Cancel pending AJAX requests in PHP app?

Question

I'm having problems canceling my XHR requests when navigating between pages. I have a page that has 8 requests that get fired off. I cancel them on click of a link outside of the current page. The page stalls as it waits on the next document to load. They XHR requests appear as cancelled in developer tools, but the new document stalls as if it is waiting for them to come back.

Here you can see the page is stalled even though all the other requests are cancelled. The new page is the only pending request...

And here you can see once the page finally did make the jump the TTFB is 52.52s. If I wait for the calls to come back before clicking away the jump is instant.

Here are the headers for the new pages once it finally loads if that helps...

I use the following frankenstein code to manage XHR requests. I have a cancelAll function towards the bottom that aborts the requests...

 XHRManager = {
Requests: [],
pendingRequests: [],
addNextRequest: function (r) {
    var timeout = 0;
    if (trace.isDevelopment()) {
        timeout = 350;
    }
    setTimeout(function () {
        if (r.url ==  XHRManager.pendingRequests[0].url && r.start ==  XHRManager.pendingRequests[0].start) {
             XHRManager.pendingRequests.splice(0, 1);
        }
        else {
            $( XHRManager.pendingRequests).each(function (ii, dd) {
                if (dd.url == r.url && dd.start == r.start) {
                     XHRManager.pendingRequests.splice(ii, 1);
                }
            });
        }
         XHRManager.startNextRequest();
        if (trace.findLocalStorage()) {
             XHRManager.showTrace = true;
            trace.show();
        }
    }, timeout);
},
requests: [],
intervals: [],
requestsInt: 0,
firstRun: true,
delay: 500,
globalTimeout: 5000,
showTrace: false,
startNextRequest: function () {
    $( XHRManager.pendingRequests).each(function (i, d) {
        if (d.start) {

        }
        if (i == 0) {
            if (trace.domWatcher.constructor == Function) {
                trace.domWatcher(d.requestNumber);
            }
            trace.log("Request #" + d.requestNumber + " started");
            d.requestType(d);
        }
    });
    if ( XHRManager.pendingRequests.length == 0) {
        if (trace.isDevelopment()) {
            trace.show();
        }
    }
},
AddToPendingRequests: function (url, params, cb, type, errCB) {
    var rI =  XHRManager.requestsInt;
     XHRManager.requestsInt++;
    var req = {url: url, params: params, cb: cb, requestNumber: rI, requestType: type};
    if (errCB) {
        req.errCB = errCB;
    }
     XHRManager.pendingRequests.push(req);
    // if(trace.findLocalStorage()){
    //    trace.show();
    //  }
    if (rI == 0 ||  XHRManager.pendingRequests.length == 1) {
         XHRManager.startNextRequest();
    }
},
writeVals: function (url, params, data, start, cb, requestNumber) {
    if ($("meta[content='development']").length > 0) {
        try {
            var response = {};
            response.requestNumber = requestNumber;
            if (data.sql != "" && data.sql != undefined) {
                response.sql = data.sql;
            }
            if (data.debug) {
                if (data.debug.sql != "" && data.debug.sql != undefined) {
                    response.sql = data.debug.sql;
                }
            }
            if (data.data != "" && data.data != undefined) {
                response.data = data.data;
            }
            else {
                if (data != "" || data != undefined) {
                    response.data = data;
                }
            }
            if (url != "" && url != undefined) {
                response.url = url;
            }
            if (params != "" && params != undefined) {
                response.params = params;
            }
            if (cb) {
                response.cb = cb.toString();
            }
            else {
                response.cb = "";
            }
            response.requestStats = {};
            response.requestStats.start = start;
            response.requestStats.end = Date();
            response.requestStats.totalTime = ((new Date(response.requestStats.end)).getTime() - (new Date(start)).getTime()) / 1000 + " sec(s)";
             XHRManager.Requests.push(response);

