function TxnProgressWidget( txnProgressContext, txnProgressStatusMessage, txnProgressPostCompletionFixup ) {

  /**************************** Private methods ********************************/
  var txnProgressStateShouldBeSkipped = function( txnProgressState ) {
    
    var skipIt = false;

    /* Step over any deploy progress states that aren't in the CLUSTER, SERVICE 
     * or SERVICE-SMOKETEST contexts. 
     */
    if( (txnProgressState.subTxnType != 'CLUSTER') && 
        (txnProgressState.subTxnType != 'SERVICE') &&
        (txnProgressState.subTxnType != 'SERVICE-SMOKETEST') ) {

      skipIt = true;
    }

    return skipIt;
  }

  var generateSingleTxnProgressStateMarkup = function( txnProgressStateTitle, txnProgressStateCssClass ) {

    globalYui.log( 'Generating: ' + txnProgressStateTitle + '-' + txnProgressStateCssClass );

    var markup = 
      '<li>' + 
        '<div class=' + txnProgressStateCssClass + '>' +
          /* TODO XXX Format this nicer by camel-casing and putting the status in parentheses. */
          txnProgressStateTitle +
        '</div>' +
      '</li>';

    globalYui.log("XXX" + markup);
    return markup;
  }

  /**************************** Public data members ********************************/
  globalYui.log( 'Got txnId:' + txnProgressContext.txnId );

  this.txnProgressContext = txnProgressContext;
  this.txnProgressStatusMessage = txnProgressStatusMessage;
  this.txnProgressPostCompletionFixup = txnProgressPostCompletionFixup;
  var requestStr = '?clusterName=' + this.txnProgressContext.clusterName + '&txnId=' + this.txnProgressContext.txnId;

  if ("deployUser" in this.txnProgressContext) {
    requestStr += '&deployUser=' + this.txnProgressContext.deployUser;
  }

  var pdpDataSourceContext = {
    source: '../php/frontend/fetchTxnProgress.php',
    schema: {
      metaFields: {
        progress: 'progress'
      }
    },

    request: requestStr, 
    pollInterval: 3000,
    maxFailedAttempts: 5
  };

  var pdpResponseHandler = {
    success: function (e, pdp) {

      /* What we're here to render. */
      var txnProgressMarkup = 
        '<img id=txnProgressLoadingImgId class=loadingImg src=../images/loading.gif />'; 

      var noNeedForFurtherPolling = false;
      var txnProgressStatusDivContent = '';
      var txnProgressStatusDivCssClass = '';

      var txnProgress = e.response.meta.progress;

      /* Guard against race conditions where txnProgress is null because the 
       * txn hasn't had time to be kicked off yet.
       */
      if (txnProgress) {

        /* The first time we get back meaningful progress data, pause the 
         * automatic polling to avoid race conditions where response N+1 
         * is made (and returns with fresh data) while request N hasn't 
         * yet been fully processed.
         *
         * We'll unpause at the end, after we've performed the rendering
         * of the updated states.
         */
        pdp.pause();

        var txnProgressStates = txnProgress.subTxns || [];
        globalYui.log(globalYui.Lang.dump(txnProgressStates));

        txnProgressMarkup = '<ul id=txnProgressStatesListId>';

        var progressStateIndex = 0;

        /* Generate markup for all the "done" states. */
        for( ; progressStateIndex < txnProgressStates.length; ++progressStateIndex ) {

          var presentTxnProgressState = txnProgressStates[ progressStateIndex ];

          /* Step over any progress states that don't deserve to be shown. */
          if( txnProgressStateShouldBeSkipped( presentTxnProgressState ) ) {
            continue;
          }

          /* The first sign of a state that isn't done, and we're outta here. */
          if( presentTxnProgressState.progress != 'COMPLETED' ) {
            break;
          }

          globalYui.log( 'Done loop - ' + progressStateIndex );

          txnProgressMarkup += generateSingleTxnProgressStateMarkup
            ( presentTxnProgressState.description, 'txnProgressStateDone' );

            globalYui.log("Currently, markup is:" + txnProgressMarkup );
        }

        /* Next, generate markup for the first "in-progress" state. */
        for( ; progressStateIndex < txnProgressStates.length; ++progressStateIndex ) {

          var presentTxnProgressState = txnProgressStates[ progressStateIndex ];

