[Lazarus] Converting Pascal Client/Server application into Web Client?

[Michael Van Canneyt]

On Fri, 1 Oct 2010, Michael Schnell wrote:

> On 10/01/2010 02:22 PM, Michael Van Canneyt wrote:
>> 
>> ExtJS has no such mechanism to my knowledge, but I'm not a reference.
>
> last time I checked some months ago it was said to be a new feature, but 
> should work, even though I did not find anybody who has tested it.
>
>> 
>>> This is necessary to overcome the asymmetry of the http protocol.
>>> 
>>> IMHO "Server-Events" are necessary to design a CGI program with some 
>>> complexity.
>> 
>> Well, I create seriously complex applications. No such mechanism was
>> necessary till now.
>
> Hmm, How to do notify the user that the  server has finished complex work in 
> the background without blocking the interface and/or creating excessive http 
> polling traffic ?

Define excessive polling ?

>
> An easy to do (non complex) example for a good application for such a method 
> is a server that allows tow client to chat.
>
>> 
>>> Moreover "server-Internal-Events" like from Timers, serial or "direct" 
>>> TCP/IP communication, thread-notification, ... are very desirable and 
>>> should be able to fire "server-events" towards the client.
>> 
>> Why ? What could you possibly need to do that the server needs to contact
>> the client ? Everything you need can be programmed in the client, in my
>> experience.
>
> Above trivial example ("chat server")
>
> Real-world: Imagine an "embedded" program automatically working with some 
> hardware interfaces and/or serial and/or TCP/IP connections. Now you want to 
> provide it with a "web" interface that allows a "user" to connect to it and 
> do configuration and monitoring. For monitoring this method is essential: 
> many "states" in the program permanently change due to the work of the 
> program and the external situation. These state changes need to be shown in 
> the remote browser with low latency and without excessive communication 
> overhead.

In my experience: keeping a connection open IS excessive communication overhead 
and very unreliable to boot.

I thought like you, 12 years ago. Till reality kicked in. Faulty switches,
company firewalls and whatnot. Not all servers/firewalls support HTTP 1.1
(for a persistent connection) etc. Too many problems.

So you end up with polling anyway. 10 polls per second is enough to fool the
human eye and doesn't stress the server too much.

Michael.