Added overview.

README.markdown

@@ -1,13 +1,51 @@
 # Vitess
 
 Vitess is a set of servers and tools meant to facilitate scaling of MySQL
-databases for the web. It's currently used as a fundamental component of
-YouTube's MySQL infrastructure, serving thousands of QPS (per server).
+databases for the web. It's been developed since 2011, and is currently used as
+a fundamental component of YouTube's MySQL infrastructure, serving thousands of
+QPS (per server).
 
+## Overview
+
+![Overview](https://raw.githubusercontent.com/youtube/vitess/master/doc/VitessOverview.png)
+
+Vitess consists of a number servers, command line utilities, and a consistent
+metadata store. Taken together, they allow you to serve more database traffic,
+and add features like sharding, which normally you would have to implement in your
+application.
+
+**vttablet** is a server that sits in front of a MySQL database, making it more
+robust and available in the face of high traffic. Among other things, it adds a
+connection pool, has a row based cache, and it rewrites SQL queries to be safer
+and nicer to the underlying database.
+
+**vtgate** is a very light proxy that routes database traffic from your app to the
+right vttablet, basing on the sharding scheme, latency required, and health of
+the vttablets. This allows the client to be very simple, as all it needs to be
+concerned about is finding the closest vtgate.
+
+The **topology** is a metadata store that contains information about running
+servers, the sharding scheme, and replication graph. It is backed by a
+consistent data store, like [Apache ZooKeeper](http://zookeeper.apache.org/).
+The topology backends are plugin based, allowing you to write your own if
+ZooKeeper doesn't fit your needs. You can explore the topology through
+**vtctld**, a webserver (not shown in the diagram).
+
+**vtctl** is a command line utility that allows a human or a script to easily
+interact with the system.
+
+All components communicate using a lightweight RPC system based on
+[BSON](http://bsonspec.org/). The RPC system is plugin based, so you can easily
+write your own backend (at Google we use a Protocol Buffers based protocol). We
+provide a client implementation for three languages: Python, Go, and Java.
+Writing a client for your language should not be difficult, as it's a matter of
+implementing only a few API calls (please send us a pull request if you do!).
+
+To learn more, please click on the documentation links below.
 [sougou](https://github.com/sougou) presented Vitess at Fosdem '14 in the go
-devroom. [Here are the
-slides](https://github.com/youtube/vitess/blob/master/doc/Vitess2014.pdf?raw=true),
-and here is the [video](http://youtu.be/qATTTSg6zXk).
+devroom
+([slides](https://github.com/youtube/vitess/blob/master/doc/Vitess2014.pdf?raw=true),
+[video](http://youtu.be/qATTTSg6zXk)).
 
 ## Trying it out
 
@@ -25,6 +63,7 @@ doc](https://github.com/youtube/vitess/blob/master/doc/GettingStarted.markdown).
  * [Vision](https://github.com/youtube/vitess/blob/master/doc/Vision.markdown)
  * [General Concepts](https://github.com/youtube/vitess/blob/master/doc/Concepts.markdown)
  * [Replication Graph](https://github.com/youtube/vitess/blob/master/doc/ReplicationGraph.markdown)
+ * [Vttablet](https://github.com/youtube/vitess/blob/master/doc/Vttablet.markdown)
  * [Serving graph](https://github.com/youtube/vitess/blob/master/doc/ServingGraph.markdown)
  * [Tools](https://github.com/youtube/vitess/blob/master/doc/Tools.markdown)
  * [Getting Started](https://github.com/youtube/vitess/blob/master/doc/GettingStarted.markdown)