事件发生时获取从Express到angular的http调用

Question

i'm building an angular app that will make about a thousand people to connect simultaneously to book a ticket. I want only "XYZ" of them to access simultaneously at the registration Angular component. The other ones will see a "waiting room" component until it's their turn. I set up the whole thing like this:

1. User enters the page.
2. I make an http call to expressjs server
3. The server checks if the "connections" collection constains less than XYZ docs
4. If true, it unlocks the user registation component and with an http post req, it creates a new doc in the db. if false it leaves it hidden and shows up the waitingroom component
5. When user leaves the page, his doc in "connections" collection gets destroyed with an http delete call.

Fully working.

The problem is now that i want to create a kind of "priority" system, because, going like that, if you just refresh you may be lucky and get access, even if you are soon arrived and there is who is waiting since 1990's. So i introduced a "priority" system. When the user makes the first http call, if user is not allowed, the server creates a timestamp and pushes it into an array.

const timestamps = []
.
.
.
// this below is in http get req
Connessione.countDocuments({},(err,count)=>{
    if(count<=nmax){
      console.log("Ok")
      res.status(200).json({allowed: true})
    }
    else{
      const timestamp = req.params.timestamp;
      timestamps.push(timestamp);
      console.log("Semo troppi")
      res.status(401).json({allowed: false})
    }
  });

The idea is to listen to db changes, and when there is just XYZ-1 in the db. Make a call to the first timestamp's angular frontend to say him: "Hey there, if you want we're done. You can go" and unlock him the access to registration component.

The problem is that i can't make continuous http requests every second from angular until there's a free place... Is there any method to send a request at the server, and when server says OK, calls angular and says "Hey dude. You can go!"? Hope you understood my question. If not ask me in the comments. Thanks in advance

Answer 1

Even i had trouble with sockets in the beginning so i'll try to explain the concept in a simple way, Whenever you write an API or Endpoint you have a one way connection ie you send request to server and it return back some response as shown below.

Event 1:
(Client) -> Request -> (Server)

Event 2:
(Client) <- Response <- (Server)

For API's, without request you cannot get response.

To overcome this issue as of now i can think of two possible ways.

1. Using Sockets , With sockets you can create a two way connection. Something like this

(Server) <-> data <-> (Client)

It means you can pass data both ways, Client to server and Server to client. So whenever an event occurs(some data is added or updated in database) one can emit or broadcast it to the client and the client can listen to the socket and receive it.

In your case as it's a two connection you can emit the data from angular and I've attached few links at the bottom. please have a look.

2. Using XML/AJAX Request , This is not a preferable method, using setInterval you can call the server in every 5 seconds or so and do the operation needed.

setInterval(ajaxCall, 5000); //5000 MS == 5 seconds

function ajaxCall() {
    //do your AJAX stuff here
}

Links:

• https://socket.io/docs/

• https://alligator.io/angular/socket-io/