Many sites have real-time notifications about a new message, etc. Despite the fact that these sites do not create a websocket connection and work only via https. How is this implemented? If the user just has a page open, all requests have already been sent to the server, there is no channel connection, how does the browser receive information about some event without refreshing the page?
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When speaking from a conceptual point of view, is it standard practice to mix WebSockets and HTTP requests when making a chat application (or any application that requires real-time communication between devices)?
Imagine a scenario with a client and a server in a chat app. What would be the best approach for connecting and sending data between the client and the server? Would it be using sockets for both sending and receiving or HTTP requests for sending (so the client would get a response and then know if the message was received), and then using WebSocket for only receiving new messages?
No this is not standard practice.
If you need real-time communication between client and server you normally just use a websocket connection and keep that one open. The client can send messages to the server and receive messages through the same connection.
Using HTTP requests for sending messages to the server and receiving new messages via websocket seems odd and just adds unnecessary complexity.
Now if your server has some endpoints to subscribe for real-time data e.g. a chat room and endpoints for getting information you don't necessary want to subscribe to e.g. information about a certain user, than you can use the appropriate protocol for each endpoint
I have a question regarding http requests and responses.
I know that I can send a request to a server from my device (I can build and send a GET request to http://google.com for example). But what if I am Google and I want to send a request from the server to the user's device? How do I do that?
I understand that when the server receives a request, it can answer it, but in this case I want the server to send the request to the user's device. Just like WhatsApp does when you receive a new message.
Thanks for the help!
There are several options for sending information from the server to a client:
Push notifications - depends on the platform you are using
Constructing a Websocket connection that allows bi-directional communication
I'm sure there are more options but those are the two that come up to my mind right away.
It really depends on your application use case. For example, a chat application would like to have a socket open between it and the server so it can update frequently on new messages, etc. On the other had some simple Calendar applications might want to use push notifications to send reminders on certain dates and times.
Please explain to me how webpush work in TCP/IP network layers (especially layer 4-5).
I understand that HTTP is stateless protocol:
the protocol is opening TCP / layer 4 connection,
'state' is 'made to work' with cookie/session,
then client send HTTP request (plaintext/compressed "HTTP/1.1 /url/here ... Content-Length: ..."),
then server respond with HTTP request (plaintext/compressed "200 OK ... ..."),
Therefore it's understandable that for a user behind NAT to be able to view webpage of a remote host (because the user behind NAT is the one initiating the connection); but the webserver cannot initiate TCP connection with the client (browser process).
However there are some exceptions like 'websocket' where client (browser) initiate a connection, then leave it open (elevate to just TCP, not HTTP anymore). In this architecture, webserver may send / initiate sending message to client (for example "you have new chat message" notification).
What I don't understand is the new term 'webpush'.
I observed that it can send notification from server to client/browser (from user, it 'feels' like the server is the one initiating the connection)
webpush can send notification anytime, even when browser is closed / not opened yet (as when the device was just freshly turned on), or when it's just connected to internet
How does it work? How do they accomplish this? Previously I think that:
either a javascript in a page is continously (ex: 5 second interval) checking if there's a new notification in server,
or a javascript initiate a websocket (browser initiate/open TCP connection) and keep it alive, when server need to send something, it's sent from webserver to client/browser through this connection
Is this correct? Or am I missing something? Since both of my guess above won't work behind NAT'd network
Is Firebase web notification also this kind of webpush?
I have searched the internet for explanation on what make it work on client side, but there seems only explanation on 'how to send webpush', 'how to market your product with webpush', those articles only explain the server side (communication of app server with push service server) or articles about marketing.
Also, I'm interested in understanding what application layer protocol they're running on (as in what text/binary data the client/server send to each other), if it's not HTTP
Web Push works because there is a persistent connection between the browser (e.g. Chrome) and the browser push service (e.g. FCM).
When your application server needs to send a notification to a browser, it cannot reach the browser directly with a connection, instead it contacts the browser push service (e.g. FCM for Chrome) and then it's the browser push service that delivers the notification to the user browser.
