Welcome back! In our first [tutorial](Tutorial%3A-A-First-Look-at-Druid.html), we introduced you to the most basic Druid setup: a single realtime node. We streamed in some data and queried it. Realtime nodes collect very recent data and periodically hand that data off to the rest of the Druid cluster. Some questions about the architecture must naturally come to mind. What does the rest of Druid cluster look like? How does Druid load available static data?
This tutorial will hopefully answer these questions!
In this tutorial, we will set up other types of Druid nodes as well as and external dependencies for a fully functional Druid cluster. The architecture of Druid is very much like the [Megazord](http://www.youtube.com/watch?v=7mQuHh1X4H4) from the popular 90s show Mighty Morphin' Power Rangers. Each Druid node has a specific purpose and the nodes come together to form a fully functional system.
Druid requires 3 external dependencies. A "deep" storage that acts as a backup data repository, a relational database such as MySQL to hold configuration and metadata information, and [Apache Zookeeper](http://zookeeper.apache.org/) for coordination among different pieces of the cluster.
For deep storage, we have made a public S3 bucket (static.druid.io) available where data for this particular tutorial can be downloaded. More on the data later.
Similar to the first tutorial, the data we will be loading is based on edits that have occurred on Wikipedia. Every time someone edits a page in Wikipedia, metadata is generated about the editor and edited page. Druid collects each individual event and packages them together in a container known as a [segment](Segments.html). Segments contain data over some span of time. We've prebuilt a segment for this tutorial and will cover making your own segments in other [pages](Tutorial%3A-Loading-Your-Data-Part-1.html).The segment we are going to work with has the following format:
Let's start up a few nodes and download our data. First things though, let's make sure we have config directory where we will store configs for our various nodes:
If you are interested in learning more about Druid configuration files, check out this [link](Configuration.html). Many aspects of Druid are customizable. For the purposes of this tutorial, we are going to use default values for most things.
Coordinator nodes are in charge of load assignment and distribution. Coordinator nodes monitor the status of the cluster and command historical nodes to assign and drop segments.
Historical nodes are the workhorses of a cluster and are in charge of loading historical segments and making them available for queries. Our Wikipedia segment will be downloaded by a historical node.
For more information about Historical nodes, see [here](Historical.html).
Broker nodes are responsible for figuring out which historical and/or realtime nodes correspond to which queries. They also merge partial results from these nodes in a scatter/gather fashion.
The MySQL dependency we introduced earlier on contains a 'segments' table that contains entries for segments that should be loaded into our cluster. The Druid coordinator compares this table with segments that already exist in the cluster to determine what should be loaded and dropped. To load our wikipedia segment, we need to create an entry in our MySQL segment table.
Usually, when new segments are created, these MySQL entries are created directly so you never have to do this by hand. For this tutorial, we can do this manually by going back into MySQL and issuing:
2013-08-08 22:48:41,959 INFO [ZkCoordinator-0] com.metamx.druid.coordination.BatchDataSegmentAnnouncer - Announcing segment[wikipedia_2013-08-01T00:00:00.000Z_2013-08-02T00:00:00.000Z_2013-08-08T21:22:48.989Z] at path[/druid/segments/127.0.0.1:8081/2013-08-08T22:48:41.959Z]
If you are intested in how data flows through the different Druid components, check out the Druid [Data Flow](Data-Flow.html). Now that you have an understanding of what the Druid cluster looks like, why not load some of your own data?
