From 7695b606c6145904295e8dbc10eb45f5f44b7c9f Mon Sep 17 00:00:00 2001
From: Zaid Abdulla <zkam83@gmail.com>
Date: Sat, 9 Nov 2013 00:16:06 +0300
Subject: [PATCH] Use locking to avoid possible unnecessary fetching and
 caching of presets and objects

---
 db.coffee | 72 +++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 62 insertions(+), 10 deletions(-)

diff --git a/db.coffee b/db.coffee
index 6a6b334..ee7b0ad 100644
--- a/db.coffee
+++ b/db.coffee
@@ -51,6 +51,53 @@ getTask = (taskId, callback) ->
   )
 
 
+getCached = (name, valueCallback, valueExpiry, callback) ->
+  redisClient.get(name, (err, res) ->
+    return callback(err) if err
+    return callback(null, JSON.parse(res)) if res?
+
+    lockAcquired = () ->
+      valueCallback((err, value) ->
+        return callback(err, value) if err
+        redisClient.setex(name, valueExpiry, JSON.stringify(value), (err) ->
+          return callback(err, value) if err
+          redisClient.del("lock.#{name}", (err) ->
+            callback(err, value)
+          )
+        )
+      )
+
+    redisClient.setnx("lock.#{name}", Date.now() + 30000, (err, res) ->
+      return callback(err) if err
+      if res == 1
+        lockAcquired()
+      else
+        setTimeout(() ->
+          redisClient.get("lock.#{name}", (err, timestamp) ->
+            return callback(err) if err
+            now = Date.now()
+            if not timestamp?
+              getCached(name, valueCallback, valueExpiry, callback)
+            else if timestamp > now
+              setTimeout(() ->
+                getCached(name, valueCallback, valueExpiry, callback)
+              , 1000)
+            else
+              redisClient.getset("lock.#{name}", now + 30000, (err, timestamp) ->
+                return callback(err) if err
+                if timestamp > now
+                  setTimeout(() ->
+                    getCached(name, valueCallback, valueExpiry, callback)
+                  , 1000)
+                else
+                  lockAcquired()
+              )
+          )
+        , 1000)
+    )
+  )
+
+
 getPresets = (callback) ->
   redisClient.mget('presets', 'objects', (err, res) ->
     presets = JSON.parse(res[0])
@@ -60,22 +107,27 @@ getPresets = (callback) ->
       return
 
     if not presets
-      presetsCollection.find().toArray((err, p) ->
+      getCached('presets', (callback) ->
+        presetsCollection.find().toArray((err, res) ->
+          callback(err, res)
+        )
+      , config.PRESETS_CACHE_DURATION, (err, res) ->
         throw err if err
-        presets = p
-
-        redisClient.setex('presets', config.PRESETS_CACHE_DURATION, JSON.stringify(presets))
+        presets = res
         callback(presets, objects) if objects
       )
 
     if not objects
-      objectsCollection.find().toArray((err, o) ->
+      getCached('objects', (callback) ->
+        objectsCollection.find().toArray((err, res) ->
+          return callback(err) if err
+          objs = {}
+          objs[r._id] = r for r in res
+          callback(null, objs)
+        )
+      , config.PRESETS_CACHE_DURATION, (err, res) ->
         throw err if err
-        objects = {}
-        for i in o
-          objects[i._id] = i
-
-        redisClient.setex('objects', config.PRESETS_CACHE_DURATION, JSON.stringify(objects))
+        objects = res
         callback(presets, objects) if presets
       )
   )