I feel ackward asking these fundamental questions given that I am not exactly new to web development. But I want to double-check my assumptions nevertheless...
I'm building the recording of unique image views in my application. When a user (not a bot) visits an image page, an Ajax call is made to a back-end process that collects the session info, compares for duplications and stores the visit. I have all my javascript references as well as this call at the bottom of the HTML, just before the </body> element:
$.get(basepath + "image/1329/record/human", function(data){
console.log("Data Loaded: " + data);
});
By default, the call to $.get is made asynchronous. Yet I want to test the following assumptions:
Is it correct that this method ensures that the call to the view recording script is non-blocking for the rest of the UI?
Is it correct that the back-end script will finish once called, regardless of whether the user navigates to another page?
According to jQuery .get reference...
This [$.get()] is a shorthand Ajax function, which is equivalent to:
$.ajax({ url: url, data: data,
success: success, dataType: dataType
});
And $.ajax is asynchronous (i.e. non-blocking) by default, that's what the A in Ajax means.
Also, back-end server code is started in the moment the server receives the request and then runs independently of the client staying on the page or not, unless you implement some kind of mechanism to stop the running service which I suppose you did not.
God bless!
The jQuery.get request you're making is asynchronous and will not block the DOM or other JavaScript from continuing. The get function is a shorthand method which uses jQuery.ajax.
The second question I don't have a solid answer for -- I expect it may depend more on how the back-end code is structured and whether it's told that the session/request has terminated.
API:
jQuery.get()
jQuery.ajax()
that is correct on both counts
Related
When my site first initializes, it queries a server to get back some data. I can't lay anything out on the page until this data gets back. With d3.js, I can use d3.json() to get my data, but because it's asynchronous, I need to put the entire page logic in the callback function. How do I request the data and wait for it to come back?
You're basically doing it the only way. The callback function has to be the one initiating the rest of your code. You don't need all your code in the callback function though, you can introduce indirection. So the callback function will call another function inside which would be what is currently in your callback function.
Using synchronous requests in JavaScript is not recommended as it blocks the whole thread and nothing gets done in the meantime. The user can also not interact well with the webpage.
If it is really what you want, you can do the following (using jQuery):
var jsonData;
jQuery.ajax({
dataType: "json",
url: "jsondatafile.json",
async: false
success: function(data){jsonData = data}
});
However it is not recommended, even by jQuery, as explained here the jQuery.ajax() documentation:
The first letter in Ajax stands for "asynchronous," meaning that the operation occurs in parallel and the order of completion is not guaranteed. The async option to $.ajax() defaults to true, indicating that code execution can continue after the request is made. Setting this option to false (and thus making the call no longer asynchronous) is strongly discouraged, as it can cause the browser to become unresponsive.
As a final note, I don't see what prevents you from using whatever function there is in the success attribute in an asynchronous way. Most of the times changing your design to use async requests will be worth it. By experience, debugging a page that uses synchronous requests is a pain (especially when the requests don't get answered...).
I understand from this site that in the case of a POST Ajax request, we need to place the request.onreadystatechange function call before the request.open command. In GET requests we typically place them in the opposite order. Why the difference?
What does "typically" mean? Where do you see documented that you are supposed to do that with GET requests? request.onreadystatechange should always come first. By defining it after the request is initiated you open yourself up to the possibility that the event is fired prior to your handler being established.
Controller:
The code that handles post back has a db call in biz layer that is long running. After 30 minutes, the browser showed the error message from the ajax call, however my db logs showed that the db method was still running - ran way past the 30 min mark.
My ? is how does the ajax error get raised when it wasn't raised in the controller [log4j did not show any errors trapped] as the controller was still waiting for feedback from the biz layer.
Does adjusting the timeout in tomcat help? I assume not as the app was still processing
My View:
function runAjax(){
$.ajax({
type: "POST",
url: "Test.html",
data: { testparam1,testparam2},
success: function(data){
document.getElementById("processdata").innerHTML="Success";
},
error: function(request){
document.getElementById("processdata").innerHTML="Error";
}
});
}
The browser, aka JavaScript, is not going to wait forever for a response. After some time (read 30min in this case), it will give up on the request, thinking the server is not responding. HTTP is not really set up to handle requests that take > 30min, at least not through the normal means.
There are some options. First, you could optimize whatever you are doing in the backend to make it faster. I mean, if it's taking 30+ min for a query to run, then you may need to rethink your SQL approach a bit.
Second, you could simply break up the steps, so that you can run each step, verify, then run the next. Depending on what you are doing, this may open up the possibility to do some work in parallel, speeding up the process. The challenge here would be in rolling back in the case when something failed.
Third, you could implement a Pub/Sub model. There are several frameworks out there to do Pub/Sub for JavaScript. I have personally used Atmosphere, which has a nice jQuery plugin and falls back to more HTTP conventional methods when the more advanced methods are not available in the browser. In this model, your View would submit the request, the backend would queue up the work and tell the View to subscribe to a particular queue/channel to get the result. The View can simply wait for the response.
