diff --git a/CHANGES.md b/CHANGES.md
index 36aad1f95daa..952a44d2595c 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -5,8 +5,10 @@ Change Log
 
 * Deprecated
    * `GoogleEarthImageryProvider` has been deprecated and will be removed in Cesium 1.37, use `GoogleEarthEnterpriseMapsProvider` instead. 
-   * The `throttleRequest` parameter for `TerrainProvider.requestTileGeometry`, `CesiumTerrainProvider.requestTileGeometry`, `VRTheWorldTerrainProvider.requestTileGeometry`, and `EllipsoidTerrainProvider.requestTileGeometry` is deprecated and will be replaced with an optional `Request` object. The `throttleRequests` parameter will be removed in 1.36, instead to throttle requests set the request's `throttle` property to `true`. 
-   * The ability to provide a Promise for the `options.url` parameter of `loadWithXhr` is deprecated. The same applies for the `url` parameter for `loadArrayBuffer`, `loadBlob`, `loadImageViaBlob`, `loadText`, `loadJson`, `loadXML`, `loadImage`, `loadCRN`, `loadKTX`, and `loadCubeMap`. This will be removed in 1.36, instead `url` must be a string.
+   * The `throttleRequest` parameter for `TerrainProvider.requestTileGeometry`, `CesiumTerrainProvider.requestTileGeometry`, `VRTheWorldTerrainProvider.requestTileGeometry`, and `EllipsoidTerrainProvider.requestTileGeometry` is deprecated and will be replaced with an optional `Request` object. The `throttleRequests` parameter will be removed in 1.37. Instead to throttle requests set the request's `throttle` property to `true`.
+   * The ability to provide a Promise for the `options.url` parameter of `loadWithXhr` and for the `url` parameter of `loadArrayBuffer`, `loadBlob`, `loadImageViaBlob`, `loadText`, `loadJson`, `loadXML`, `loadImage`, `loadCRN`, `loadKTX`, and `loadCubeMap` is deprecated. This will be removed in 1.37, instead `url` must be a string.
+
+* Added an `options.request` parameter to `loadWithXhr` and a `request` parameter to `loadArrayBuffer`, `loadBlob`, `loadImageViaBlob`, `loadText`, `loadJson`, `loadJsonp`, `loadXML`, `loadImageFromTypedArray`, `loadImage`, `loadCRN`, and `loadKTX`.
 
 ### 1.34 - 2017-06-01
 
diff --git a/Source/Core/CesiumTerrainProvider.js b/Source/Core/CesiumTerrainProvider.js
index 677b47897af2..11fb68504de8 100644
--- a/Source/Core/CesiumTerrainProvider.js
+++ b/Source/Core/CesiumTerrainProvider.js
@@ -493,7 +493,7 @@ define([
      * @param {Number} x The X coordinate of the tile for which to request geometry.
      * @param {Number} y The Y coordinate of the tile for which to request geometry.
      * @param {Number} level The level of the tile for which to request geometry.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      *
      * @returns {Promise.<TerrainData>|undefined} A promise for the requested geometry.  If this method
      *          returns undefined instead of a promise, it is an indication that too many requests are already
@@ -534,7 +534,7 @@ define([
         }
 
         if (typeof request === 'boolean') {
-            deprecationWarning('throttleRequests', 'The throttleRequest parameter for requestTileGeometry was deprecated in Cesium 1.35.  It will be removed in 1.36.');
+            deprecationWarning('throttleRequests', 'The throttleRequest parameter for requestTileGeometry was deprecated in Cesium 1.35.  It will be removed in 1.37.');
             request = new Request({
                 throttle : request,
                 throttleByServer : request,
diff --git a/Source/Core/EllipsoidTerrainProvider.js b/Source/Core/EllipsoidTerrainProvider.js
index 31189d7ec512..58adf683ca83 100644
--- a/Source/Core/EllipsoidTerrainProvider.js
+++ b/Source/Core/EllipsoidTerrainProvider.js
@@ -152,7 +152,7 @@ define([
      * @param {Number} x The X coordinate of the tile for which to request geometry.
      * @param {Number} y The Y coordinate of the tile for which to request geometry.
      * @param {Number} level The level of the tile for which to request geometry.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      *
      * @returns {Promise.<TerrainData>|undefined} A promise for the requested geometry.  If this method
      *          returns undefined instead of a promise, it is an indication that too many requests are already
diff --git a/Source/Core/GoogleEarthEnterpriseMetadata.js b/Source/Core/GoogleEarthEnterpriseMetadata.js
index be0b51f72dd0..d5ee5dd6185b 100644
--- a/Source/Core/GoogleEarthEnterpriseMetadata.js
+++ b/Source/Core/GoogleEarthEnterpriseMetadata.js
@@ -294,7 +294,7 @@ define([
      *
      * @param {String} [quadKey=''] The quadkey to retrieve the packet for.
      * @param {Number} [version=1] The cnode version to be used in the request.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      *
      * @private
      */
@@ -373,7 +373,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      *
      * @returns {Promise<GoogleEarthEnterpriseTileInformation>} A promise that resolves to the tile info for the requested quad key
      *
@@ -409,7 +409,7 @@ define([
                         throttle : request.throttle,
                         throttleByServer : request.throttleByServer,
                         type : request.type,
-                        distance : request.distance
+                        priorityFunction : request.priorityFunction
                     });
                     return populateSubtree(that, quadKey, subtreeRequest);
                 });
@@ -439,7 +439,7 @@ define([
                     throttle : request.throttle,
                     throttleByServer : request.throttleByServer,
                     type : request.type,
-                    distance : request.distance
+                    priorityFunction : request.priorityFunction
                 });
                 return populateSubtree(that, quadKey, subtreeRequest);
             })
diff --git a/Source/Core/GoogleEarthEnterpriseTerrainProvider.js b/Source/Core/GoogleEarthEnterpriseTerrainProvider.js
index 5ed226e6c56a..d5e2928633b5 100644
--- a/Source/Core/GoogleEarthEnterpriseTerrainProvider.js
+++ b/Source/Core/GoogleEarthEnterpriseTerrainProvider.js
@@ -346,7 +346,7 @@ define([
      * @param {Number} x The X coordinate of the tile for which to request geometry.
      * @param {Number} y The Y coordinate of the tile for which to request geometry.
      * @param {Number} level The level of the tile for which to request geometry.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<TerrainData>|undefined} A promise for the requested geometry.  If this method
      *          returns undefined instead of a promise, it is an indication that too many requests are already
      *          pending and the request will be retried later.
@@ -448,7 +448,7 @@ define([
             sharedRequest = terrainRequests[q];
         } else { // Create new request for terrain
             if (typeof request === 'boolean') {
-                deprecationWarning('throttleRequests', 'The throttleRequest parameter for requestTileGeometry was deprecated in Cesium 1.35.  It will be removed in 1.36.');
+                deprecationWarning('throttleRequests', 'The throttleRequest parameter for requestTileGeometry was deprecated in Cesium 1.35.  It will be removed in 1.37.');
                 request = new Request({
                     throttle : request,
                     throttleByServer : request,
diff --git a/Source/Core/Heap.js b/Source/Core/Heap.js
index a455ce6d3cd2..77da477e03dc 100644
--- a/Source/Core/Heap.js
+++ b/Source/Core/Heap.js
@@ -18,14 +18,16 @@ define([
      * @constructor
      * @private
      *
-     * @param {Function} comparator The comparator to use for the heap. If comparator(a, b) is less than 0, sort a to a lower index than b, otherwise sort to a higher index.
+     * @param {Object} options Object with the following properties:
+     * @param {Function} options.comparator The comparator to use for the heap. If comparator(a, b) is less than 0, sort a to a lower index than b, otherwise sort to a higher index.
      */
-    function Heap(comparator) {
+    function Heap(options) {
         //>>includeStart('debug', pragmas.debug);
-        Check.defined('comparator', comparator);
+        Check.typeOf.object('options', options);
+        Check.defined('options.comparator', options.comparator);
         //>>includeEnd('debug');
 