          /* Step over any progress states that don't deserve to be shown. */
          if( txnProgressStateShouldBeSkipped( presentTxnProgressState ) ) {
            continue;
          }

          /* The first state that shouldn't be skipped is marked as being
           * "in-progress", even if presentTxnProgressState.progress is
           * not explicitly set to "IN_PROGRESS".
           *
           * This is to take care of race conditions where the poll to the 
           * backend is made at a time when the previous state has 
           * "COMPLETED" but the next state hasn't been started yet (which
           * means it's "PENDING") - if we were explicitly looking for
           * "IN_PROGRESS", there'd be nothing to show in this loop and it
           * would run to the end of txnProgressStates hunting for that
           * elusive "IN_PROGRESS", thus not even showing any of the 
           * "PENDING" states, causing a momentary jitter in the rendering
           * (see AMBARI-344 for an example).
           */
          globalYui.log( 'In-progress/failed - ' + progressStateIndex );

          /* Decide upon what CSS class to assign to the currently-in-progress
           * state - if an error was marked as having been encountered, assign
           * the fitting .txnProgressStateError, else just annoint it with
           * .txnProgressStateInProgress 
           */
          var currentProgressStateCssClass = 'txnProgressStateInProgress';

          /* The 2 possible indications of error are:
           * 
           * a) presentTxnProgressState.progress is 'IN_PROGRESS' but 
           *    txnProgress.encounteredError is true.
           * b) presentTxnProgressState.progress is 'FAILED'.
           */
          if( (txnProgress.encounteredError) || 
              (presentTxnProgressState.progress == 'FAILED') ) {

            currentProgressStateCssClass = 'txnProgressStateError';
          }

          /* And generate markup for this "in-progress" state. */
          txnProgressMarkup += generateSingleTxnProgressStateMarkup
            ( presentTxnProgressState.description, currentProgressStateCssClass );

          /* It's important to manually increment progressStateIndex here, 
           * to set it up correctly for the upcoming loop.
           */
          ++progressStateIndex;

          /* Remember, we only care for the FIRST "in-progress" state.
           *
           * Any following "in-progress" states will all be marked as 
           * "pending", so as to avoid the display from becoming 
           * disorienting (with multiple states "in-progress").
           */
          break;
        }

        /* Finally, generate markup for all the "pending" states. */
        for( ; progressStateIndex < txnProgressStates.length; ++progressStateIndex ) {

          var presentTxnProgressState = txnProgressStates[ progressStateIndex ];

          /* Step over any progress states that don't deserve to be shown. */
          if( txnProgressStateShouldBeSkipped( presentTxnProgressState ) ) {
            continue;
          }

          globalYui.log( 'Pending loop - ' + progressStateIndex );
          txnProgressMarkup += generateSingleTxnProgressStateMarkup
            ( presentTxnProgressState.description, 'txnProgressStatePending' );
        }

        txnProgressMarkup += '</ul>';

        /* Make sure we have some progress data to show - if not, 
         * we'll just show a loading image until this is non-null.
         *
         * The additional check for txnProgress.processRunning is to account 
         * for cases where there are no subTxns (because it's all a no-op at 
         * the backend) - the loading image should only be shown as long as 
         * the backend is still working; after that, we should break out of
         * the loading image loop and let the user know that there was
         * nothing to be done.
         */
        if( txnProgress.subTxns == null ) {
          if( txnProgress.processRunning == 0 ) {
            txnProgressMarkup = 
              '<br/>' + 
              '<div class=txnNoOpMsg>' + 
                'Nothing to do for this transaction; enjoy the freebie!' +
              '</div>' + 
              '<br/>';
          } 
          else {
            txnProgressMarkup = 
              '<img id=txnProgressLoadingImgId class=loadingImg src=../images/loading.gif />';
          }
        }