Forth, you could post the request, have an interrum page that simply keeps refreshing via JavaScript until the backend says "Ok, we are done!". Lots of older web applications used to do this for long-running transactions.
I am sure there are probably 1000 more options you could use, but if your request will always take that long to run, I think AJAX is not the tool for you.
I have 10 divs on my page and each div will render its own ajax request when the page loads. I know i can make max 2 ajax requests and then i have to wait (based on the browser) before the next request gets fired. I was wondering what will be the best way to design such a page.
Should i create ajax request inside the divs so that i can pass the div as a context to the ajax reponse? something like this:
<div id="request1">
make an ajax request
</div>
<div id="request2">
make an ajax request
</div>
and so on......
is there any chance that result may get mixed up and wrong div will render the result from the different request?
--Edit--
I cannot make a single call as they all make calls to separate service and that service may or may not be available.
AJAX is Asynchronous, that way, if you call 10 AJAX requests using either $.get, $.post or $.ajax, those requests will fire independently without waiting for the previous ones. So unless you have a special requirements that need to avoid that, just go ahead
Why don't you send only one ajax request when the page loads, and let your server side script return the data needed for the 10 divs in form of json? That would reduce the number of requests sent to the server and the work would be a lot cleaner as well.
Edit : ok since this is no longer and option. You can queue the requests one after another, if each request you are sending, depends on each other (for eg: you might set a flag in the first request, which again gets check in a later request) you can queue them. I have been using this plugin for quite a while now, and it has come in handy)
You might be able to use in your case so check it out.
http://www.protofunc.com/scripts/jquery/ajaxManager/
I have a page which fires Ajax requests for validations at server side. I need to perform an action when all the ajax requests have finished loading or are completed.
For this, I am using Ext.Ajax.isLoading() in a recursive function in following way:
function chechValid(){
if(Ext.Ajax.isLoading()){
checkValid();
}else{
//Code for Action 1
}
}//EOF
checkValid();
//Code for Action 2
The problem is that when I do this, browsers give the following errors:
Mozill FF - too much recursions
IE - Stack overflow at line:18134
If this recursion is a heavy thing for the browsers, then how to perform a task when all the Ajax requests have finished loading?
Using delay is not what I want as, if delay is used then browser begins executing the other code (like 'Code for Action 2' as shared above) which is not what is expected.
The main aim is that the browser shouldn't execute anything unless all the Ajax requests are complete and once completed then it should perform a particular action.
Any suggestions/help on this one?
Thanks in Advance.
PS: Using ExtJs 4.0.7
(Updated)More Detail about the actual situation:-
Here is brief description of the situtaion being faced - There is a form, in which I need to perform server side validations on various fields. I am doing so by firing an ajax request on blur event. Depending upon the server response of validation Ajax fired on blur, fields are marked invalid and form submission is not allowed. (Avoiding 'change' event as that causes alot of overhead on server due to high number of Ajas requests and also leads to fluctuating effects on a field when response from various such Ajax requests are received).
Things are working fine except in one case - when user modifies the value of a field and instead of 'tab'bing out from the field she directly clicks at the save button. In such a case, though, the blur event gets fired but the processing of 'Save' doesn't wait for Ajax Validation response and submits the form. Thus, I somehow need to check if Ajax requests have finihed loading and the process the saving of form. requestComplete would unfortunately not serve the purpose here. And if try using the recursion, then of course, the browser is hung due to high usage of resources. Same case occurs if I try using a pause script work around ( as shared here - Javascript Sleep).
Any possible workaround for this one?
TIA
Your method will lead to infinite recursion.
A better way is to register a callback function in Ext.Ajax.requestcomplete, something like this (not tested):
Ext.Ajax.on('requestcomplete', function(conn, response, options) {
if (!Ext.Ajax.isLoading()) {
//your action...
}
}
};
Unless I am misunderstanding the issue couldn't you create a couple of globals. I know globals are bad, but in this case it will save you quite a bit of headache. One global would be "formReady" and initially set it to false, the other would be "ajaxActive" and set to false. You would also add an onSubmit method that would validate that "formReady" was true and if not alert the user that validation was occurring (or you could set a timeout for form submission again and have a second validation that checks to see if "ajaxActive" is true). When the AJAX call is made it would set the variable "ajaxActive" to true and once complete would set formReady to true. You could also potentially resubmit the form automatically if the response from the AJAX was that the form was good.
Ext.Ajax.request() returns a transaction object when you call it, which is unique and allows you to recognise and abort specific Ajax requests.
By just calling Ext.Ajax.isLoading() without a specified transaction object, it defaults to the last request, which is why you have to call it recursively at the moment.
If it were me, I'd create an array of these transaction objects as you fire them off, and pass each of those in as optional parameters to the Ext.Ajax.isLoading() function to check if a particular request has finished. If it has, you can remove that transaction object from the array, and only progress with the save when your array is empty.
This would get round your recursion problem, since you've always got a finite number of requests that you're waiting on.
if (Object.keys(Ext.Ajax.requests).length === 0) console.log("No active requests");