-        this._comparator = comparator;
+        this._comparator = options.comparator;
         this._array = [];
         this._length = 0;
         this._maximumLength = undefined;
@@ -78,6 +80,19 @@ define([
                     this._array.length = value;
                 }
             }
+        },
+
+        /**
+         * The comparator to use for the heap. If comparator(a, b) is less than 0, sort a to a lower index than b, otherwise sort to a higher index.
+         *
+         * @memberof Heap.prototype
+         *
+         * @type (Function}
+         */
+        comparator : {
+            get : function() {
+                return this._comparator;
+            }
         }
     });
 
@@ -156,6 +171,7 @@ define([
 
         var array = this._array;
         var comparator = this._comparator;
+        var maximumLength = this._maximumLength;
 
         var index = this._length++;
         if (index < array.length) {
@@ -176,9 +192,9 @@ define([
 
         var removedElement;
 
-        if (defined(this._maximumLength) && (this._length > this._maximumLength)) {
-            removedElement = array[this.maximumLength];
-            this._length = this._maximumLength;
+        if (defined(maximumLength) && (this._length > maximumLength)) {
+            removedElement = array[maximumLength];
+            this._length = maximumLength;
         }
 
         return removedElement;
diff --git a/Source/Core/Request.js b/Source/Core/Request.js
index 7661249d0569..44a263bc7837 100644
--- a/Source/Core/Request.js
+++ b/Source/Core/Request.js
@@ -14,7 +14,7 @@ define([
     'use strict';
 
     /**
-     * Stores information for making a request. This should not be constructed directly. Instead, call the appropriate load function, like `loadWithXhr`, `loadImage`, etc.
+     * Stores information for making a request. In general this does not need to be constructed directly.
      *
      * @alias Request
      * @constructor
@@ -22,11 +22,12 @@ define([
      * @param {Object} [options] An object with the following properties:
      * @param {Boolean} [options.url] The url to request.
      * @param {Function} [options.requestFunction] The actual function that makes the request. The function takes no arguments and returns a promise for the requested data.
+     * @param {Function} [options.cancelFunction] Function to call when a request is cancelled. The function takes no arguments.
+     * @param {Function} [options.priorityFunction] Function that is called when the request is updated. The function takes no arguments and returns the updated priority value.
+     * @param {Number} [options.priority=0.0] The initial priority of the request.
      * @param {Boolean} [options.throttle=false] Whether to throttle and prioritize the request. If false, the request will be sent immediately. If true, the request will be throttled and sent based on priority.
      * @param {Boolean} [options.throttleByServer=false] Whether to throttle the request by server.
      * @param {RequestType} [options.type=RequestType.OTHER] The type of request.
-     * @param {Number} [options.distance=0.0] The distance from the camera, used to prioritize requests.
-     * @param {Number} [options.screenSpaceError=0.0] The screen space error, used to prioritize requests.
      */
     function Request(options) {
         options = defaultValue(options, defaultValue.EMPTY_OBJECT);
@@ -45,11 +46,35 @@ define([
          * The actual function that makes the request. The function takes no arguments and returns a promise for the requested data.
          *
          * @type {Function}
-         *
-         * @private
          */
         this.requestFunction = options.requestFunction;
 
+        /**
+         * Function to call when a request is cancelled. The function takes no arguments.
+         *
+         * @type {Function}
+         */
+        this.cancelFunction = options.cancelFunction;
+
+        /**
+         * Function that is called when the request is updated. The function takes no arguments and returns the updated priority value.
+         *
+         * @type {Function}
+         */
+        this.priorityFunction = options.priorityFunction;
+
+        /**
+         * Priority is a unit-less value where lower values represent higher priority.
+         * For world-based objects, this is usually the distance from the camera.
+         * A request that does not have a priority function defaults to a priority of 0.
+         *
+         * If priorityFunction is defined, this value is updated every frame with the result of that call.
+         *
+         * @type {Number}
+         * @default 0.0
+         */
+        this.priority = defaultValue(options.priority, 0.0);
+
         /**
          * Whether to throttle and prioritize the request. If false, the request will be sent immediately. If true, the
          * request will be throttled and sent based on priority.
@@ -84,52 +109,22 @@ define([
         this.type = defaultValue(options.type, RequestType.OTHER);
 
         /**
-         * The distance from the camera, used to prioritize requests. This value may be edited continually to update
-         * the request's priority.
-         *
-         * @type {Number}
-         *
-         * @default 0.0
-         */
-        this.distance = defaultValue(options.distance, 0.0);
-
-        /**
-         * The screen space error, used to prioritize requests. This value may be edited continually to update
-         * the request's priority.
-         *
-         * @type {Number}
-         *
-         * @default 0.0
-         */
-        this.screenSpaceError = defaultValue(options.screenSpaceError, 0.0);
-
-        /**
-         * The request server, derived from the url.
+         * A key used to identify the server that a request is going to. It is derived from the url's authority and scheme.
          *
          * @type {String}
          *
          * @private
          */
-        this.server = undefined;
+        this.serverKey = undefined;
 
         /**
          * The current state of the request.
          *
          * @type {RequestState}
-         *
-         * @private
+         * @readonly
          */
         this.state = RequestState.UNISSUED;
 
-        /**
-         * Reference to the underlying XMLHttpRequest so that it may be aborted in RequestScheduler.
-         *
-         * @type {Object}
-         *
-         * @private
-         */
-        this.xhr = undefined;
-
         /**
          * The requests's deferred promise.
          *
diff --git a/Source/Core/RequestScheduler.js b/Source/Core/RequestScheduler.js
index 2c302924d78b..264838a5b826 100644
--- a/Source/Core/RequestScheduler.js
+++ b/Source/Core/RequestScheduler.js
@@ -24,7 +24,7 @@ define([
     'use strict';
 
     function sortRequests(a, b) {
-        return a.distance - b.distance;
+        return a.priority - b.priority;
     }
 
     var statistics = {
@@ -37,16 +37,17 @@ define([
     };
 
     var priorityHeapLength = 20;
-    var requestHeap = new Heap(sortRequests);
+    var requestHeap = new Heap({
+        comparator : sortRequests
+    });
     requestHeap.maximumLength = priorityHeapLength;
     requestHeap.reserve(priorityHeapLength);
-    var activeRequests = [];
 
+    var activeRequests = [];
     var numberOfActiveRequestsByServer = {};
 
     var pageUri = typeof document !== 'undefined' ? new Uri(document.location.href) : new Uri();
 
-
     /**
      * Tracks the number of active requests and prioritizes incoming requests.
      *
@@ -67,7 +68,7 @@ define([
     /**
      * The maximum number of simultaneous active requests per server. Un-throttled requests do not observe this limit.
      * @type {Number}
-     * @default 50
+     * @default 6
      */
     RequestScheduler.maximumRequestsPerServer = 6;
 
@@ -76,7 +77,7 @@ define([
      * @type {Boolean}
      * @default true
      */
-    RequestScheduler.debugThrottle = true;
+    RequestScheduler.throttleRequests = true;
 
     /**
      * When true, log statistics to the console every frame
@@ -128,69 +129,14 @@ define([
         }
     });
 
