Sketch in cache

.gitignore

@@ -18,5 +18,9 @@ etl/**/*.xls
 etl/**/*.xlsx
 etl/**/*.zip
 
+# Cache
+app/tilecache/**/*.png
+app/tilecache/**/*.mbtiles
+
 # Notes
 TODO

app/src/tiles/cache.js

@@ -0,0 +1,72 @@
+/**
+ * Cache tiles (PNG images generated from database)
+ *
+ * Frequency of change:
+ * - base layer tiles change rarely - on changes to underlying geometry table
+ * - visualisation layer tiles change frequently - with almost any edit to the buildings table
+ *
+ * Cost of generation and storage:
+ * - low zoom tiles are more expensive to render, containing more features from the database
+ * - high zoom tiles are cheaper to rerender, and changes are more visible
+ * - there are many more high zoom tiles than low: 4 tiles at zoom level n+1 for each tile
+ *   at zoom level n
+ *
+ */
+import fs from 'fs';
+import path from 'path';
+
+// const CACHE_PATH = process.env.CACHE_PATH
+const CACHE_PATH = '/home/tom/projects/colouring-london/colouring-london/app/tilecache'
+
+function get(tileset, z, x, y, cb){
+    if (!should_try_cache(tileset, z)) {
+        cb("Skip cache", null)
+        return
+    }
+    const dir = `${CACHE_PATH}/${tileset}/${z}/${x}`
+    const fname = `${dir}/${y}.png`
+    fs.readFile(fname, cb)
+}
+
+function put(im, tileset, z, x, y, cb){
+    if (!should_try_cache(tileset, z)) {
+        cb("Skip cache")
+        return
+    }
+    const dir = `${CACHE_PATH}/${tileset}/${z}/${x}`
+    const fname = `${dir}/${y}.png`
+    fs.writeFile(fname, im, 'binary', (err) => {
+        if (err && err.code === 'ENOENT') {
+            try {
+                console.log("trying")
+                mkdir_recursive(dir);
+                fs.writeFile(fname, im, 'binary', cb);
+            } catch (error) {
+                console.log("mkdir error")
+                cb(err);
+            }
+        } else {
+            cb(err)
+        }
+    });
+}
+
+// for node >10 we could drop this in favour of fs.mkdir(dir, { recursive: true }, (err) => {})
+function mkdir_recursive(dir) {
+    const parent = path.dirname(dir);
+    if (!fs.existsSync(parent)) {
+        mkdir_recursive(parent)
+    }
+    fs.mkdirSync(dir);
+}
+
+function should_try_cache(tileset, z) {
+    if (tileset === 'base_light' ||  tileset === 'base_light') {
+        // cache for higher zoom levels (unlikely to change)
+        return z <= 15
+    }
+    // else cache for lower zoom levels (change slowly)
+    return z <= 12
+}
+
+export { get, put };

app/src/tiles/tileserver.js

@@ -4,6 +4,7 @@
  */
 import express from 'express';
 
+import { get, put } from './cache';
 import { render_tile } from './tile';
 import { strictParseInt } from '../parse';
 
@@ -51,12 +52,27 @@ function handle_tile_request(tileset, req, res) {
         return {error:'Bad parameter'}
     }
 
-    render_tile(tileset, int_z, int_x, int_y, undefined, function(err, im) {
-        if (err) throw err
+    get(tileset, int_z, int_x, int_y, (err, im) => {
+        if (err) {
+            render_tile(tileset, int_z, int_x, int_y, undefined, (err, im) => {
+                if (err) throw err
 
-        res.writeHead(200, {'Content-Type': 'image/png'})
-        res.end(im)
+                put(im, tileset, z, x, y, (err) => {
+                    if (err) {
+                        console.error(err)
+                    }
+
+                    res.writeHead(200, {'Content-Type': 'image/png'})
+                    res.end(im)
+                })
+
+            })
+        } else {
+            res.writeHead(200, {'Content-Type': 'image/png'})
+            res.end(im)
+        }
     })
+
 }
 
 function handle_highlight_tile_request(req, res) {