        }
        catch (e) {
            trace.log(e);
        }
    }
},
_create: function (r) {
    var xm =  XHRManager;
    var start = Date();
    var req = $.get(r.url, r.params, r.cb)
        .done(function (data) {
             XHRManager.writeVals(r.url, r.params, data, start, r.cb, r.requestNumber);
            if (trace.isDevelopment() && trace.isOn()) {
                 XHRManager.addNextRequest(r);
            }
        });
    xm.requests.push(req);
},
_createAjax: function (r) {
    var xm =  XHRManager;
    var start = Date();
    if (r.type == "PUT" || r.type == "DELETE") {
        var req = $.ajax({
            type: r.type,
            xhrFields: {
                withCredentials: true
            },
            url: r.url,
            data: r.params,
            success: function (data) {
                 XHRManager.writeVals(r.url, r.params, r.data, r.start, r.cb, r.requestNumber);
                r.cb(data);
                if (trace.isDevelopment() && trace.isOn()) {
                     XHRManager.addNextRequest(r);
                }
            },
            error: r.errCB
        });
        xm.requests.push(req);
    }
    else {
        var req = $.ajax({
            type: r.type,
            xhrFields: {
                withCredentials: true
            },
            dataType: 'json',
            json: 'json',
            url: r.url,
            data: r.params,
            success: function (data) {
                 XHRManager.writeVals(r.url, r.params, data, start, r.cb, r.requestNumber);
                r.cb(data);
                if (trace.isDevelopment() && trace.isOn()) {
                     XHRManager.addNextRequest(r);
                }
            },
            error: r.errCB
        });
        xm.requests.push(req);
    }
},
_createJSON: function (r) {
    var start = Date();
    var xm =  XHRManager;
    var req = $.getJSON(r.url, r.params, r.cb)
        .done(function (data) {
             XHRManager.writeVals(r.url, r.params, data, start, r.cb, r.requestNumber);
            if (trace.isDevelopment() && trace.isOn()) {
                 XHRManager.addNextRequest(r);
            }
        });
    xm.requests.push(req);
},
create: function (url, params, cb) {
    if (trace.isDevelopment() && trace.isOn()) {
         XHRManager.AddToPendingRequests(url, params, cb,  XHRManager._create);
    }
    else {
        var r = {};
        r.url = url;
        r.params = params;
        r.cb = cb;
         XHRManager._create(r);
    }
},
createAjax: function (url, params, type, cb, errCB) {
    if (trace.isDevelopment() && trace.isOn()) {
         XHRManager.AddToPendingRequests(url, params, cb,  XHRManager._createAjax, errCB);
    }
    else {
        var r = {};
        r.url = url;
        r.params = params;
        r.cb = cb;
        r.type = type;
        r.errCB = errCB;
         XHRManager._createAjax(r);
    }

},
createJSON: function (url, params, cb) {
    if (trace.isDevelopment() && trace.isOn()) {
         XHRManager.AddToPendingRequests(url, params, cb,  XHRManager._createJSON);
    }
    else {
        var r = {};
        r.url = url;
        r.params = params;
        r.cb = cb;
         XHRManager._createJSON(r);
    }
},
remove: function (xhr) {
    var xm =  XHRManager;
    var index = xm.requests.indexOf(xhr);
    if (index > -1) {
        xm.requests.splice(index, 1);
    }
    index = xm.intervals.indexOf(xhr.interval);
    if (index > -1) {
        xm.intervals.splice(index, 1);
    }
},
cancelAll: function () {
    var xm =  XHRManager;
    $(xm.requests).each(function () {
        var t = this;
        t.abort();
    });
    $(xm.intervals).each(function () {
        var t = this;
        clearInterval(t);
    });
    xm.requests = [];
    xm.intervals = [];
}
};

The site uses jQuery, PHP, Zend Framework 2, and SQL, Apache. What am I missing?

Answer 1

Probable causal chain

1. the server does not realise the XHR requests are cancelled, and so the corresponding PHP processes keep running
2. these PHP processes use sessions, and prevent concurrent access to this session until they terminate

Possible solutions

Adressing either of the above two points breaks the chain and may fix the problem:

1. (a) ignore_user_abort is FALSE by default, but you could be using a non-standard setting. Change this setting back to FALSE in you php.ini or call ignore_user_abort(false) in the scripts that handle these interruptible requests.

Drawback: the script just terminates. Any work in progress is dropped, possibly leaving the system in a dirty state.

1. (b) By default, PHP will not detect that the user has aborted the connection until an attempt is made to send information to the client. Do echo something periodically during the course of your long-running script.

Drawback: this dummy data might corrupt the normal output of your script. And here too, the script may leave the system in a dirty state.

2. A PHP sessions is stored as a file on the server. On session_start(), the script opens the session file in write mode, effectively acquiring an exclusive lock on it. Subsequent requests that use the same session are put on hold until the lock is released. This happens when the script terminates, unless you close the session explicitely. Call session_write_close() or session_abort() as early as possible.

Drawback: when closed, the session cannot be written anymore (unless you reopen the session, but this is somewhat inelegant a hack). Also the script does keep running, possibly wasting resources.

I definitely recommend the last option.

Answer 2

Are you storing your Ajax Request on a variable?. If not, that's what you need to do to completely cancel a request

var xhr = $.ajax({
    type: "POST",
    url: "anyScript.php",
    data: "data1=0&data2=1",
    success: function(msg){
       //Success Function
    }
});

//here you abort the request
xhr.abort()