-    /**
-     * Get the server name from a given url.
-     *
-     * @param {String} url The url.
-     * @returns {String} The server name.
-     */
-    RequestScheduler.getServer = function(url) {
-        //>>includeStart('debug', pragmas.debug);
-        Check.typeOf.string('url', url);
-        //>>includeEnd('debug');
-
-        var uri = new Uri(url).resolve(pageUri);
-        uri.normalize();
-        var serverName = uri.authority;
-        if (!/:/.test(serverName)) {
-            serverName = serverName + ':' + (uri.scheme === 'https' ? '443' : '80');
-        }
-
-        var length = numberOfActiveRequestsByServer[serverName];
-        if (!defined(length)) {
-            numberOfActiveRequestsByServer[serverName] = 0;
-        }
-
-        return serverName;
-    };
-
-    /**
-     * For testing only.
-     *
-     * @private
-     */
-    RequestScheduler.clearForSpecs = function() {
-        while (requestHeap.length > 0) {
-            var request = requestHeap.pop();
-            cancelRequest(request);
+    function updatePriority(request) {
+        if (defined(request.priorityFunction)) {
+            request.priority = request.priorityFunction();
         }
-        var length = activeRequests.length;
-        for (var i = 0; i < length; ++i) {
-            cancelRequest(activeRequests[i]);
-        }
-        activeRequests.length = 0;
-        numberOfActiveRequestsByServer = {};
-
-        // Clear stats
-        statistics.numberOfAttemptedRequests = 0;
-        statistics.numberOfActiveRequests = 0;
-        statistics.numberOfCancelledRequests = 0;
-        statistics.numberOfCancelledActiveRequests = 0;
-        statistics.numberOfFailedRequests = 0;
-        statistics.numberOfActiveRequestsEver = 0;
-    };
-
-    /**
-     * For testing only.
-     *
-     * @private
-     */
-    RequestScheduler.numberOfActiveRequestsByServer = function(serverName) {
-        return numberOfActiveRequestsByServer[serverName];
-    };
+    }
 
-    function serverHasOpenSlots(server) {
-        return numberOfActiveRequestsByServer[server] < RequestScheduler.maximumRequestsPerServer;
+    function serverHasOpenSlots(serverKey) {
+        return numberOfActiveRequestsByServer[serverKey] < RequestScheduler.maximumRequestsPerServer;
     }
 
     function issueRequest(request) {
@@ -204,14 +150,11 @@ define([
     function getRequestReceivedFunction(request) {
         return function(results) {
             if (request.state === RequestState.CANCELLED) {
+                // If the data request comes back but the request is cancelled, ignore it.
                 return;
             }
-
-            if (request.state !== RequestState.IGNORED) {
-                --statistics.numberOfActiveRequests;
-                --numberOfActiveRequestsByServer[request.server];
-            }
-
+            --statistics.numberOfActiveRequests;
+            --numberOfActiveRequestsByServer[request.serverKey];
             request.state = RequestState.RECEIVED;
             request.deferred.resolve(results);
         };
@@ -220,15 +163,12 @@ define([
     function getRequestFailedFunction(request) {
         return function(error) {
             if (request.state === RequestState.CANCELLED) {
+                // If the data request comes back but the request is cancelled, ignore it.
                 return;
             }
-
-            if (request.state !== RequestState.IGNORED) {
-                ++statistics.numberOfFailedRequests;
-                --statistics.numberOfActiveRequests;
-                --numberOfActiveRequestsByServer[request.server];
-            }
-
+            ++statistics.numberOfFailedRequests;
+            --statistics.numberOfActiveRequests;
+            --numberOfActiveRequestsByServer[request.serverKey];
             request.state = RequestState.FAILED;
             request.deferred.reject(error);
         };
@@ -240,8 +180,7 @@ define([
         activeRequests.push(request);
         ++statistics.numberOfActiveRequests;
         ++statistics.numberOfActiveRequestsEver;
-        ++numberOfActiveRequestsByServer[request.server];
-
+        ++numberOfActiveRequestsByServer[request.serverKey];
         request.requestFunction().then(getRequestReceivedFunction(request)).otherwise(getRequestFailedFunction(request));
         return promise;
     }
@@ -250,32 +189,30 @@ define([
         var active = request.state === RequestState.ACTIVE;
         request.state = RequestState.CANCELLED;
         ++statistics.numberOfCancelledRequests;
-        request.deferred.reject('Cancelled');
+        request.deferred.reject();
 
         if (active) {
-            // Despite the Request being cancelled, the xhr request is still in flight.
-            // When it resolves getRequestReceivedFunction or getRequestFailedFunction will ignore it.
             --statistics.numberOfActiveRequests;
-            --numberOfActiveRequestsByServer[request.server];
+            --numberOfActiveRequestsByServer[request.serverKey];
             ++statistics.numberOfCancelledActiveRequests;
-            if (defined(request.xhr)) {
-                request.xhr.abort();
-            }
+        }
+
+        if (defined(request.cancelFunction)) {
+            request.cancelFunction();
         }
     }
 
     /**
-     * Issuers of a request should update properties of requests. At the end of the frame,
-     * RequestScheduler.update is called to start, cancel, or defer requests.
+     * Sort requests by priority and start requests.
      */
     RequestScheduler.update = function() {
+        var i;
         var request;
 
         // Loop over all active requests. Cancelled, failed, or received requests are removed from the array to make room for new requests.
-        // If an active request is cancelled, its XMLHttpRequest will be aborted.
         var removeCount = 0;
         var activeLength = activeRequests.length;
-        for (var i = 0; i < activeLength; ++i) {
+        for (i = 0; i < activeLength; ++i) {
             request = activeRequests[i];
             if (request.cancelled) {
                 // Request was explicitly cancelled
@@ -293,7 +230,12 @@ define([
         }
         activeRequests.length -= removeCount;
 
-        // Resort the heap since priority may have changed. Distance and sse are updated prior to getting here.
+        // Update priority of issued requests and resort the heap
+        var issuedRequests = requestHeap.internalArray;
+        var issuedLength = requestHeap.length;
+        for (i = 0; i < issuedLength; ++i) {
+            updatePriority(issuedRequests[i]);
+        }
         requestHeap.resort();
 
         // Get the number of open slots and fill with the highest priority requests.
@@ -309,7 +251,7 @@ define([
                 continue;
             }
 
-            if (request.throttleByServer && !serverHasOpenSlots(request.server)) {
+            if (request.throttleByServer && !serverHasOpenSlots(request.serverKey)) {
                 // Open slots are available, but the request is throttled by its server. Cancel and try again later.
                 cancelRequest(request);
                 continue;
@@ -322,6 +264,33 @@ define([
         updateStatistics();
     };
 
+    /**
+     * Get the server key from a given url.
+     *
+     * @param {String} url The url.
+     * @returns {String} The server key.
+     */
+    RequestScheduler.getServerKey = function(url) {
+        //>>includeStart('debug', pragmas.debug);
+        Check.typeOf.string('url', url);
+        //>>includeEnd('debug');
+
+        var uri = new Uri(url).resolve(pageUri);
+        uri.normalize();
+        var serverKey = uri.authority;
+        if (!/:/.test(serverKey)) {
+            // If the authority does not contain a port number, add port 443 for https or port 80 for http
+            serverKey = serverKey + ':' + (uri.scheme === 'https' ? '443' : '80');
+        }
+
+        var length = numberOfActiveRequestsByServer[serverKey];
+        if (!defined(length)) {
+            numberOfActiveRequestsByServer[serverKey] = 0;
+        }
+
+        return serverKey;
+    };
+
     /**
      * Issue a request. If request.throttle is false, the request is sent immediately. Otherwise the request will be
      * queued and sorted by priority before being sent.
@@ -344,11 +313,11 @@ define([
 
