How does facebook gmail send the real time notification

Question

I have read some posts about this topic and the answers are comet  reverse ajax  http streaming  server push  etc   How does incoming mail notification on Gmail works   How is GMail Chat able to make AJAX requests without client interaction   I would like to know if there are any code references that I can follow to write a very simple example  Many posts or websites just talk about the technology  It is hard to find a complete sample code  Also  it seems many methods can be used to implement the comet  e g  Hidden IFrame  XMLHttpRequest  In my opinion  using XMLHttpRequest is a better choice  What do you think of the pros and cons of different methods  Which one does Gmail use   I know it needs to do it both in server side and client side  Is there any PHP and Javascript sample code

User · Answer

According to a slideshow about Facebook s Messaging system  Facebook uses the comet technology to  push  message to web browsers  Facebook s comet server is built on the open sourced Erlang web server mochiweb    In the picture below  the phrase  channel clusters  means  comet servers      Many other big web sites build their own comet server  because there are differences between every company s need  But build your own comet server on a open source comet server is a good approach   You can try icomet  a C1000K C   comet server built with libevent  icomet also provides a JavaScript library  it is easy to use as simple as   var comet   new iComet       sign url   http       app host     sign obj     obj      sub url   http       icomet host     sub       callback  function msg              on server push         alert msg content               icomet supports a wide range of Browsers and OSes  including Safari iOS  Mac   IEs Windows   Firefox  Chrome  etc

User · Answer

Facebook uses MQTT instead of HTTP  Push is better than polling  Through HTTP we need to poll the server continuously but via MQTT server pushes the message to clients    Comparision between MQTT and HTTP  http   www youtube com watch v -KNPXPmx88E  Note  my answers best fits for mobile devices

User · Answer

One important issue with long polling is error handling  There are two types of errors    The request might timeout in which case the client should reestablish the connection immediately  This is a normal event in long polling when no messages have arrived   A network error or an execution error  This is an actual error which the client should gracefully accept and wait for the server to come back on-line     The main issue is that if your error handler reestablishes the connection immediately also for a type 2 error  the clients would DOS the server   Both answers with code sample miss this   function longPoll              var shouldDelay   false             ajax               url   poll php               async  true                by default  it s async  but                dataType   json            or the dataType you are working with             timeout  10000              IMPORTANT  this is a 10 seconds timeout             cache  false             done function  data  textStatus  jqXHR                    do something with data                fail function  jqXHR  textStatus  errorThrown                 shouldDelay   textStatus      timeout               always function                    in case of network error  throttle otherwise we DOS ourselves  If it was a timeout  its normal operation  go again              var delay   shouldDelay   10000  0              window setTimeout longPoll  delay                 longPoll      fire first handler

User · Answer

The way Facebook does this is pretty interesting    A common method of doing such notifications is to poll a script on the server  using AJAX  on a given interval  perhaps every few seconds   to check if something has happened  However  this can be pretty network intensive  and you often make pointless requests  because nothing has happened   The way Facebook does it is using the comet approach  rather than polling on an interval  as soon as one poll completes  it issues another one   However  each request to the script on the server has an extremely long timeout  and the server only responds to the request once something has happened   You can see this happening if you bring up Firebug s Console tab while on Facebook  with requests to a script possibly taking minutes   It is quite ingenious really  since this method cuts down immediately on both the number of requests  and how often you have to send them  You effectively now have an event framework that allows the server to  fire  events   Behind this  in terms of the actual content returned from those polls  it s a JSON response  with what appears to be a list of events  and info about them  It s minified though  so is a bit hard to read   In terms of the actual technology  AJAX is the way to go here  because you can control request timeouts  and many other things   I d recommend  Stack overflow cliche here  using jQuery to do the AJAX  it ll take a lot of the cross-compability problems away   In terms of PHP  you could simply poll an event log database table in your PHP script  and only return to the client when something happens  There are  I expect  many ways of implementing this   Implementing   Server Side   There appear to be a few implementations of comet libraries in PHP  but to be honest  it really is very simple  something perhaps like the following pseudocode   while  has event happened         sleep 5      echo json encode get events        The has event happened function would just check if anything had happened in an events table or something  and then the get events function would return a list of the new rows in the table  Depends on the context of the problem really  Don t forget to change your PHP max execution time  otherwise it will timeout early    Client Side   Take a look at the jQuery plugin for doing Comet interaction    Project homepage  http   plugins jquery com project Comet Google Code  https   code google com archive p jquerycomet  - Appears to have some sort of example usage in the subversion repository    That said  the plugin seems to add a fair bit of complexity   it really is very simple on the client  perhaps  with jQuery  something like   function doPoll          get  events php       function result            each result events  function event      iterate over the events             do something with your event                 doPoll             this effectively causes the poll to run again as         soon as the response comes back        json          document  ready function           ajaxSetup          timeout  1000 60  set a global AJAX timeout of a minute             doPoll       do the first poll       The whole thing depends a lot on how your existing architecture is put together

User · Answer

Update  As I continue to recieve upvotes on this  I think it is reasonable to remember that this answer is 4 years old  Web has grown in a really fast pace  so please be mindful about this answer     I had the same issue recently and researched about the subject   The solution given is called long polling  and to correctly use it you must be sure that your AJAX request has a  large  timeout and to always make this request after the current ends  timeout  error or success    Long Polling - Client  Here  to keep code short  I will use jQuery    function pollTask             ajax            url    api Polling           async  true                by default  it s async  but            dataType   json            or the dataType you are working with         timeout  10000              IMPORTANT  this is a 10 seconds timeout         cache  false         done function  eventList                 Handle your data here        var data         for  var eventName in eventList                 data   eventList eventName               dispatcher handle eventName  data      handle the  eventName  with  data                    always pollTask        It is important to remember that  from jQuery docs        In jQuery 1 4 x and below  the XMLHttpRequest object will be in an   invalid state if the request times out  accessing any object members   may throw an exception  In Firefox 3 0  only  script and JSONP   requests cannot be cancelled by a timeout  the script will run even if   it arrives after the timeout period    Long Polling - Server  It is not in any specific language  but it would be something like this   function handleRequest              while   anythingHappened      hasTimedOut      sleep 2           return events          Here  hasTimedOut will make sure your code does not wait forever  and anythingHappened  will check if any event happend  The sleep is for releasing your thread to do other stuff while nothing happens  The events will return a dictionary of events  or any other data structure you may prefer  in JSON format  or any other you prefer    It surely solves the problem  but  if you are concerned about scalability and perfomance as I was when researching  you might consider another solution I found   Solution  Use sockets    On client side  to avoid any compatibility issues  use socket io  It tries to use socket directly  and have fallbacks to other solutions when sockets are not available   On server side  create a server using NodeJS  example here   The client will subscribe to this channel  observer  created with the server  Whenever a notification has to be sent  it is published in this channel and the subscriptor  client  gets notified   If you don t like this solution  try APE  Ajax Push Engine    Hope I helped

[comet] How does facebook, gmail send the real time notification?

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