Improve behavior to reduce rate limiting issues

lib/api.mjs

@@ -3,6 +3,7 @@ import util from 'util';
 import fetch from 'node-fetch';
 
 import { WATCH_BASE } from './util.mjs';
+import { retryable, exclusive } from './retry.mjs';
 import { TemporaryError } from './retry.mjs';
 
 const log = debug('youtube-maze:api');
@@ -16,13 +17,21 @@ const PLAYER_CONFIG_REGEX = /ytplayer\.config = (\{.*?\});/;
  * @param String videoId Identifier of the video to fetch.
  * @return Object The player configuration object.
  */
-export const getPlayerConfig = async (videoId) =>
+const bareGetPlayerConfig = async (videoId) =>
 {
     const url = util.format(WATCH_BASE, encodeURIComponent(videoId));
     log(`Fetching ${videoId} (${url})`);
-
     const res = await fetch(url);
-    debugger;
+
+    if (res.status === 429)
+    {
+        throw new TemporaryError('Too many requests');
+    }
+    else if (res.status !== 200)
+    {
+        throw new Error(`Invalid YouTube HTTP status: ${res.status}`);
+    }
+
     const body = await res.text();
 
     // Look for the initial player response object to check whether the
@@ -60,9 +69,12 @@ reason: "${response.playabilityStatus.reason}"`);
         throw new Error(`Video ${videoId} has actual id ${actualId}`);
     }
 
+    log(`Done fetching ${videoId}`);
     return config;
 };
 
+export const getPlayerConfig = exclusive(retryable(bareGetPlayerConfig));
+
 /**
  * Get metadata about a YouTube video.
  *

lib/explore.mjs

@@ -1,13 +1,10 @@
 import util from 'util';
 import * as api from './api.mjs';
-import { WATCH_BASE, PromiseQueue, escapeQuotes } from './util.mjs';
-import { retryable } from '../lib/retry.mjs';
+import { WATCH_BASE, escapeQuotes } from './util.mjs';
 
 const GRAPH_NODE = '    "%s" [label="%s", URL="%s", fontcolor=blue]';
 const GRAPH_LINK = '    "%s" -> "%s"';
 
-const retryPlayerConfig = retryable(api.getPlayerConfig);
-
 /**
  * Explore the video graph starting from the given root.
  *
@@ -19,23 +16,22 @@ export const exploreVideos = async videoId =>
 {
     // Store metadata about each visited video
     const videosNodes = Object.create(null);
-    videosNodes[videoId] = {};
 
     // List of videos linked from each video either through a card or an
     // endscreen item
     const nextVideos = Object.create(null);
-    nextVideos[videoId] = new Set();
 
-    // Pending video requests
-    const queue = new PromiseQueue();
-    queue.add(retryPlayerConfig(videoId));
+    // Videos that still need to be explored
+    const queue = [videoId];
 
-    while (!queue.empty())
+    while (queue.length > 0)
     {
-        const config = await queue.next();
+        const currentId = queue.shift();
+        const config = await api.getPlayerConfig(currentId);
         const meta = api.getVideoMeta(config);
 
-        videosNodes[meta.videoId] = meta;
+        videosNodes[currentId] = meta;
+        nextVideos[currentId] = new Set();
 
         // Add links between this video and the linked ones
         api.getEndScreenVideos(config)
@@ -47,9 +43,7 @@ export const exploreVideos = async videoId =>
         {
             if (!(nextId in videosNodes))
             {
-                videosNodes[nextId] = {};
-                nextVideos[nextId] = new Set();
-                queue.add(retryPlayerConfig(nextId));
+                queue.push(nextId);
             }
         }
     }

lib/retry.mjs

@@ -4,8 +4,10 @@ import { sleep } from './util.mjs';
 const log = debug('youtube-maze:retry');
 
 /**
- * An error that is expected to be temporary such that the initial action
- * may be retried.
+ * An error that is expected to be temporary.
+ *
+ * If this error is raised, the initial action may be retried after a short
+ * period of time, and may eventually succeed.
  */
 export class TemporaryError extends Error
 {
@@ -32,11 +34,11 @@ export const retryable = (func, retries = 3, cooldown = 1000) =>
 {
     return async (...args) =>
     {
+        let remRetries = retries;
+        let curCooldown = cooldown;
+
         while (true)
         {
-            let remRetries = retries;
-            let curCooldown = cooldown;
-
             try
             {
                 const result = await func(...args);
@@ -62,3 +64,33 @@ Retrying in ${curCooldown} ms (${remRetries} retries remaining)`);
         }
     };
 };
+
+/**
+ * Make an async function mutually exclusive.
+ *
+ * Only one execution of the async function may happen at the same time.
+ * In the meantime, other requests are added to a queue.
+ *
+ * @param function func Async function to call.
+ * @param number cooldown Time to wait before two executions.
+ */
+export const exclusive = (func, cooldown = 2000) =>
+{
+    let pending = null;
+
+    return async (...args) =>
+    {
+        if (pending === null)
+        {
+            pending = func(...args);
+        }
+        else
+        {
+            pending = pending
+                .then(() => sleep(cooldown))
+                .then(() => func(...args));
+        }
+
+        return pending;
+    };
+};

lib/util.mjs

@@ -3,52 +3,6 @@ import util from 'util';
 export const YOUTUBE_BASE = 'https://www.youtube.com/%s';
 export const WATCH_BASE = util.format(YOUTUBE_BASE, 'watch?v=%s');
 
-/**
- * Hold a queue of promises from which results can be extracted.
- */
-export class PromiseQueue
-{
-    constructor()
-    {
-        this.pending = new Set();
-    }
-
-    /**
-     * Add a new promise to the queue.
-     *
-     * @param Promise promise Promise to be added.
-     */
-    add(promise)
-    {
-        const wrapped = promise.then(res => {
-            this.pending.delete(wrapped);
-            return res;
-        });
-
-        this.pending.add(wrapped);
-    }
-
-    /**
-     * Check whether there is no promise pending in the queue.
-     *
-     * @return boolean
-     */
-    empty()
-    {
-        return this.pending.size === 0;
-    }
-
-    /**
-     * Extract the next available result from the promise queue.
-     *
-     * @return any Next result.
-     */
-    next()
-    {
-        return Promise.race(this.pending);
-    }
-}
-
 /**
  * Escape double quotes in a string.
  */