Send SVG to clients, add simple caching

lib/explore.mjs

@@ -58,13 +58,13 @@ export const exploreVideos = async videoId =>
 };
 
 /**
- * Convert a video graph to the DOT format.
+ * Convert a video graph to the GraphViz format.
  *
  * @param Object videosNodes Videos of the graph.
  * @param Object nextVideos For each video, list of next videos.
- * @return string DOT representation of the graph.
+ * @return string GraphViz representation of the graph.
  */
-export const graphToDOT = (videosNodes, nextVideos) =>
+export const toGraphviz = (videosNodes, nextVideos) =>
 {
     // Convert videosNodes
     const nodesStr = Object.entries(videosNodes).map(([id, {title}]) =>
@@ -80,7 +80,7 @@ export const graphToDOT = (videosNodes, nextVideos) =>
             .join('\n')
     ).join('\n');
 
-    return `digraph maze {
+    return `digraph "" {
 ${nodesStr}
 
 ${nextStr}}`;

lib/graphviz.mjs

@@ -0,0 +1,35 @@
+import { spawn } from 'child_process';
+import xml2js from 'xml2js';
+
+const parser = new xml2js.Parser();
+const builder = new xml2js.Builder({ headless: true });
+
+const invokeDot = (input, format) => new Promise((resolve, reject) =>
+{
+    const dot = spawn('dot', [`-T${format}`]);
+    let stdout = '';
+
+    dot.stdout.on('data', data => stdout += data);
+
+    dot.on('close', code => {
+        if (code !== 0)
+        {
+            reject(new Error(`dot returned error code ${code}`));
+        }
+        else
+        {
+            resolve(stdout);
+        }
+    });
+
+    dot.on('error', reject);
+    dot.stdin.write(input);
+    dot.stdin.end();
+});
+
+export const toSVG = async input =>
+{
+    const svg = await invokeDot(input, 'svg');
+    const parsed = await parser.parseStringPromise(svg);
+    return builder.buildObject(parsed);
+};

package-lock.json

@@ -1,6 +1,6 @@
 {
-  "name": "server",
-  "version": "0.0.0",
+  "name": "youtube-maze",
+  "version": "0.3.0",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
@@ -335,6 +335,11 @@
       "resolved": "https://registry.npmjs.org/safer-buffer/-/safer-buffer-2.1.2.tgz",
       "integrity": "sha512-YZo3K82SD7Riyi0E1EQPojLz7kpepnSQI9IyPbHHg1XXXevb5dJI7tpyN2ADxGcQbHG7vcyRHk0cbwqcQriUtg=="
     },
+    "sax": {
+      "version": "1.2.4",
+      "resolved": "https://registry.npmjs.org/sax/-/sax-1.2.4.tgz",
+      "integrity": "sha512-NqVDv9TpANUjFm0N8uM5GxL36UgKi9/atZw+x7YFnQ8ckwFGKrl4xX4yWtrey3UJm5nP1kUbnYgLopqWNSRhWw=="
+    },
     "send": {
       "version": "0.16.2",
       "resolved": "https://registry.npmjs.org/send/-/send-0.16.2.tgz",
@@ -399,6 +404,20 @@
       "version": "1.1.2",
       "resolved": "https://registry.npmjs.org/vary/-/vary-1.1.2.tgz",
       "integrity": "sha1-IpnwLG3tMNSllhsLn3RSShj2NPw="
+    },
+    "xml2js": {
+      "version": "0.4.23",
+      "resolved": "https://registry.npmjs.org/xml2js/-/xml2js-0.4.23.tgz",
+      "integrity": "sha512-ySPiMjM0+pLDftHgXY4By0uswI3SPKLDw/i3UXbnO8M/p28zqexCUoPmQFrYD+/1BzhGJSs2i1ERWKJAtiLrug==",
+      "requires": {
+        "sax": ">=0.6.0",
+        "xmlbuilder": "~11.0.0"
+      }
+    },
+    "xmlbuilder": {
+      "version": "11.0.1",
+      "resolved": "https://registry.npmjs.org/xmlbuilder/-/xmlbuilder-11.0.1.tgz",
+      "integrity": "sha512-fDlsI/kFEx7gLvbecc0/ohLG50fugQp8ryHzMTuW9vSa1GJ0XYWKnhsUx7oie3G98+r56aTQIUB4kht42R3JvA=="
     }
   }
 }

package.json

@@ -10,6 +10,7 @@
     "debug": "~2.6.9",
     "express": "~4.16.1",
     "morgan": "~1.9.1",
-    "node-fetch": "^2.6.1"
+    "node-fetch": "^2.6.1",
+    "xml2js": "^0.4.23"
   }
 }

routes/mazes.mjs

@@ -1,22 +1,50 @@
 import express from 'express';
-import { exploreVideos, graphToDOT } from '../lib/explore.mjs';
-import { retryable } from '../lib/retry.mjs';
+import { exploreVideos, toGraphviz } from '../lib/explore.mjs';
+import { toSVG } from '../lib/graphviz.mjs';
 
 const router = express.Router();
 export default router;
 
-const retryExploreVideos = retryable(exploreVideos);
+const statusPending = Symbol('PENDING');
+const statusError = Symbol('ERROR');
+const cache = Object.create(null);
 
 router.get('/:videoId', async (req, res) => {
-    try
+    const {videoId} = req.params;
+
+    if (videoId in cache)
     {
-        const graph = await retryExploreVideos(req.params.videoId);
-        const dot = graphToDOT(...graph);
-        res.send(dot);
+        if (cache[videoId] === statusPending)
+        {
+            res.header('Refresh', '5');
+            res.send('Exploration in progress… Please wait.');
+        }
+        else if (cache[videoId] === statusError)
+        {
+            res.status(500).send('Error');
+        }
+        else
+        {
+            res.send(cache[videoId]);
+        }
     }
-    catch (err)
+    else
     {
-        console.error(err);
-        res.status(500).send('Error');
+        cache[videoId] = statusPending;
+        res.header('Refresh', '1');
+        res.send('Exploration in progress… Please wait.');
+
+        try
+        {
+            const graph = await exploreVideos(req.params.videoId);
+            const graphviz = toGraphviz(...graph);
+            const svg = await toSVG(graphviz);
+            cache[videoId] = svg;
+        }
+        catch (err)
+        {
+            console.error(err);
+            cache[videoId] = statusError;
+        }
     }
 });