you need to integrate with, proxies, etc.
There are many libraries available on both the client and server sides, many of which support more than one of these transports.
For example (not an exhaustive list):
socket.io for nodejs
WebSocket
Adobe® Flash® Socket
AJAX long polling
AJAX multipart streaming
Forever Iframe
JSONP Polling
SignalR for an asp/.net backend
WebSockets
Server-Sent Events
ForeverFrame
Long Polling
Atmosphere for a java backend
WebSockets
Server Side Events (SSE)
Long-Polling
Forever frame
JSONP
IMO - Websockets is NOT overkill for this type of problem and would lend itself nicely to this type of application.

Without specifically discussing frameworks or knowing what is running in the backend of your server(s), we have a few options to consider for the frontend:
Websockets
Websockets are designed for bidirectional communication, although it is kind of shocking how many users are surfing the web in a browser that doesn't support websockets. I always recommend a fallback for this, such as the other methods listed below.
SSE
SSE is an HTML5 spec and is still shaky at best. Try scrolling on a page while when an SSE event fires... It may be a little easier on the backend, put it sometimes hangs on the client side since it runs inside the same thread that the DOM is running in.
Long Polling
Keeps your connection open. It doesn't scale well with PHP, but performs swimmingly with Python+Twisted on the backend, or Node.Js
Good Old Ajax
Keep your requests small, and you still have a scalable solution. Yes, a full GET request is the most expensive, but is supported in just about every browser rolled out the past ten years. It is also worth noting that GET requests are easy to scale horizontally with more hardware.
In a perfect world:
You would break up your application into a few components, operating behind a reverse proxy such as Nginx. Then use Node.Js + Socket.IO handle the realtime aspects of your app.
Another option would be to use small Ajax requests, and offer websocket support for the browsers that support it. This is advice specifically for PHP in the backend.

WebSocket is certainly not overkill. On the contrary. With websockets, you have a bi-directional communication channel; this means, that the server can initiate communication whenever it seems fit (e.g. when sensor data changes).
In a previous project, I have used node.js together with socket.io, to monitor 50+ sensors. Data was updated in real-time in a browser. The data was visualized using smoothie.js.
Whenever a sensor value was updated, it was communicated to the browser. Some sensors only updated once a minute, others once a second, ...
Polling would have been overkill, because it would retrieve all data for all sensors, even from those that were not updated yet.

I had a similar problem and did a lot of research on this. As I understand it, there are three main options:
Short polling: Have an endpoint that your javascript client pings every second. This is the worst option, because the pings add latency up to one second to your communication, and depending on how you implement, the endpoint could query the database every second, adding unnecessary overhead.
Long polling: Have an endpoint that your javascript client pings that holds the connection until a) the event occurs or b) the connection times out. If the endpoint returns a response, the client gets the event information. If the endpoint does not return a response, no event has occurred, and the client sends a new request. This is a good option because the events can immediately trigger the response to the client, assuming you have an asynchronous interprocess communication layer (like 0MQ) to send the message without any sort of polling.
Websocket: Have your javascript client connect to a websocket server, which will send a message to your client immediately upon the event trigger.
I think a websocket is your best option, because it accommodates immediate communication of the event without all the request/response overhead. And most importantly, this is exactly what websockets are designed to do! As such, you will probably have to write the least amount of custom code with this solution.

There are two great commercial services that might work for you.
Firebase - a javascript hierarchical database and realtime
messaging/ synchronization platform, uses websockets and has other fallbacks
PubNub - a real time message passing and queue system, uses websockets

If I wanted to create an AJAX chat what communication technique should be used to maintain scalability?

I put together an AJAX chat a while back ASP.NET MVC and jQuery. The javascript would hit the server about every 7 seconds to check for new messages. Obviously this was horrible on performance as the chat grew and included more and more users. The site traffic grew exponentially with so many requests going on. A user could leave the computer on all day and not even be there and they would still be making hits every 7 seconds.
Is there a better way to do this? I have heard of something called "push" but I haven't really been able to wrap my head around it. I think I just need pointed in the right direction.
1.) What is the best way to develop an AJAX chat and have it be scalable?
2.) What is push and how would I just that with jQuery?

1.) What is the best way to develop an AJAX chat and have it be scalable?
I agree with #freakish about the complexity and potential lack of scaling of IIS.
However, there is a relatively new Microsoft option in the works called SignalR which could become a core part of ASP.NET. More details in this related SO Question:
AJAX Comet - Is there any solution Microsoft is working on or supports to allow it to be scalable?
2.) What is push and how would I just that with jQuery?
Partially answered elsewhere, but it's a long-held persistent connection between the server and the client which means the server can instantly 'push' data to the client when it has new data available.
jQuery does support making AJAX requests but the core library doesn't support expose ways of doing HTTP Long-Polling or HTTP Streaming. More information in this SO answer to 'Long Polling/HTTP Streaming General Questions'.

Server push is a technology that allows the server to push data back to the client without forcing client to make many requests (like every 7 seconds). It is not really a matter of javascript but rather good server scripting. The upcoming HTML5 will make it simple due to server-sent events and/or WebSockets. This will be a true TCP connection between different machines.
But if you intend to make a webpage compatible with older browsers, then the most common technique is the long polling. The client sends request to the server and the server does not respond to it until it has new data. If it does then the response is made and the client immediatly after receiving data calls the server with new request. In practice however this requires the server to be well-written (for example it has to maintain thousands of idle requests at the same time) and can become a rather big challenge for developers.
I hope this helps. :) Good luck!

The technique you should use is the real-time persistent long running connections over a web page using WebSockets. You can use this library.

Node.js is becoming quite popular to build something like this and supports socket connections so you could push the data out only when there is a new message. But that would be learning something completely new.
Another nice potential would be to use MVC's OutputCacheAttribute and use the SQL dependency option so your AJAX page could be cached and would only be a new request when a new chat message appears. Also you would want your controller to be an Asynchronous controller to help reduce the load on IIS.
Enjoy, optimization is always fun and very time consuming!

Quick AJAX responses from Rails application

I have a need to send alerts to a web-based monitoring system written in RoR. The brute force solution is to frequently poll a lightweight controller with javascript. Naturally, the downside is that in order to get a tight response time on the alerts, I'd have to poll very frequently (every 5 seconds).
One idea I had was to have the AJAX-originated polling thread sleep on the server side until an alert arrived on the server. The server would then wake up the sleeping thread and get a response back to the web client that would be shown immediately. This would have allowed me to cut the polling interval down to once every 30 seconds or every minute while improving the time it took to alert the user.
One thing I didn't count on was that mongrel/rails doesn't launch a thread per web request as I had expected it to. That means that other incoming web requests block until the first thread's sleep times out.
I've tried tinkering around with calling "config.threadsafe!" in my configuration, but that doesn't seem to change the behavior to a thread per request model. Plus, it appears that running with config.threadsafe! is a risky proposition that could require a great deal more testing and rework on my existing application.
Any thoughts on the approach I took or better ways to go about getting the response times I'm looking for without the need to deluge the server with requests?

You could use Rails Metal to improve the controller performance or maybe even separate it out entirely into a Sinatra application (Sinatra can handle some serious request throughput).
Another idea is to look into a push solution using Juggernaut or similar.

One approach you could consider is to have (some or all of) your requests create deferred monitoring jobs in an external queue which would in turn periodically notify the monitoring application.

What you need is Juggernaut which is a Rails plugin that allows your app to initiate a connection and push data to the client. In other words your app can have a real time connection to the server with the advantage of instant updates.