Lift eats Comet update failure

[Donald McLean]

I have a Comet actor on a page and discovered that a recent change in my code was pushing an update to the client that was invalid.

Unfortunately, Lift was eating the error message and so I had no way of knowing where the problem lie and no easy way of figuring it out.

Soooo, have I egregiously overlooked something and the error messages are actually supposed to be somewhere (log on server)? Or does Lift need to be modified so that these error messages DO come out somewhere?

Thank you,

Donald

[Diego Medina]

What kind of error was it?

Was the invalid update,  invalid JavaScript?

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[Donald McLean]

Hi Diego,

I'm not sure what the error was (it was getting eaten) - I had to figure out what was causing it by backing out specific changes until it worked and then figuring out what was wrong.

The problem turned out to be with this:

JsRaw("console.log('[doReinstallCallback] Generated from lift.';")

There was a missing close paren immediately before the semi-colon. This statement became part of a JavaScript function that was created on the Lift side and contained an ajaxCall.

Thank you,

Donald

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[Diego Medina]

this would not happen on the server side, so Lift doesn't have a way to know there was an error and log it on the server side.

most of the time when I have some error in my js that goes out using comet, it results in the client getting an error (javascript alert message) that says something along the lines of, cannot contact server ...

[Donald McLean]

What I don't understand is:

a) No error messages come out on the client (Lift client side stuff eating them?)

b) The server is repeatedly re-sending the update. Of course, it keeps failing and so the app is effectively dead (sometimes reloading the page helps).

[Joe Barnes]

If I recall correctly, I've seen these errors show up in my Chrome console rather than as alert messages.  See if they show up there for you.

Joe

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[Diego Medina]

On Thu, Jul 17, 2014 at 3:34 PM, Donald McLean <dmcl...@gmail.com> wrote:

What I don't understand is:

a) No error messages come out on the client (Lift client side stuff eating them?)

this I haven't seen, this is where  I normally see an error mesage, what lift version? at least 2.5.1 showed a message

 

b) The server is repeatedly re-sending the update. Of course, it keeps failing and so the app is effectively dead (sometimes reloading the page helps).

it retries 3 times and then it gives up, again, this was on 2.5.1

[Antonio Salazar Cardozo]

Nope, this is actually a much more pernicious problem that I've seen many times. It is indeed a bit of a pain, but it's also a bit

hard to solve. When the JS hits the client, jQuery attempts to eval it as JavaScript. But, when it goes to do that, the JavaScript

fails to parse, so nothing gets to run! This means that the JS sent down from the server never runs at all. However, the jQuery

AJAX handler does invoke the comet failure function—which is designed to deal with connection errors, not JS parse errors.

The reaction to a comet request's failure is to try again (I think in 10s?), so we try another comet request. However, it's the JS

that comes from the server that lets us know what the latest update we've seen from the server is. Since the JS didn't run, our

version didn't update, which means the comet sees that we're out of date and sends us the original update as well as anything

else that has happened since then! Net-net, we get the bad JS again, and the cycle starts over.

We may be able to update the failure handler to look for what error jQuery tells us it got—but the problem is that our AJAX stuff

is designed to work with jQuery, YUI, and one or two other libraries, which means we'd have to make sure they all reported a JS

parse error the same way, or that we implemented handling for all the different ways in the failure callback. Either way, things get

pretty nasty.

Hope that sheds some light onto why this is happening, despite not offering an immediate solution. Thanks for bringing it up; this

has bit me before (though not since we switched to using events+JSON to send things down to the client as a matter of course),

and it can be quite frustrating.

Thanks,

Antonio

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[Joe Barnes]

Ah, that makes sense.  The failures I've seen in my console have been from valid JS hitting undefined, etc.

I understand your point regarding handling the failures from different libraries.  If we merely wanted to log the failure in the browser console, do you suppose that would be more manageable? 

Joe

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[Antonio Salazar Cardozo]

No matter our approach, we'd need to detect the error, and that's something that I think is only provided in the failure callback,

maybe? But it's definitely worth investigating (and maybe all the libraries report a JS parse error the same way—I haven't had a

chance to investigate that either).

Thanks,

Antonio

[fmpwizard]

@Antonio: would you say we need a ticket for this or is it already in your master plan to tackle this? :)

[Antonio Salazar Cardozo]

A ticket is actually a great idea! :) This is an issue that keeps slipping off my radar because it's only annoying

when it happens, and it happens to me very rarely because I structure almost all comet JavaScript as a single

JsCmd that dispatches an event on the client. So I haven't run into it in a loong time.

A ticket would ensure it gets to stay on the radar :)

Thanks,

Antonio

[Andreas Joseph Krogh]

På torsdag 21. august 2014 kl. 16:57:13, skrev Antonio Salazar Cardozo <savedf...@gmail.com>:

A ticket is actually a great idea! :) This is an issue that keeps slipping off my radar because it's only annoying

when it happens, and it happens to me very rarely because I structure almost all comet JavaScript as a single

JsCmd that dispatches an event on the client. So I haven't run into it in a loong time.

 

A ticket would ensure it gets to stay on the radar :)

Thanks,

Antonio

 

Just had to: https://www.youtube.com/watch?v=HXKOsajNZY4

 

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[Diego Medina]

https://github.com/lift/framework/issues/1618

 

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[Donald McLean]

Thank you!

On Thu, Aug 21, 2014 at 12:10 PM, Diego Medina <di...@fmpwizard.com> wrote:

https://github.com/lift/framework/issues/1618