         ++statistics.numberOfAttemptedRequests;
 
-        if (!defined(request.server)) {
-            request.server = RequestScheduler.getServer(request.url);
+        if (!defined(request.serverKey)) {
+            request.serverKey = RequestScheduler.getServerKey(request.url);
         }
 
-        if (!RequestScheduler.debugThrottle || !request.throttle) {
+        if (!RequestScheduler.throttleRequests || !request.throttle) {
             return startRequest(request);
         }
 
@@ -357,13 +326,14 @@ define([
             return undefined;
         }
 
-        if (request.throttleByServer && !serverHasOpenSlots(request.server)) {
+        if (request.throttleByServer && !serverHasOpenSlots(request.serverKey)) {
             // Server is saturated. Try again later.
             return undefined;
         }
 
         // Insert into the priority heap and see if a request was bumped off. If this request is the lowest
         // priority it will be returned.
+        updatePriority(request);
         var removedRequest = requestHeap.insert(request);
 
         if (defined(removedRequest)) {
@@ -408,8 +378,47 @@ define([
         clearStatistics();
     }
 
-    // For testing
-    RequestScheduler._requestHeap = requestHeap;
+    /**
+     * For testing only. Clears any requests that may not have completed from previous tests.
+     *
+     * @private
+     */
+    RequestScheduler.clearForSpecs = function() {
+        while (requestHeap.length > 0) {
+            var request = requestHeap.pop();
+            cancelRequest(request);
+        }
+        var length = activeRequests.length;
+        for (var i = 0; i < length; ++i) {
+            cancelRequest(activeRequests[i]);
+        }
+        activeRequests.length = 0;
+        numberOfActiveRequestsByServer = {};
+
+        // Clear stats
+        statistics.numberOfAttemptedRequests = 0;
+        statistics.numberOfActiveRequests = 0;
+        statistics.numberOfCancelledRequests = 0;
+        statistics.numberOfCancelledActiveRequests = 0;
+        statistics.numberOfFailedRequests = 0;
+        statistics.numberOfActiveRequestsEver = 0;
+    };
+
+    /**
+     * For testing only.
+     *
+     * @private
+     */
+    RequestScheduler.numberOfActiveRequestsByServer = function(serverKey) {
+        return numberOfActiveRequestsByServer[serverKey];
+    };
+
+    /**
+     * For testing only.
+     *
+     * @private
+     */
+    RequestScheduler.requestHeap = requestHeap;
 
     return RequestScheduler;
 });
diff --git a/Source/Core/RequestState.js b/Source/Core/RequestState.js
index 7cd28c4e65db..23498ef57fc7 100644
--- a/Source/Core/RequestState.js
+++ b/Source/Core/RequestState.js
@@ -6,16 +6,58 @@ define([
     'use strict';
 
     /**
-     * @private
+     * State of the request.
+     *
+     * @exports RequestState
      */
     var RequestState = {
-        UNISSUED : 0,   // Initial unissued state.
-        ISSUED : 1,     // Issued but not yet active. Will become active when open slots are available.
-        ACTIVE : 2,     // Actual http request has been sent.
-        RECEIVED : 3,   // Request completed successfully.
-        CANCELLED : 4,  // Request was cancelled, either explicitly or automatically because of low priority.
-        FAILED : 5,     // Request failed.
-        IGNORED : 6     // For RequestScheduler.clearForSpecs - lets requests finish but doesn't contribute to statistics.
+        /**
+         * Initial unissued state.
+         *
+         * @type Number
+         * @constant
+         */
+        UNISSUED : 0,
+
+        /**
+         * Issued but not yet active. Will become active when open slots are available.
+         *
+         * @type Number
+         * @constant
+         */
+        ISSUED : 1,
+
+        /**
+         * Actual http request has been sent.
+         *
+         * @type Number
+         * @constant
+         */
+        ACTIVE : 2,
+
+        /**
+         * Request completed successfully.
+         *
+         * @type Number
+         * @constant
+         */
+        RECEIVED : 3,
+
+        /**
+         * Request was cancelled, either explicitly or automatically because of low priority.
+         *
+         * @type Number
+         * @constant
+         */
+        CANCELLED : 4,
+
+        /**
+         * Request failed.
+         *
+         * @type Number
+         * @constant
+         */
+        FAILED : 5
     };
 
     return freezeObject(RequestState);
diff --git a/Source/Core/TerrainProvider.js b/Source/Core/TerrainProvider.js
index bfba9cf039d2..6bdc2e000016 100644
--- a/Source/Core/TerrainProvider.js
+++ b/Source/Core/TerrainProvider.js
@@ -198,7 +198,7 @@ define([
      * @param {Number} x The X coordinate of the tile for which to request geometry.
      * @param {Number} y The Y coordinate of the tile for which to request geometry.
      * @param {Number} level The level of the tile for which to request geometry.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      *
      * @returns {Promise.<TerrainData>|undefined} A promise for the requested geometry.  If this method
      *          returns undefined instead of a promise, it is an indication that too many requests are already
diff --git a/Source/Core/VRTheWorldTerrainProvider.js b/Source/Core/VRTheWorldTerrainProvider.js
index 79523b4afe62..5339deb9c6c8 100644
--- a/Source/Core/VRTheWorldTerrainProvider.js
+++ b/Source/Core/VRTheWorldTerrainProvider.js
@@ -260,7 +260,7 @@ define([
      * @param {Number} x The X coordinate of the tile for which to request geometry.
      * @param {Number} y The Y coordinate of the tile for which to request geometry.
      * @param {Number} level The level of the tile for which to request geometry.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<TerrainData>|undefined} A promise for the requested geometry.  If this method
      *          returns undefined instead of a promise, it is an indication that too many requests are already
      *          pending and the request will be retried later.
@@ -281,7 +281,7 @@ define([
         }
 