This is possible because the browser constantly tries to keep an open connection with the server (e.g. FCM for Chrome). This means that there isn't any problem for NAT, since it's the clients that starts the connection. Also consider that any TCP connection is bi-directional: so any side of the connection can start sending data at any time. Don't confuse higher level protocols like HTTP with a normal TCP connection.
If you want more details I have written this article that explains in simple words how Web Push works. You can also read the standards: Push API and IETF Web Push in particular.
Note: Firebase (FCM) is two different things, even if that is not clear from the documentation. It is both the browser push service required to deliver notifications to Chrome (like Mozilla autopush for Firefox, Windows Push Notification Services for Edge and Apple Push Notification service for Safari), but it is also a proprietary service with additional features to send notifications to any browser (like Pushpad, Onesignal and many others).
https://developer.mozilla.org/en-US/docs/Web/API/EventSource
The EventSource interface is web content's interface to server-sent events. An EventSource instance opens a persistent connection to an HTTP server, which sends events in text/event-stream format. The connection remains open until closed by calling EventSource.close().
From what I understand server-sent events require persistent HTTP connection (Connection: keep-alive) so similarly to keeping the connection alive like in case of web sockets.
If the connection is persistent, why server-sent events are unidirectional? Web socket connections are persistent as well.
In this case, what happens if I send a request to my HTTP service and I have persistent connection opened due to EventSource. Will it re-use HTTP connection opened by EventSource or open a new connection?
If it re-uses the connection opened by EventSource how is it considered unidirectional?
Might be trivial, but I had to ask because it is not clear. Because nothing mentions what happens to subsequent HTTP requests when there's existing connection opened by EventSource.
For example, it seems possible to me to implement centralized chat app using SSE:
User 1 sends message to User 2(by sending it to HTTP server). Server sends event to user 2 with a new message, user 2 sends another request to HTTP server with message for User 1, server sends event to user 1.
How is that not considered bi-directional?
Related:
What's the behavioral difference between HTTP Stay-Alive and Websockets?
SSE is unidirectional because when you open a SSE connection, only the server can send data to the client (browser, etc.). The client cannot send any data. SSE is a bit older than WebSockets, hence may be the difference between the unidirectional and bi-directional support between these two technos.
In your use-case, if you open a SSE connection (which is an HTTP connection), only the server will be able to send data. If you wish to send a request to your HTTP service, you will need to open a new "classical" HTTP connection. You will see your browser opening two HTTP connections: 1 for the SSE connection and 1 for the classical HTTP request (short live).
You can implement a chat with SSE. You can have a SSE connection (hence HTTP) to let the user receives the messages from the server. And you can use POST HTTP requests to enable the user to send his/her messages.
Note that most of the browsers can open around 6 HTTP/1.x connections to the same host. So, if you use 1 SSE connection, it will remain potentially 5 HTTP/1.x connections. This is only true with HTTP/1.x. With HTTP 2.x, the connections to the same host are multiplexed: so, in theory, you can send as many HTTP requests at the same time as you wish or you can open as many SSE connections as you wish and thus, by passing the limitation of the 6 connections.
You can have a look at this article (https://streamdata.io/blog/push-sse-vs-websockets/) and this video (https://www.youtube.com/watch?v=NDDp7BiSad4) to get an insight about this technology and whether it could fit your needs. They summarize pros & cons of both SSE and WebSockets.
I am new to SignalR and I have a question on SignalR communication when we introduce a load balancer.
Lets assume we want to execute a void method on server side which receives some data as a parameter from client. Server takes that data and processes further. Lets say after processing for a while, it identifies that it has to send the notification back to client.
Case 1(Between client and server): Client calls void method on server side(Hub) by passing some data. Connection gets disconnected. Server processes the client data further. When it identifies that it has to push the notification back to client, it recreates the connection and pushes back the data to client.
Case 2(Between client and server with load balancer in between): How does the above scenario(Case 1) work here?. When server sends the push notification back to load balancer after processing client data, how does it know to which client it has to send the notification back?
You should read the scaleout docs. Short version: messages get sent to all servers, so if the client reconnects (it's not the server that establishes the connection!) before the connection times out , it will get the message.
Quote from the docs:
The cursor mechanism works even if a client is routed to a different
server on reconnect. The backplane is aware of all the servers, and it
doesn’t matter which server a client connects to.