        /* We can break this polling cycle in one of 2 ways: 
         * 
         * 1) If we are explicitly told by the backend that we're done.
         */
        if( txnProgress.processRunning == 0 ) {

          noNeedForFurtherPolling = true;
          /* Be optimistic and assume that no errors were encountered (we'll
           * get more in touch with reality further below).
           */
          txnProgressStatusDivContent = this.txnProgressStatusMessage.success;
          txnProgressStatusDivCssClass = 'statusOk';
        }

        /* 2) If we encounter an error.
         *
         * Note how this is placed after the previous check, so as to serve
         * as an override in case the backend explicitly told us that we're
         * done, but an error was encountered in that very last progress report.
         */
        if( txnProgress.encounteredError ) {

          noNeedForFurtherPolling = true;
          txnProgressStatusDivContent = this.txnProgressStatusMessage.failure;
          txnProgressStatusDivCssClass = 'statusError';
        }
      }

      /* Render txnProgressMarkup before making any decisions about the
       * future state of pdp. 
       */
      globalYui.log('About to generate markup: ' + txnProgressMarkup);
      globalYui.one('#txnProgressDynamicRenderDivId').setContent( txnProgressMarkup );

      /* And before checking out, decide whether we're done with this txn 
       * or whether any more polling is required.
       */
      if (noNeedForFurtherPolling) {

        /* We've made all the progress we could have, so stop polling. */
        pdp.stop();

        var txnProgressStatusDiv = globalYui.one('#txnProgressStatusDivId');
        
        txnProgressStatusDiv.addClass(txnProgressStatusDivCssClass);
        txnProgressStatusDiv.one('#txnProgressStatusMessageDivId').setContent(txnProgressStatusDivContent);
        txnProgressStatusDiv.setStyle('display', 'block');

        /* Run the post-completion fixups. */
        if (txnProgressStatusDivCssClass == 'statusOk') {
          if (this.txnProgressPostCompletionFixup.success) {
            this.txnProgressPostCompletionFixup.success(this);
          }
        }
        else if (txnProgressStatusDivCssClass == 'statusError') {
          if (this.txnProgressPostCompletionFixup.failure) {
            this.txnProgressPostCompletionFixup.failure(this);
          }
        }
      }
      else {
        /* There's still more progress to be made, so unpause. */
        pdp.unPause();
      }

    }.bind(this),

    failure: function (e, pdp) {
      alert('Failed to fetch more progress!');
    }.bind(this)
  };

  this.periodicDataPoller = new PeriodicDataPoller( pdpDataSourceContext, pdpResponseHandler );
}

TxnProgressWidget.prototype.show = function() {
  
  /* Start with a clean slate for #txnProgressStatusDivId, regardless of 
   * the mess previous uses might have left it in.
   */
  var txnProgressStatusDiv = globalYui.one('#txnProgressStatusDivId');
  txnProgressStatusDiv.one('#txnProgressStatusMessageDivId').setContent('');
  txnProgressStatusDiv.one('#txnProgressStatusActionsDivId').setContent('');
  /* Remove the CSS statusOk/statusError classes from txnProgressStatusDiv 
   * as well - sure would be nice to remove all classes that match a 
   * pattern, but oh well. 
   */
  txnProgressStatusDiv.removeClass('statusOk');
  txnProgressStatusDiv.removeClass('statusError');

  /* Similarly, set a clean slate for #txnProgressDynamicRenderDivId as well. */
  globalYui.one('#txnProgressDynamicRenderDivId').setContent
    ( '<img id=txnProgressLoadingImgId class=loadingImg src=../images/loading.gif />' );

  globalYui.one('#blackScreenDivId').setStyle('display', 'block');
  globalYui.one('#txnProgressCoreDivId').setStyle('display','block');  

  this.periodicDataPoller.start();
}

TxnProgressWidget.prototype.hide = function() {

  this.periodicDataPoller.stop();

  globalYui.one('#txnProgressStatusDivId').setStyle('display', 'none'); 
  globalYui.one('#txnProgressCoreDivId').setStyle('display','none');
  globalYui.one('#blackScreenDivId').setStyle('display', 'none');
}