         if (typeof request === 'boolean') {
-            deprecationWarning('throttleRequests', 'The throttleRequest parameter for requestTileGeometry was deprecated in Cesium 1.35.  It will be removed in 1.36.');
+            deprecationWarning('throttleRequests', 'The throttleRequest parameter for requestTileGeometry was deprecated in Cesium 1.35.  It will be removed in 1.37.');
             request = new Request({
                 throttle : request,
                 throttleByServer : request,
diff --git a/Source/Core/loadArrayBuffer.js b/Source/Core/loadArrayBuffer.js
index d86038eff303..cc56d573ff6c 100644
--- a/Source/Core/loadArrayBuffer.js
+++ b/Source/Core/loadArrayBuffer.js
@@ -15,7 +15,7 @@ define([
      *
      * @param {String} url The URL of the binary data.
      * @param {Object} [headers] HTTP headers to send with the requests.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<ArrayBuffer>|undefined} a promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      * @example
diff --git a/Source/Core/loadBlob.js b/Source/Core/loadBlob.js
index aad892afdbd2..0e136d22c37c 100644
--- a/Source/Core/loadBlob.js
+++ b/Source/Core/loadBlob.js
@@ -15,7 +15,7 @@ define([
      *
      * @param {String} url The URL of the data.
      * @param {Object} [headers] HTTP headers to send with the requests.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Blob>|undefined} a promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      * @example
diff --git a/Source/Core/loadCRN.js b/Source/Core/loadCRN.js
index f01270bb6ffd..eac7121e1004 100644
--- a/Source/Core/loadCRN.js
+++ b/Source/Core/loadCRN.js
@@ -28,7 +28,7 @@ define([
      *
      * @param {String|ArrayBuffer} urlOrBuffer The URL of the binary data or an ArrayBuffer.
      * @param {Object} [headers] HTTP headers to send with the requests.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<CompressedTextureBuffer>|undefined} A promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      * @exception {RuntimeError} Unsupported compressed format.
diff --git a/Source/Core/loadImage.js b/Source/Core/loadImage.js
index b726ed800564..79c0c95673ae 100644
--- a/Source/Core/loadImage.js
+++ b/Source/Core/loadImage.js
@@ -35,7 +35,7 @@ define([
      * @param {Boolean} [allowCrossOrigin=true] Whether to request the image using Cross-Origin
      *        Resource Sharing (CORS).  CORS is only actually used if the image URL is actually cross-origin.
      *        Data URIs are never requested using CORS.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image>|undefined} a promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      *
@@ -65,7 +65,7 @@ define([
         if (typeof url !== 'string') {
             // Returning a promise here is okay because it is unlikely that anyone using the deprecated functionality is also
             // providing a Request object marked as throttled.
-            deprecationWarning('url promise', 'url as a Promise is deprecated and will be removed in Cesium 1.36');
+            deprecationWarning('url promise', 'url as a Promise is deprecated and will be removed in 1.37');
             return url.then(function(url) {
                 return makeRequest(url, allowCrossOrigin, request);
             });
diff --git a/Source/Core/loadImageViaBlob.js b/Source/Core/loadImageViaBlob.js
index 7f2e12ecc23f..9d2d32924534 100644
--- a/Source/Core/loadImageViaBlob.js
+++ b/Source/Core/loadImageViaBlob.js
@@ -28,7 +28,7 @@ define([
      * @exports loadImageViaBlob
      *
      * @param {String} url The source URL of the image.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image>|undefined} a promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      *
diff --git a/Source/Core/loadJson.js b/Source/Core/loadJson.js
index f3215e59448b..1fd1de5133c0 100644
--- a/Source/Core/loadJson.js
+++ b/Source/Core/loadJson.js
@@ -30,7 +30,7 @@ define([
      * @param {Object} [headers] HTTP headers to send with the request.
      * 'Accept: application/json,&#42;&#47;&#42;;q=0.01' is added to the request headers automatically
      * if not specified.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Object>|undefined} a promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      *
diff --git a/Source/Core/loadJsonp.js b/Source/Core/loadJsonp.js
index 5e721769340e..c63f3b58b0b0 100644
--- a/Source/Core/loadJsonp.js
+++ b/Source/Core/loadJsonp.js
@@ -33,7 +33,7 @@ define([
      * @param {Object} [options.parameters] Any extra query parameters to append to the URL.
      * @param {String} [options.callbackParameterName='callback'] The callback parameter name that the server expects.
      * @param {Proxy} [options.proxy] A proxy to use for the request. This object is expected to have a getURL function which returns the proxied URL, if needed.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Object>|undefined} a promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      *
diff --git a/Source/Core/loadKTX.js b/Source/Core/loadKTX.js
index 9be69bb2bb80..cd8600b10d14 100644
--- a/Source/Core/loadKTX.js
+++ b/Source/Core/loadKTX.js
@@ -39,7 +39,7 @@ define([
      *
      * @param {String|ArrayBuffer} urlOrBuffer The URL of the binary data or an ArrayBuffer.
      * @param {Object} [headers] HTTP headers to send with the requests.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<CompressedTextureBuffer>|undefined} A promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      * @exception {RuntimeError} Invalid KTX file.
diff --git a/Source/Core/loadText.js b/Source/Core/loadText.js
index a92607405edf..8ca386fe1bfa 100644
--- a/Source/Core/loadText.js
+++ b/Source/Core/loadText.js
@@ -15,7 +15,7 @@ define([
      *
      * @param {String} url The URL to request.
      * @param {Object} [headers] HTTP headers to send with the request.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<String>|undefined} a promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      *
diff --git a/Source/Core/loadWithXhr.js b/Source/Core/loadWithXhr.js
index d2102b50bac5..789b737607ac 100644
--- a/Source/Core/loadWithXhr.js
+++ b/Source/Core/loadWithXhr.js
@@ -74,7 +74,7 @@ define([
         if (typeof url !== 'string') {
             // Returning a promise here is okay because it is unlikely that anyone using the deprecated functionality is also
             // providing a Request object marked as throttled.
-            deprecationWarning('url promise', 'options.url as a Promise is deprecated and will be removed in Cesium 1.36');
+            deprecationWarning('url promise', 'options.url as a Promise is deprecated and will be removed in Cesium 1.37');
             return url.then(function(url) {
                 return makeRequest(options, url);
             });
@@ -95,7 +95,12 @@ define([
         request.url = url;
         request.requestFunction = function() {
             var deferred = when.defer();
-            request.xhr = loadWithXhr.load(url, responseType, method, data, headers, deferred, overrideMimeType);
+            var xhr = loadWithXhr.load(url, responseType, method, data, headers, deferred, overrideMimeType);
+            if (defined(xhr)) {
+                request.cancelFunction = function() {
+                    xhr.abort();
+                };
+            }
             return deferred.promise;
         };
 
diff --git a/Source/Core/loadXML.js b/Source/Core/loadXML.js
index e43ef0064367..780ff6ad5eea 100644
--- a/Source/Core/loadXML.js
+++ b/Source/Core/loadXML.js
@@ -15,7 +15,7 @@ define([
      *
      * @param {String} url The URL to request.
      * @param {Object} [headers] HTTP headers to send with the request.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<XMLDocument>|undefined} a promise that will resolve to the requested data when loaded. Returns undefined if <code>request.throttle</code> is true and the request does not have high enough priority.
      *
      *
diff --git a/Source/Scene/ArcGisMapServerImageryProvider.js b/Source/Scene/ArcGisMapServerImageryProvider.js
index 0970ce28b2ed..fa53f26b766e 100644
--- a/Source/Scene/ArcGisMapServerImageryProvider.js
+++ b/Source/Scene/ArcGisMapServerImageryProvider.js
@@ -579,7 +579,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/BingMapsImageryProvider.js b/Source/Scene/BingMapsImageryProvider.js
index 3dea11dc2da9..fdcf1531b7c6 100644
--- a/Source/Scene/BingMapsImageryProvider.js
+++ b/Source/Scene/BingMapsImageryProvider.js
@@ -522,7 +522,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/GlobeSurfaceTile.js b/Source/Scene/GlobeSurfaceTile.js
index 90b0da7a0020..1e27a4df4f80 100644
--- a/Source/Scene/GlobeSurfaceTile.js
+++ b/Source/Scene/GlobeSurfaceTile.js
@@ -252,6 +252,12 @@ define([
         });
     }
 
+    function createPriorityFunction(surfaceTile, frameState) {
+        return function() {
+            return surfaceTile.tileBoundingRegion.distanceToCamera(frameState);
+        };
+    }
+
     GlobeSurfaceTile.processStateMachine = function(tile, frameState, terrainProvider, imageryLayerCollection, vertexArraysToDestroy) {
         var surfaceTile = tile.data;
         if (!defined(surfaceTile)) {
@@ -261,8 +267,10 @@ define([
             surfaceTile.tileBoundingRegion = createTileBoundingRegion(tile);
         }
 
