first attempt at modularizing

backend.js

@@ -3,6 +3,7 @@ var crypto = require('crypto');
 var mapnik = require('mapnik');
 var util = require('util');
 var sm = new (require('sphericalmercator'))();
+var queue = require('d3-queue');
 
 module.exports = Backend;
 
@@ -75,7 +76,7 @@ Backend.prototype.getTile = function(z, x, y, callback) {
 
     var size = 0;
     var headers = {};
-    var lookbacks = 0;
+    var lookbacks = true;
 
     // Overzooming support.
     if (bz > backend._maxzoom) {
@@ -85,7 +86,32 @@ Backend.prototype.getTile = function(z, x, y, callback) {
         headers['x-vector-backend-object'] = 'overzoom';
     }
 
-    source.getTile(bz, bx, by, function sourceGet(err, body, head) {
+    function testAsync(callback) {
+        // do nothing
+        return callback();
+    }
+
+    function loadAsync(lz, lx, ly, callback) {
+        source.getTile(lz, lx, ly, function (err, body, head) {
+            if (err && err.message !== 'Tile does not exist') return callback(err);
+            return callback(null, {
+                err: err,
+                body: body,
+                head: head,
+                z: lz,
+                x: lx,
+                y: ly
+            });
+        });
+    }
+
+    function loadMultipleWrapper(err, body, head, callback) {
+        return callback(err, body, head)
+    }
+
+    source.getTile(bz, bx, by, sourceGet);
+
+    function sourceGet(err, body, head) {
         if (typeof backend._fillzoom === 'number' &&
             err && err.message === 'Tile does not exist' &&
             bz > backend._fillzoom) {
@@ -96,13 +122,26 @@ Backend.prototype.getTile = function(z, x, y, callback) {
             return source.getTile(bz, bx, by, sourceGet);
         } else if (typeof backend._lookback === 'number' &&
             err && err.message === 'Tile does not exist' &&
-            lookbacks <= backend._lookback) {
-            lookbacks += 1
-            bz = bz - 1;
-            bx = Math.floor(x / Math.pow(2, z - bz));
-            by = Math.floor(y / Math.pow(2, z - bz));
+            lookbacks === true) {
+            lookbacks == false;
+            var q = new queue.queue();
             headers['x-vector-backend-object'] = 'fillzoom';
-            return source.getTile(bz, bx, by, sourceGet);
+            for (var lb = 1; lb <= backend._lookback; lb++) {
+                bz-= 1
+                bx = Math.floor(x / Math.pow(2, z - bz));
+                by = Math.floor(y / Math.pow(2, z - bz));
+                q.defer(loadAsync, bz, bx, by);
+            }
+            return q.awaitAll(function(err, data) {
+                data = data.filter(function(d, i) {
+                    return d === null || i === 0;
+                });
+                bz = data[0].z;
+                bx = data[0].x;
+                by = data[0].y;
+                source.getTile(bz, bx, by, sourceGet);
+            });
+
         }
         if (err && err.message !== 'Tile does not exist') return callback(err);
 
@@ -135,7 +174,7 @@ Backend.prototype.getTile = function(z, x, y, callback) {
             headers = head || {};
             return makevtile(body);
         }
-    });
+    };
 
     function makevtile(data, type) {
         // If no last modified is provided, use epoch.

package.json

@@ -14,6 +14,7 @@
   ],
   "dependencies": {
     "aws-sdk": "^2.2.30",
+    "d3-queue": "^2.0.3",
     "s3urls": "^1.3.0",
     "mapnik": "~3.5.0",
     "tilelive": "~5.12.0",

test/test.js

@@ -144,7 +144,7 @@ var tests = {
     // Image sources.
     'i@2x': ['0.0.0', '1.0.0'],
     // Image sources for multiple lookback
-    l: ['0.0.0', '1.0.0', '1.1.1', '2.0.0', '2.3.3'],
+    l: ['0.0.0', '1.0.0', '1.1.1', '2.0.0', '2.3.3', '3.1.1'],
     // Invalid tiles that are empty
     invalid: ['1.1.0', '1.1.1'],