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@@ -903,3 +903,18 @@ A relational database like SQL is a collection of data items organized in tables
 * **Durability** - Once a transaction has been committed, it will remain so
 
 There are many techniques to scale a relational database: **master-slave replication**, **master-master replication**, **federation**, **sharding**, **denormalization**, and **SQL tuning**.
+
+#### Master-slave replication
+
+The master serves reads and writes, replicating writes to one or more slaves, which serve only reads.  Slaves can also replicate to additional slaves in a tree-like fashion.  If the master goes offline, the system can continue to operate in read-only mode until a slave is promoted to a master or a new master is provisioned.
+
+<p align="center">
+  <img src="http://i.imgur.com/C9ioGtn.png">
+  <br/>
+  <i><a href=http://www.slideshare.net/jboner/scalability-availability-stability-patterns/>Source: Scalability, availability, stability, patterns</a></i>
+</p>
+
+##### Disadvantage(s): master-slave replication
+
+* Additional logic is needed to promote a slave to a master.
+* See [Disadvantage(s): replication](#disadvantages-replication) for points related to **both** master-slave and master-master.