-        // Update distance while the tile loads
-        tile._distance = surfaceTile.tileBoundingRegion.distanceToCamera(frameState);
+        if (!defined(tile._priorityFunction)) {
+            // The priority function is used to prioritize requests among all requested tiles
+            tile._priorityFunction = createPriorityFunction(surfaceTile, frameState);
+        }
 
         if (tile.state === QuadtreeTileLoadState.START) {
             prepareNewTile(tile, terrainProvider, imageryLayerCollection);
@@ -328,6 +336,7 @@ define([
 
             if (isDoneLoading) {
                 tile.state = QuadtreeTileLoadState.DONE;
+                tile._priorityFunction = undefined;
             }
         }
     };
@@ -360,7 +369,7 @@ define([
         var suspendUpsampling = false;
 
         if (defined(loaded)) {
-            loaded.processLoadStateMachine(frameState, terrainProvider, tile.x, tile.y, tile.level, tile._distance);
+            loaded.processLoadStateMachine(frameState, terrainProvider, tile.x, tile.y, tile.level, tile._priorityFunction);
 
             // Publish the terrain data on the tile as soon as it is available.
             // We'll potentially need it to upsample child tiles.
diff --git a/Source/Scene/GoogleEarthEnterpriseImageryProvider.js b/Source/Scene/GoogleEarthEnterpriseImageryProvider.js
index 79c0f4d56df8..3dfb87bbcf20 100644
--- a/Source/Scene/GoogleEarthEnterpriseImageryProvider.js
+++ b/Source/Scene/GoogleEarthEnterpriseImageryProvider.js
@@ -432,7 +432,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
@@ -457,7 +457,7 @@ define([
                     throttle : request.throttle,
                     throttleByServer : request.throttleByServer,
                     type : request.type,
-                    distance : request.distance
+                    priorityFunction : request.priorityFunction
                 });
                 metadata.populateSubtree(x, y, level, metadataRequest);
                 return undefined; // No metadata so return undefined so we can be loaded later
diff --git a/Source/Scene/GoogleEarthEnterpriseMapsProvider.js b/Source/Scene/GoogleEarthEnterpriseMapsProvider.js
index ab4adf47350e..4251d3d2d40a 100644
--- a/Source/Scene/GoogleEarthEnterpriseMapsProvider.js
+++ b/Source/Scene/GoogleEarthEnterpriseMapsProvider.js
@@ -538,7 +538,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/GridImageryProvider.js b/Source/Scene/GridImageryProvider.js
index b98e628a84ad..7bfa4f16485e 100644
--- a/Source/Scene/GridImageryProvider.js
+++ b/Source/Scene/GridImageryProvider.js
@@ -321,7 +321,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/Imagery.js b/Source/Scene/Imagery.js
index d832d9f210ac..07414e2ac3e7 100644
--- a/Source/Scene/Imagery.js
+++ b/Source/Scene/Imagery.js
@@ -87,15 +87,10 @@ define([
         return this.referenceCount;
     };
 
-    Imagery.prototype.processStateMachine = function(frameState, needGeographicProjection, distance) {
+    Imagery.prototype.processStateMachine = function(frameState, needGeographicProjection, priorityFunction) {
         if (this.state === ImageryState.UNLOADED) {
             this.state = ImageryState.TRANSITIONING;
-            this.imageryLayer._requestImagery(this, distance);
-        }
-
-        if (defined(this.request) && this.request.state === RequestState.ISSUED) {
-            // Update distance while loading to prioritize request
-            this.request.distance = distance;
+            this.imageryLayer._requestImagery(this, priorityFunction);
         }
 
         if (this.state === ImageryState.RECEIVED) {
diff --git a/Source/Scene/ImageryLayer.js b/Source/Scene/ImageryLayer.js
index 038826aaea09..39d7014322fd 100644
--- a/Source/Scene/ImageryLayer.js
+++ b/Source/Scene/ImageryLayer.js
@@ -662,9 +662,9 @@ define([
      * @private
      *
      * @param {Imagery} imagery The imagery to request.
-     * @param {Number} [distance] The distance of the tile from the camera.
+     * @param {Function} [priorityFunction] The priority function used for sorting the imagery request.
      */
-    ImageryLayer.prototype._requestImagery = function(imagery, distance) {
+    ImageryLayer.prototype._requestImagery = function(imagery, priorityFunction) {
         var imageryProvider = this._imageryProvider;
 
         var that = this;
@@ -710,7 +710,7 @@ define([
                 throttle : true,
                 throttleByServer : true,
                 type : RequestType.IMAGERY,
-                distance : distance
+                priorityFunction : priorityFunction
             });
             imagery.request = request;
             imagery.state = ImageryState.TRANSITIONING;
diff --git a/Source/Scene/ImageryProvider.js b/Source/Scene/ImageryProvider.js
index b73b5adfddb0..36c5a2eb573e 100644
--- a/Source/Scene/ImageryProvider.js
+++ b/Source/Scene/ImageryProvider.js
@@ -266,7 +266,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
@@ -307,7 +307,7 @@ define([
      *
      * @param {ImageryProvider} imageryProvider The imagery provider for the URL.
      * @param {String} url The URL of the image.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/MapboxImageryProvider.js b/Source/Scene/MapboxImageryProvider.js
index 5c32d2082b6a..ac2949df0c54 100644
--- a/Source/Scene/MapboxImageryProvider.js
+++ b/Source/Scene/MapboxImageryProvider.js
@@ -317,7 +317,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/QuadtreePrimitive.js b/Source/Scene/QuadtreePrimitive.js
index c5b736112777..faab9989d1fa 100644
--- a/Source/Scene/QuadtreePrimitive.js
+++ b/Source/Scene/QuadtreePrimitive.js
@@ -100,7 +100,6 @@ define([
         this._tileLoadQueueLow = []; // low priority tiles were refined past or are non-visible parts of quads.
         this._tileReplacementQueue = new TileReplacementQueue();
         this._levelZeroTiles = undefined;
-        this._levelZeroTilesReady = false;
         this._loadQueueTimeSlice = 5.0;
 
         this._addHeightCallbacks = [];
diff --git a/Source/Scene/QuadtreeTile.js b/Source/Scene/QuadtreeTile.js
index 5ebea3fe0051..65c45f7274a1 100644
--- a/Source/Scene/QuadtreeTile.js
+++ b/Source/Scene/QuadtreeTile.js
@@ -67,6 +67,7 @@ define([
         // distance - for example, by using the natural ordering of a quadtree.
         // QuadtreePrimitive gets/sets this private property.
         this._distance = 0.0;
+        this._priorityFunction = undefined;
 
         this._customData = [];
         this._frameUpdated = undefined;
diff --git a/Source/Scene/SingleTileImageryProvider.js b/Source/Scene/SingleTileImageryProvider.js
index ca9486e0f1f4..9e869c6b5b62 100644
--- a/Source/Scene/SingleTileImageryProvider.js
+++ b/Source/Scene/SingleTileImageryProvider.js
@@ -370,7 +370,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/TileCoordinatesImageryProvider.js b/Source/Scene/TileCoordinatesImageryProvider.js
index 52e4ebc2b2be..0ecf383b481f 100644
--- a/Source/Scene/TileCoordinatesImageryProvider.js
+++ b/Source/Scene/TileCoordinatesImageryProvider.js
@@ -240,7 +240,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/TileImagery.js b/Source/Scene/TileImagery.js
index 282b44cda6a7..d3ffdd839df3 100644
--- a/Source/Scene/TileImagery.js
+++ b/Source/Scene/TileImagery.js
@@ -50,7 +50,7 @@ define([
         var loadingImagery = this.loadingImagery;
         var imageryLayer = loadingImagery.imageryLayer;
 
