Add Cache-aside section

Author: donnemartin

README.md

@@ -1273,3 +1273,42 @@ Suggestions of what to cache:
 * Fully rendered web pages
 * Activity streams
 * User graph data
+
+### When to update the cache
+
+Since you can only store a limited amount of data in cache, you'll need to determine which cache update strategy works best for your use case.
+
+#### Cache-aside
+
+<p align="center">
+  <img src="http://i.imgur.com/ONjORqk.png">
+  <br/>
+  <i><a href=http://www.slideshare.net/tmatyashovsky/from-cache-to-in-memory-data-grid-introduction-to-hazelcast>Source: From cache to in-memory data grid</a></i>
+</p>
+
+The application is responsible for reading and writing from storage.  The cache does not interact with storage directly.  The application does the following:
+
+* Look for entry in cache, resulting in a cache miss
+* Load entry from the database
+* Add entry to cache
+* Return entry
+
+```
+def get_user(self, user_id):
+    user = cache.get("user.{0}", user_id)
+    if user is None:
+        user = db.query("SELECT * FROM users WHERE user_id = {0}", user_id)
+        if user is not None:
+            cache.set(key, json.dumps(user))
+    return user
+```
+
+[Memcached](https://memcached.org/) is generally used in this manner.
+
+Subsequent reads of data added to cache are fast.  Cache-aside is also referred to as lazy loading.  Only requested data is cached, which avoids filling up the cache with data that isn't requested.
+
+##### Disadvantage(s): cache-aside
+
+* Each cache miss results in three trips, which can cause a noticeable delay.
+* Data can become stale if it is updated in the database.  This issue is mitigated by setting a time-to-live (TTL) which forces an update of the cache entry, or by using write-through.
+* When a node fails, it is replaced by a new, empty node, increasing latency.