Lookback

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;
 
@@ -35,6 +36,9 @@ function Backend(opts, callback) {
         backend._fillzoom = 'fillzoom' in info && !isNaN(parseInt(info.fillzoom, 10)) ?
             parseInt(info.fillzoom, 10) :
             undefined;
+        backend._lookback = 'lookback' in info && !isNaN(parseInt(info.lookback, 10)) ?
+            parseInt(info.lookback, 10) :
+            undefined;
         backend._source = source;
         if (callback) callback(null, backend);
     }
@@ -72,6 +76,7 @@ Backend.prototype.getTile = function(z, x, y, callback) {
 
     var size = 0;
     var headers = {};
+    var lookbacks = true;
 
     // Overzooming support.
     if (bz > backend._maxzoom) {
@@ -81,6 +86,20 @@ Backend.prototype.getTile = function(z, x, y, callback) {
         headers['x-vector-backend-object'] = 'overzoom';
     }
 
+    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
+            });
+        });
+    }
+
     source.getTile(bz, bx, by, function sourceGet(err, body, head) {
         if (typeof backend._fillzoom === 'number' &&
             err && err.message === 'Tile does not exist' &&
@@ -90,6 +109,25 @@ Backend.prototype.getTile = function(z, x, y, callback) {
             by = Math.floor(y / Math.pow(2, z - bz));
             headers['x-vector-backend-object'] = 'fillzoom';
             return source.getTile(bz, bx, by, sourceGet);
+        } else if (typeof backend._lookback === 'number' &&
+            err && err.message === 'Tile does not exist' &&
+            lookbacks === true) {
+            lookbacks = false;
+            var q = new queue.queue();
+            for (var lb = 1; lb <=  Math.min(backend._lookback, z); lb++) {
+                q.defer(loadAsync, bz - lb, Math.floor(x / Math.pow(2, lb)), Math.floor(y / Math.pow(2, lb)));
+            }
+            return q.awaitAll(function(err, lookbackTiles) {
+                if (err) return callback(err);
+                lookbackTiles = lookbackTiles.filter(function(t, i) {
+                    return t === null || i === lookbackTiles.length - 1;
+                });
+                headers['x-vector-backend-object'] = 'fillzoom';
+                bz = lookbackTiles[0].z;
+                bx = lookbackTiles[0].x;
+                by = lookbackTiles[0].y;
+                sourceGet(lookbackTiles[0].err, lookbackTiles[0].body, lookbackTiles[0].head);
+            });
         }
         if (err && err.message !== 'Tile does not exist') return callback(err);
 

package.json

@@ -14,6 +14,7 @@
   ],
   "dependencies": {
     "aws-sdk": "^2.2.30",
+    "d3-queue": "^2.0.3",
     "s3urls": "^1.3.0",
     "mapnik": "~3.5.13",
     "tilelive": "~5.12.0",

test/backend.js

@@ -70,6 +70,7 @@ tilelive.protocols['test:'] = Testsource;
         c: new Backend({ source: new Testsource('b'), minzoom:0, maxzoom: 2, fillzoom: 1, scale: 2, legacy: true }),
         h: new Backend({ source: new Testsource('b'), minzoom:0, maxzoom: 2, fillzoom: 1, scale: 2 }),
         i: new Backend({ source: new Testsource('i'), minzoom:0, maxzoom: 1 }),
+        l: new Backend({ source: new Testsource('i'), minzoom:0, maxzoom: 4, lookback: 3}),
         invalid: new Backend({ source: new Testsource('invalid'), minzoom:0, maxzoom: 1 }),
         iv: new Backend({ source: new Testsource('i'), minzoom:0, maxzoom: 1, vector_layers: [{id:'custom_layer_name'}] }),
         gz: new Backend({ source: new Testsource('gz'), minzoom:0, maxzoom: 0 }),
@@ -90,6 +91,8 @@ tilelive.protocols['test:'] = Testsource;
         h: ['0.0.0', '1.0.0', '1.0.1', '1.1.0', '1.1.1', '2.1.1', '2.1.2', '3.2.2', '3.2.3', '3.2.4'],
         // wraps image source with vector tiles.
         i: ['0.0.0', '1.0.0', '1.0.1', '1.1.0', '1.1.1', '2.0.0', '2.0.1'],
+        // test for multiple lookback
+        l: ['0.0.0', '1.0.0', '1.0.1', '1.1.0', '1.1.1', '2.0.0'],
         // wraps image source with vector tiles, with named vector layer.
         iv: ['0.0.0'],
         // loads gzip compressed protobuf.

test/expected/backend-l.1.0.1.json

@@ -0,0 +1,13 @@
+[
+  {
+    "version": 2,
+    "name": "_image",
+    "extent": 4096,
+    "features": [
+      {
+        "raster": 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+        "properties": {}
+      }
+    ]
+  }
+]