-        loadingImagery.processStateMachine(frameState, !this.useWebMercatorT, tile._distance);
+        loadingImagery.processStateMachine(frameState, !this.useWebMercatorT, tile._priorityFunction);
 
         if (loadingImagery.state === ImageryState.READY) {
             if (defined(this.readyImagery)) {
@@ -92,7 +92,7 @@ define([
                 // Push the ancestor's load process along a bit.  This is necessary because some ancestor imagery
                 // tiles may not be attached directly to a terrain tile.  Such tiles will never load if
                 // we don't do it here.
-                closestAncestorThatNeedsLoading.processStateMachine(frameState, !this.useWebMercatorT, tile._distance);
+                closestAncestorThatNeedsLoading.processStateMachine(frameState, !this.useWebMercatorT, tile._priorityFunction);
                 return false; // not done loading
             } else {
                 // This imagery tile is failed or invalid, and we have the "best available" substitute.
diff --git a/Source/Scene/TileTerrain.js b/Source/Scene/TileTerrain.js
index 8e1a4b12c0ff..98c315518c93 100644
--- a/Source/Scene/TileTerrain.js
+++ b/Source/Scene/TileTerrain.js
@@ -93,14 +93,9 @@ define([
         tile.data.occludeePointInScaledSpace = Cartesian3.clone(mesh.occludeePointInScaledSpace, surfaceTile.occludeePointInScaledSpace);
     };
 
-    TileTerrain.prototype.processLoadStateMachine = function(frameState, terrainProvider, x, y, level, distance) {
+    TileTerrain.prototype.processLoadStateMachine = function(frameState, terrainProvider, x, y, level, priorityFunction) {
         if (this.state === TerrainState.UNLOADED) {
-            requestTileGeometry(this, terrainProvider, x, y, level, distance);
-        }
-
-        if (defined(this.request) && this.request.state === RequestState.ISSUED) {
-            // Update distance while loading to prioritize request
-            this.request.distance = distance;
+            requestTileGeometry(this, terrainProvider, x, y, level, priorityFunction);
         }
 
         if (this.state === TerrainState.RECEIVED) {
@@ -112,7 +107,7 @@ define([
         }
     };
 
-    function requestTileGeometry(tileTerrain, terrainProvider, x, y, level, distance) {
+    function requestTileGeometry(tileTerrain, terrainProvider, x, y, level, priorityFunction) {
         function success(terrainData) {
             tileTerrain.data = terrainData;
             tileTerrain.state = TerrainState.RECEIVED;
@@ -149,7 +144,7 @@ define([
                 throttle : true,
                 throttleByServer : true,
                 type : RequestType.TERRAIN,
-                distance : distance
+                priorityFunction : priorityFunction
             });
             tileTerrain.request = request;
             tileTerrain.data = terrainProvider.requestTileGeometry(x, y, level, request);
diff --git a/Source/Scene/UrlTemplateImageryProvider.js b/Source/Scene/UrlTemplateImageryProvider.js
index 5ab353eab4ee..6d1edf87cca8 100644
--- a/Source/Scene/UrlTemplateImageryProvider.js
+++ b/Source/Scene/UrlTemplateImageryProvider.js
@@ -601,7 +601,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/WebMapServiceImageryProvider.js b/Source/Scene/WebMapServiceImageryProvider.js
index 93f3c83dc431..93e98bc4a010 100644
--- a/Source/Scene/WebMapServiceImageryProvider.js
+++ b/Source/Scene/WebMapServiceImageryProvider.js
@@ -433,7 +433,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Source/Scene/WebMapTileServiceImageryProvider.js b/Source/Scene/WebMapTileServiceImageryProvider.js
index 2da7cd3fb36d..21986a61866f 100644
--- a/Source/Scene/WebMapTileServiceImageryProvider.js
+++ b/Source/Scene/WebMapTileServiceImageryProvider.js
@@ -432,7 +432,7 @@ define([
      * @param {Number} x The tile X coordinate.
      * @param {Number} y The tile Y coordinate.
      * @param {Number} level The tile level.
-     * @param {Request} [request] The request object.
+     * @param {Request} [request] The request object. Intended for internal use only.
      * @returns {Promise.<Image|Canvas>|undefined} A promise for the image that will resolve when the image is available, or
      *          undefined if there are too many active requests to the server, and the request
      *          should be retried later.  The resolved image may be either an
diff --git a/Specs/Core/HeapSpec.js b/Specs/Core/HeapSpec.js
index a3af65f71848..4d41e7e48fb0 100644
--- a/Specs/Core/HeapSpec.js
+++ b/Specs/Core/HeapSpec.js
@@ -30,7 +30,9 @@ defineSuite([
     }
 
     it('maintains heap property on insert', function() {
-        var heap = new Heap(comparator);
+        var heap = new Heap({
+            comparator : comparator
+        });
         var pass = true;
         for (var i = 0; i < length; ++i) {
             heap.insert(Math.random());
@@ -41,7 +43,9 @@ defineSuite([
     });
 
     it('maintains heap property on pop', function() {
-        var heap = new Heap(comparator);
+        var heap = new Heap({
+            comparator : comparator
+        });
         var i;
         for (i = 0; i < length; ++i) {
             heap.insert(Math.random());
@@ -55,7 +59,9 @@ defineSuite([
     });
 
     it('limited by maximum length', function() {
-        var heap = new Heap(comparator);
+        var heap = new Heap({
+            comparator : comparator
+        });
         heap.maximumLength = length / 2;
         var pass = true;
         for (var i = 0; i < length; ++i) {
@@ -69,7 +75,9 @@ defineSuite([
     });
 
     it('pops in sorted order', function() {
-        var heap = new Heap(comparator);
+        var heap = new Heap({
+            comparator : comparator
+        });
         var i;
         for (i = 0; i < length; ++i) {
             heap.insert(Math.random());
@@ -85,7 +93,9 @@ defineSuite([
     });
 
     it('insert returns the removed element when maximumLength is set', function() {
-        var heap = new Heap(comparator);
+        var heap = new Heap({
+            comparator : comparator
+        });
         heap.maximumLength = length;
 
         var i;
@@ -123,7 +133,9 @@ defineSuite([
         }
 
         var i;
-        var heap = new Heap(comparator);
+        var heap = new Heap({
+            comparator : comparator
+        });
         for (i = 0; i < length; ++i) {
             heap.insert({
                 distance : i / (length - 1),
diff --git a/Specs/Core/RequestSchedulerSpec.js b/Specs/Core/RequestSchedulerSpec.js
index 63a7e1a90aa7..4dfff78ecff2 100644
--- a/Specs/Core/RequestSchedulerSpec.js
+++ b/Specs/Core/RequestSchedulerSpec.js
@@ -59,17 +59,17 @@ defineSuite([
 
     it('getServer throws if url is undefined', function() {
         expect(function() {
-            RequestScheduler.getServer();
+            RequestScheduler.getServerKey();
         }).toThrowDeveloperError();
     });
 
     it('getServer with https', function() {
-        var server = RequestScheduler.getServer('https://foo.com/1');
+        var server = RequestScheduler.getServerKey('https://foo.com/1');
         expect(server).toEqual('foo.com:443');
     });
 
     it('getServer with http', function() {
-        var server = RequestScheduler.getServer('http://foo.com/1');
+        var server = RequestScheduler.getServerKey('http://foo.com/1');
         expect(server).toEqual('foo.com:80');
     });
 
@@ -151,7 +151,7 @@ defineSuite([
         }
 
         var url = 'http://foo.com/1';
-        var server = RequestScheduler.getServer(url);
+        var server = RequestScheduler.getServerKey(url);
 
         function createRequest() {
             return new Request({
@@ -217,12 +217,12 @@ defineSuite([
             return deferred.promise;
         }
 
