Multiple Ajax Requests ServerSide - Part 2 (Possible Partial Solution)

Hello again,

Sorry to take your time again, but i came up with a possible partial solution to the problem, and i would like to share with you, so you may consider it and tell me if i'm doing something wrong.

I came up with a level system, based on a certain amount of rows enough to avoid making so many AJAX requests, and overlapping just a small part to ensure the infinite scroll position.

I still have two problems to solve:
1 - When a new request is made and rows are updated, the position of the table is higher/lower than it should be, by 1 or 2 lines. I think this problem is happening because Scroller is forcing start row to be an odd one.
2 - Cache the small part left so no overlapping exists at all.

First, in Settings (this.s), after "loaderVisible: false", i define a start level = 1;
Then i update the _fnScroll function as it follows (changes are marked with (AM):

/**

• Scrolling function - fired whenever the scrolling position is changed.
• This method needs to use the stored values to see if the table should be
• redrawn as we are moving towards the end of the information that is
• currently drawn or not. If needed, then it will redraw the table based on
• the new position.
• @returns {void}
• @private
  */

"_fnScroll": function ()
{
var
that = this,
heights = this.s.heights,
iScrollTop = this.dom.scroller.scrollTop,
iTopRow;

    if ( this.s.skip ) {
        return;
    }

    if ( this.s.ingnoreScroll ) {
        return;
    }

    /* If the table has been sorted or filtered, then we use the redraw that
     * DataTables as done, rather than performing our own
     */
    if ( this.s.dt.bFiltered || this.s.dt.bSorted ) {
        this.s.lastScrollTop = 0;
        return;
    }

    /* Update the table's information display for what is now in the viewport */
    this._fnInfo();

    /* We don't want to state save on every scroll event - that's heavy
     * handed, so use a timeout to update the state saving only when the
     * scrolling has finished
     */
    clearTimeout( this.s.stateTO );
    this.s.stateTO = setTimeout( function () {
        that.s.dt.oApi._fnSaveState( that.s.dt );
    }, 250 );

    /* Check if the scroll point is outside the trigger boundary which would required
     * a DataTables redraw
     */
    if ( iScrollTop < this.s.redrawTop || iScrollTop > this.s.redrawBottom ) {

                    var preRows = Math.ceil( ((this.s.displayBuffer-1)/2) * this.s.viewportRows );

        if ( Math.abs( iScrollTop - this.s.lastScrollTop ) > heights.viewport || this.s.ani ) {
            iTopRow = parseInt(this._domain( 'physicalToVirtual', iScrollTop ) / heights.row, 10) - preRows;
            this.s.topRowFloat = (this._domain( 'physicalToVirtual', iScrollTop ) / heights.row);
        }
        else {
            iTopRow = this.fnPixelsToRow( iScrollTop - preRows);
            this.s.topRowFloat = this.fnPixelsToRow( iScrollTop, false );
        }

                    /* At the start of the table */
                    if ( iTopRow <= 0 ) {
            iTopRow = 0;
        }

                    /* At the end of the table */
        else if ( iTopRow + this.s.dt._iDisplayLength > this.s.dt.fnRecordsDisplay() ) {
            iTopRow = this.s.dt.fnRecordsDisplay() - this.s.dt._iDisplayLength;
            if ( iTopRow < 0 ) {
                iTopRow = 0;
            }
        }

                    // For the row-striping classes (odd/even) we want only to start
                    // on evens otherwise the stripes will change between draws and
                    // look rubbish
        else if ( iTopRow % 2 !== 0 ) {
            iTopRow++;
        }

                    // (AM) Set level boundary line based on the maximum number of rows
                    // necessary to ensure the minimum of Ajax requests
                    // and also the infinite scrolling behavior at the same time.
                    var levelBoundary = this.s.dt._iDisplayLength - preRows;

                    // (AM) Level changes when boundary line is crossed
                    var level = iTopRow > levelBoundary ? Math.ceil( iTopRow / levelBoundary ) : 1;

                    // (AM) Only request new data when level changes
                    if (level !== this.s.level) {

                            // (AM) Local displayStart definition
                            var displayStart = null;

                            // (AM) Scroll Up/Down Start Position
                            if (level > this.s.level) {
                                displayStart = iTopRow;
                            } else {
                                displayStart = (iTopRow - (this.s.dt._iDisplayLength - preRows));
                                if (displayStart < 0) {
                                    displayStart = 0;
                                }
                            }

                            // (AM) Update Level
                            this.s.level = level;

            /* Cache the new table position for quick lookups */
            this.s.tableTop = $(this.s.dt.nTable).offset().top;
            this.s.tableBottom = $(this.s.dt.nTable).height() + this.s.tableTop;

            var draw =  function () {
                                if ( that.s.scrollDrawReq === null ) {
                                        that.s.scrollDrawReq = iScrollTop;
                                }

                                // (AM) Assign displayStart
                                that.s.dt._iDisplayStart = displayStart;

                                if ( that.s.dt.oApi._fnCalculateEnd ) { // Removed in 1.10
                                        that.s.dt.oApi._fnCalculateEnd( that.s.dt );
                                }
                                that.s.dt.oApi._fnDraw( that.s.dt );
            };

            /* Do the DataTables redraw based on the calculated start point - note that when
             * using server-side processing we introduce a small delay to not DoS the server...
             */
            if ( this.s.dt.oFeatures.bServerSide ) {
                clearTimeout( this.s.drawTO );
                this.s.drawTO = setTimeout( draw, this.s.serverWait );
            }
            else {
                draw();
            }

            if ( this.dom.loader && ! this.s.loaderVisible ) {
                this.dom.loader.css( 'display', 'block' );
                this.s.loaderVisible = true;
            }
        }
    }

    this.s.lastScrollTop = iScrollTop;
    this.s.stateSaveThrottle();
}

Start by reading the Scroller settings and the _fnScroll function shown in the issue, including the level and displayStart calculations. Reproduce server-side scrolling and investigate the reported one- or two-row position shift and overlapping requests; done means the scrolling position remains stable while unnecessary AJAX requests are avoided.