What are Long-Polling Websockets Server-Sent Events SSE and Comet

Question

I have tried reading some articles  but I am not very clear on the concepts yet   Would someone like to take a shot at explaining to me what these technologies are    Long Polling Server-Sent Events Websockets Comet   One thing that I came across every time was  the server keeps a connection open and pushes data to the client  How is the connection kept open  and how does the client get the pushed data   How does the client use the data  maybe some code might help    Now  which one of them should I use for a real-time app  I have been hearing a lot about websockets  with socket io  a node js library   but why not PHP

User · Answer

You can easily use Node JS in your web app only for real-time communication  Node JS is really powerful when it s about WebSockets  Therefore  PHP Notifications via Node js  would be a great concept   See this example  Creating a Real-Time Chat App with PHP and Node js

User · Answer

I have tried to make note about these and have collected and written examples from a java perspective  HTTP for Java Developers Reverse Ajax - Old style Async Handling on server side Reverse Ajax - New style Server Sent Events Putting it here for any java developer who is looking into the same subject

User · Answer

In the examples below the client is the browser and the server is the webserver hosting the website   Before you can understand these technologies  you have to understand classic HTTP web traffic first   Regular HTTP    A client requests a webpage from a server  The server calculates the response The server sends the response to the client       Ajax Polling    A client requests a webpage from a server using regular HTTP  see HTTP above   The client receives the requested webpage and executes the JavaScript on the page which requests a file from the server at regular intervals  e g  0 5 seconds   The server calculates each response and sends it back  just like normal HTTP traffic      Ajax Long-Polling    A client requests a webpage from a server using regular HTTP  see HTTP above   The client receives the requested webpage and executes the JavaScript on the page which requests a file from the server  The server does not immediately respond with the requested information but waits until there s new information available  When there s new information available  the server responds with the new information  The client receives the new information and immediately sends another request to the server  re-starting the process       HTML5 Server Sent Events  SSE    EventSource    A client requests a webpage from a server using regular HTTP  see HTTP above   The client receives the requested webpage and executes the JavaScript on the page which opens a connection to the server  The server sends an event to the client when there s new information available     Real-time traffic from server to client  mostly that s what you ll need You ll want to use a server that has an event loop Connections with servers from other domains are only possible with correct CORS settings If you want to read more  I found these very useful   article    article    article    tutorial        HTML5 Websockets    A client requests a webpage from a server using regular http  see HTTP above   The client receives the requested webpage and executes the JavaScript on the page which opens a connection with the server  The server and the client can now send each other messages when new data  on either side  is available    Real-time traffic from the server to the client and from the client to the server You ll want to use a server that has an event loop With WebSockets it is possible to connect with a server from another domain  It is also possible to use a third party hosted websocket server  for example Pusher or others  This way you ll only have to implement the client side  which is very easy  If you want to read more  I found these very useful   article    article   tutorial        Comet   Comet is a collection of techniques prior to HTML5 which use streaming and long-polling to achieve real time applications  Read more on wikipedia or this article        Now  which one of them should I use for a realtime app  that I need to   code   I have been hearing a lot about websockets  with socket io  a   node js library   but why not PHP     You can use PHP with WebSockets  check out Ratchet

User · Answer

Tieme put a lot of effort into his excellent answer  but I think the core of the OP s question is how these technologies relate to PHP rather than how each technology works  PHP is the most used language in web development besides the obvious client side HTML  CSS  and Javascript  Yet PHP has 2 major issues when it comes to real-time applications   PHP started as a very basic CGI  PHP has progressed very far since its early stage  but it happened in small steps  PHP already had many millions of users by the time it became the embed-able and flexible C library that it is today  most of whom were dependent on its earlier model of execution  so it hasn t yet made a solid attempt to escape the CGI model internally  Even the command line interface invokes the PHP library  libphp5 so on Linux  php5ts dll on Windows  etc  as if it still a CGI processing a GET POST request  It still executes code as if it just has to build a  quot page quot  and then end its life cycle  As a result  it has very little support for multi-thread or event-driven programming  within PHP userspace   making it currently unpractical for real-time  multi-user applications   Note that PHP does have extensions to provide event loops  such as libevent  and threads  such as pthreads  in PHP userspace  but very  very  few of the applications use these   PHP still has significant issues with garbage collection  Although these issues have been consistently improving  likely its greatest step to end the life cycle as described above   even the best attempts at creating long-running PHP applications require being restarted on a regular basis  This also makes it unpractical for real-time applications   PHP 7 will be a great step to fix these issues as well  and seems very promising as a platform for real-time applications

[php] What are Long-Polling, Websockets, Server-Sent Events (SSE) and Comet?

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