-        function createRequest(distance) {
+        function createRequest(priority) {
             var request = new Request({
                 url : 'http://foo.com/1',
                 requestFunction : requestFunction,
                 throttle : true,
-                distance : distance
+                priority : priority
             });
             requests.push(request);
             return request;
@@ -271,11 +271,11 @@ defineSuite([
         expect(promise).toBeDefined();
 
         if (dataOrBlobUri) {
-            expect(request.server).toBeUndefined();
+            expect(request.serverKey).toBeUndefined();
             expect(statistics.numberOfActiveRequests).toBe(0);
         } else {
             expect(statistics.numberOfActiveRequests).toBe(1);
-            expect(RequestScheduler.numberOfActiveRequestsByServer(request.server)).toBe(1);
+            expect(RequestScheduler.numberOfActiveRequestsByServer(request.serverKey)).toBe(1);
         }
 
         deferreds[0].resolve();
@@ -284,7 +284,7 @@ defineSuite([
             expect(request.state).toBe(RequestState.RECEIVED);
             expect(statistics.numberOfActiveRequests).toBe(0);
             if (!dataOrBlobUri) {
-                expect(RequestScheduler.numberOfActiveRequestsByServer(request.server)).toBe(0);
+                expect(RequestScheduler.numberOfActiveRequestsByServer(request.serverKey)).toBe(0);
             }
         });
     }
@@ -362,28 +362,23 @@ defineSuite([
         return promise.then(function() {
             fail('should not be called');
         }).otherwise(function(error) {
-            expect(error).toBe('Cancelled');
+            expect(request.state).toBe(RequestState.CANCELLED);
         });
     });
 
     it('cancels an active request', function() {
         var statistics = RequestScheduler.statistics;
-        var aborted = true;
-        var mockXhr = {
-            abort : function() {
-                aborted = true;
-            }
-        };
+        var cancelFunction = jasmine.createSpy('cancelFunction');
 
         function requestFunction() {
-            request.xhr = mockXhr;
             return when.defer().promise;
         }
 
         var request = new Request({
             throttle : true,
             url : 'https://foo.com/1',
-            requestFunction : requestFunction
+            requestFunction : requestFunction,
+            cancelFunction : cancelFunction
         });
 
         var promise = RequestScheduler.request(request);
@@ -396,13 +391,13 @@ defineSuite([
         expect(request.state).toBe(RequestState.CANCELLED);
         expect(statistics.numberOfCancelledRequests).toBe(1);
         expect(statistics.numberOfCancelledActiveRequests).toBe(1);
-        expect(RequestScheduler.numberOfActiveRequestsByServer(request.server)).toBe(0);
-        expect(aborted).toBe(true);
+        expect(RequestScheduler.numberOfActiveRequestsByServer(request.serverKey)).toBe(0);
+        expect(cancelFunction).toHaveBeenCalled();
 
         return promise.then(function() {
             fail('should not be called');
         }).otherwise(function(error) {
-            expect(error).toBe('Cancelled');
+            expect(request.state).toBe(RequestState.CANCELLED);
         });
     });
 
@@ -436,47 +431,58 @@ defineSuite([
         });
     });
 
-    it('prioritizes requests by distance', function() {
-        var currentDistance = 0.0;
+    it('prioritizes requests', function() {
+        var currentPriority = 0.0;
 
-        function getRequestFunction(distance) {
+        function getRequestFunction(priority) {
             return function() {
-                expect(distance).toBeGreaterThan(currentDistance);
-                currentDistance = distance;
+                expect(priority).toBeGreaterThan(currentPriority);
+                currentPriority = priority;
                 return when.resolve();
             };
         }
 
-        function createRequest(distance) {
+        function createRequest(priority) {
             return new Request({
                 throttle : true,
                 url : 'https://foo.com/1',
-                requestFunction : getRequestFunction(distance),
-                distance : distance
+                requestFunction : getRequestFunction(priority),
+                priority : priority
             });
         }
 
         var length = RequestScheduler.priorityHeapLength;
         for (var i = 0; i < length; ++i) {
-            var distance = Math.random();
-            RequestScheduler.request(createRequest(distance));
+            var priority = Math.random();
+            RequestScheduler.request(createRequest(priority));
         }
 
         RequestScheduler.update();
-        expect(currentDistance).toBeGreaterThan(0.0); // Ensures that the expect in getRequestFunction is actually called
+        expect(currentPriority).toBeGreaterThan(0.0); // Ensures that the expect in getRequestFunction is actually called
     });
 
     it('updates priority', function() {
+        var invertPriority = false;
+
+        function getPriorityFunction(priority) {
+            return function() {
+                if (invertPriority) {
+                    return 1.0 - priority;
+                }
+                return priority;
+            };
+        }
+
         function requestFunction() {
             return when.resolve();
         }
 
-        function createRequest(distance) {
+        function createRequest(priority) {
             return new Request({
                 throttle : true,
                 url : 'https://foo.com/1',
                 requestFunction : requestFunction,
-                distance : distance
+                priorityFunction : getPriorityFunction(priority)
             });
         }
 
@@ -484,16 +490,17 @@ defineSuite([
         var request;
         var length = RequestScheduler.priorityHeapLength;
         for (i = 0; i < length; ++i) {
-            var distance = i / (length - 1);
-            request = createRequest(distance);
+            var priority = i / (length - 1);
+            request = createRequest(priority);
             request.testId = i;
             RequestScheduler.request(request);
         }
 
+        // Update priorities while not letting any requests go through
         RequestScheduler.maximumRequests = 0;
         RequestScheduler.update();
 
-        var requestHeap = RequestScheduler._requestHeap;
+        var requestHeap = RequestScheduler.requestHeap;
         var requests = [];
         var currentTestId = 0;
         while (requestHeap.length > 0) {
@@ -507,11 +514,9 @@ defineSuite([
             requestHeap.insert(requests[i]);
         }
 
-        for (i = 0; i < length; ++i) {
-            requests[i].distance = 1.0 - requests[i].distance; // Invert priority
-        }
-
+        invertPriority = true;
         RequestScheduler.update();
+
         while (requestHeap.length > 0) {
             request = requestHeap.pop();
             expect(request.testId).toBeLessThanOrEqualTo(currentTestId);
@@ -524,12 +529,12 @@ defineSuite([
             return when.resolve();
         }
 
-        function createRequest(distance) {
+        function createRequest(priority) {
             return new Request({
                 throttle : true,
                 url : 'https://foo.com/1',
                 requestFunction : requestFunction,
-                distance : distance
+                priority : priority
             });
         }
 
@@ -625,14 +630,14 @@ defineSuite([
         }
     });
 
-    it('debugThrottle', function() {
+    it('throttleRequests', function() {
         RequestScheduler.maximumRequests = 0;
 
         function requestFunction() {
             return when.resolve();
         }
 
-        RequestScheduler.debugThrottle = true;
+        RequestScheduler.throttleRequests = true;
         var request = new Request({
             throttle : true,
             url : 'https://foo.com/1',
@@ -641,7 +646,7 @@ defineSuite([
         var promise = RequestScheduler.request(request);
         expect(promise).toBeUndefined();
 
-        RequestScheduler.debugThrottle = false;
+        RequestScheduler.throttleRequests = false;
         request = new Request({
             throttle : true,
             url : 'https://foo.com/1',
@@ -650,7 +655,7 @@ defineSuite([
         promise = RequestScheduler.request(request);
         expect(promise).toBeDefined();
 
-        RequestScheduler.debugThrottle = true;
+        RequestScheduler.throttleRequests = true;
     });
 
     it('debugShowStatistics', function() {
@@ -693,5 +698,7 @@ defineSuite([
         for (var i = 0; i < length; ++i) {
             deferreds[i].resolve();
         }
+
+        RequestScheduler.debugShowStatistics = false;
     });
 });