In what situations would AJAX long short polling be preferred over HTML5 WebSockets

Question

I am building a small chat application for friends, but unsure about how to get information in a timely manner that is not as manual or as rudimentary as forcing a page refresh.

Currently, I am implementing this using simple AJAX, but this has the disadvantage of regularly hitting the server when a short timer elapses.

In researching long/short polling, I ran across HTML5 WebSockets. This seems easy to implement, but I'm not sure if there are some hidden disadvantages. For example, I think WebSockets is only supported by certain browsers. Are there other disadvantages to WebSockets that I should be aware of?

Since it seems like both technologies do the same thing, in what sorts of scenarios would one prefer to use one over the other? More specifically, has HTML5 WebSockets made AJAX long/short polling obsolete, or are there compelling reasons to prefer AJAX over WebSockets?

User · Answer

XHR polling A Request is answered when the event occurs  could be straight away  or after a delay   Subsequent requests will need to made to receive further events      The browser makes an asynchronous request of the server    which may wait for data to be available before responding  The   response can contain encoded data  typically XML or JSON  or   Javascript to be executed by the client  At the end of the processing   of the response  the browser creates and sends another XHR  to await   the next event  Thus the browser always keeps a request outstanding   with the server  to be answered as each event occurs  Wikipedia  Server Sent Events Client sends request to server  Server sends new data to webpage at any time      Traditionally  a web page has to send a request to the server to   receive new data  that is  the page requests data from the server    With server-sent events  it s possible for a server to send new data   to a web page at any time  by pushing messages to the web page  These   incoming messages can be treated as Events   data inside the web page  Mozilla  WebSockets After the initial handshake  via HTTP protocol   Communication is done bidirectionally using the WebSocket protocol       The handshake starts with an HTTP request response  allowing servers   to handle HTTP connections as well as WebSocket connections on the   same port  Once the connection is established  communication switches   to a bidirectional binary protocol which does not conform to the HTTP   protocol  Wikipedia

User · Answer

One contending technology you ve omitted is Server-Sent Events   Event Source   What are Long-Polling  Websockets  Server-Sent Events  SSE  and Comet  has a good discussion of all of these   Keep in mind that some of these are easier than others to integrate with on the server side

User · Answer

WebSockets is definitely the future   Long polling is a dirty workaround to prevent creating connections for each request like AJAX does -- but long polling was created when WebSockets didn t exist  Now due to WebSockets   long polling is going away    WebRTC allows for peer-to-peer communication   I recommend learning WebSockets   Comparison   of different communication techniques on the web   AJAX - request  rarr  response  Creates a connection to the server  sends request headers with optional data  gets a response from the server  and closes the connection   Supported in all major browsers  Long poll - request  rarr  wait  rarr  response  Creates a connection to the server like AJAX does  but maintains a keep-alive connection open for some time  not long though   During connection  the open client can receive data from the server  The client has to reconnect periodically after the connection is closed  due to timeouts or data eof  On server side it is still treated like an HTTP request  same as AJAX  except the answer on request will happen now or some time in the future  defined by the application logic   support chart  full    wikipedia WebSockets - client  harr  server  Create a TCP connection to the server  and keep it open as long as needed  The server or client can easily close the connection  The client goes through an HTTP compatible handshake process  If it succeeds  then the server and client can exchange data in both directions at any time  It is efficient if the application requires frequent data exchange in both ways  WebSockets do have data framing that includes masking for each message sent from client to server  so data is simply encrypted   support chart  very good    wikipedia WebRTC - peer  harr  peer  Transport to establish communication between clients and is transport-agnostic  so it can use UDP  TCP or even more abstract layers  This is generally used for high volume data transfer  such as video audio streaming  where reliability is secondary and a few frames or reduction in quality progression can be sacrificed in favour of response time and  at least  some data transfer  Both sides  peers  can push data to each other independently  While it can be used totally independent from any centralised servers  it still requires some way of exchanging endPoints data  where in most cases developers still use centralised servers to  link  peers  This is required only to exchange essential data for establishing a connection  after which a centralised server is not required  support chart  medium    wikipedia Server-Sent Events - client  larr  server  Client establishes persistent and long-term connection to server  Only the server can send data to a client  If the client wants to send data to the server  it would require the use of another technology protocol to do so  This protocol is HTTP compatible and simple to implement in most server-side platforms  This is a preferable protocol to be used instead of Long Polling  support chart  good  except IE    wikipedia   Advantages   The main advantage of WebSockets server-side  is that it is not an HTTP request  after handshake   but a proper message based communication protocol  This enables you to achieve huge performance and architecture advantages  For example  in node js  you can share the same memory for different socket connections  so they can each access shared variables  Therefore  you don t need to use a database as an exchange point in the middle  like with AJAX or Long Polling with a language like PHP   You can store data in RAM  or even republish between sockets straight away   Security considerations  People are often concerned about the security of WebSockets  The reality is that it makes little difference or even puts WebSockets as better option  First of all  with AJAX  there is a higher chance of MITM  as each request is a new TCP connection that is traversing through internet infrastructure  With WebSockets  once it s connected it is far more challenging to intercept in between  with additionally enforced frame masking when data is streamed from client to server as well as additional compression  which requires more effort to probe data  All modern protocols support both  HTTP and HTTPS  encrypted    P S   Remember that WebSockets generally have a very different approach of logic for networking  more like real-time games had all this time  and not like http

User · Answer

For chat applications or any other application that is in constant conversation with the server  WebSockets are the best option  However  you can only use WebSockets with a server that supports them  so that may limit your ability to use them if you cannot install the required libraries  In which case  you would need to use Long Polling to obtain similar functionality

[javascript] In what situations would AJAX long/short polling be preferred over HTML5 WebSockets?

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