Thanks a lot for your response Jeremy. I can see a lot of site that does update infos without the user to have to click "refresh" (I think Facebook does?). Do they do polling? I think this approach would be fine for me, my app if not very fast paced. Then I don't need to add a event handler to happstack-state ;) This is done with something like: <META HTTP-EQUIV="*Refresh*" CONTENT="*n* "> ? Thanks, Corentin On Mon, Jan 17, 2011 at 5:41 PM, Jeremy Shaw wrote:

Hello,

The problem is that clients are not 'connected' to the web server. The way it works (more or less) is:

1. client connects to server and sends a Request 2. server sends a Response 3. connection is terminated.

So, once the page has been loaded there is no connection between the web-server and the client. Hence there is no way for the server to send anything to the clients.

In HTML 5, there is something known as web sockets with does allow you to have a persistent connection to the clients. However, it does not yet have broad browser support. In fact, some browsers have temporarily removed support due to a security flaw in the design specification.

There are a variety of hacks collectively known as 'comet' for trying to create a persistent connection on top of the existing HTTP 1.1 standard:

http://en.wikipedia.org/wiki/Comet_(programming)http://en.wikipedia.org/wiki/Comet_%28programming%29

That deals with trying to have the server push updates to the client.

A different approach is to have the clients pull new data from the server by polling the server for changes every now and then. But that depends on how much latency you can afford for the updates. For example, if updating only once a minute is fine versus must update every second.

Once you decide how to get the data to the client, then you can figure out how to track changes to the MACID database.

- jeremy

On Jan 17, 2011, at 4:58 AM, Corentin Dupont wrote:

Hello again, I have another question for happstack users/experts:

I have a program with happstack-state and a web server with happstack-server. What I'd like is, whenever the MACID state is changed, that the web page is refreshed for every clients connected on the web server.

So I think the question is 2 fold: - How to add an event handler on happstack-state for that? - How to ask to the web server to refresh every clients?

I did not found infos about that in the API.

Thanks a lot, Corentin

Indeed, I tried with <META HTTP-EQUIV="*Refresh*" CONTENT="*n*"> ? and it's unusable. It make blink the page, ungrey the "stop" button for a second and make the fields loose the focus so it's impossible to type in. I'll try with XMLHTTPRequest. On Mon, Jan 17, 2011 at 6:30 PM, Chris Smith wrote:

On Mon, 2011-01-17 at 18:25 +0100, Corentin Dupont wrote:

...

Thanks a lot for your response Jeremy. I can see a lot of site that does update infos without the user to have to click "refresh" (I think Facebook does?). Do they do polling?

While I'm not familiar with Facebook, I'd guess that today, most such sites are all doing polling. Especially the high-volume ones, for which leaving a connection open for every current user would be impractical.

Polling doesn't have to be done with page refreshes. It can also be done with JavaScript using, e.g., the XMLHTTPRequest object. Then it would be pretty transparent to you.

-- Chris

On 17 January 2011 21:50, Jeremy Shaw wrote:

On Jan 17, 2011, at 2:19 PM, Corentin Dupont wrote:

Indeed, I tried with <META HTTP-EQUIV="Refresh" CONTENT="n"> ? and it's unusable. It make blink the page, ungrey the "stop" button for a second and make the fields loose the focus so it's impossible to type in.

I'll try with XMLHTTPRequest.

Right. Using the jQuery library should make it easier to do ajax requests and modify the DOM on the fly, http://jquery.com/ - jeremy _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

A nice variation of polling is "long polling": http://en.wikipedia.org/wiki/Push_technology#Long_polling This can easily be accomplished in Haskell by having an MVar per session. Initially an empty MVar is created per new session. When a client makes a request, the server thread that handles the request takes the MVar belonging to the session. This thread will block until the MVar is filled. When the server has an update it will fill all the MVars. This causes all the blocked threads to continue with sending a response to the client notifying it about the update. Like Jeremy said it's a good idea to make these update requests asynchronous. Regards, Bas

Hi, I've not really followed the thread, but on the client side, for long-polling, you issue an XMLHttpRequest that will receive an answer from the server only when the server has something to push to the client (maybe done with a thread waiting on an MVar). XHR is asynchronous as you have to supply a callback to process the server's response. Usually an XHR request is done from an already loaded page and will update part of that page with the content of the server's response. So no blank page. (So no timer needed either, this is done asynchronously with callbacks.) HTH, Thu 2011/1/19 Corentin Dupont :

Thanks. There seems to be several technologies to realize this push or polling.

Is what you explained feasible on the user's side of happstack-server (I mean, if I can do it myself)? If I take an empty MVar in my ServerPartTs, which are read over client's request, I think that nothing will be sent back to the browser and it will remain blank! Is that to be combined with an HTTP refresh timer on the client side?

By sessions, you mean sessions that I create myself for every client connected?

Regards, Corentin

On Wed, Jan 19, 2011 at 2:21 PM, Bas van Dijk wrote:

...

On 17 January 2011 21:50, Jeremy Shaw wrote:

...

On Jan 17, 2011, at 2:19 PM, Corentin Dupont wrote:

Indeed, I tried with <META HTTP-EQUIV="Refresh" CONTENT="n"> ? and it's unusable. It make blink the page, ungrey the "stop" button for a second and make the fields loose the focus so it's impossible to type in.

I'll try with XMLHTTPRequest.

Right. Using the jQuery library should make it easier to do ajax requests and modify the DOM on the fly, http://jquery.com/ - jeremy _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

A nice variation of polling is "long polling":

http://en.wikipedia.org/wiki/Push_technology#Long_polling

This can easily be accomplished in Haskell by having an MVar per session. Initially an empty MVar is created per new session. When a client makes a request, the server thread that handles the request takes the MVar belonging to the session. This thread will block until the MVar is filled. When the server has an update it will fill all the MVars. This causes all the blocked threads to continue with sending a response to the client notifying it about the update.

Like Jeremy said it's a good idea to make these update requests asynchronous.

Regards,

Bas

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