Cloud provisioning with Apache Zookeeper


A few months back we explored how Java apps could be provisioned on the cloud using YARN. Based on the interest that post generated, let us re-visit the topic and explore provisioning with a variant architecture leveraging MRv1, HDFS and Zookeeper.

In a paper by S.Krishnan, J.C. Counio, the authors have described an approach used at Yahoo to create an elastic web server cloud farm based on Hadoop.  The authors describe 2 primary use cases for the system which they name as ‘Pepper’:
(1)   Web Feed processing – Dynamic provisioning of web apps to process high frequency small size feeds like news, sports, finance.
(2)   Online Clustering – Extract features and assign posts to clusters determined earlier by offline clustering.

In the implemented design,
- Zookeeper is leveraged to act as central registry where the information of the tasks that run the web applications is maintained.
- A Job Manager module exposes an API to deploy web apps (WAR file) on a dedicated location in HDFS.
- A Map Web Engine runs as a map task on Hadoop nodes and starts, monitors, stop the web app on the web server installed on the node. The web server could be an embedded Jetty server or other compatible server. Each web server registers itself with Zookeeper and generates its individual log4j logs which are aggregated on a common location in the cluster.
- A Proxy router acts as a single point of entry for client requests and forwards request to one of the available web servers.


What stands out in this implementation is how Apache Zookeeper has been leveraged for cloud provisioning. We understand that Apache ZooKeeper aids in cross-node synchronization. It “acts as a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services”.

The key operations performed by Zookeeper in this design, include the following, but are not limited to:
- Act as a monitor for the web servers
- Maintain a list of available web servers
- Maintain information of jobs that run the web applications
- Associate each web application with a free port
- Register the web applications with the central registry
- End the session initiated by the web application if it turns faulty

Some of the key characteristics of Zookeeper come to fore here making it a good choice:
- First-In-First-Out (FIFO) order queue – utilizing a pipeline architecture for storing a list of web applications that requires to be processed
 - Caching - priority web application can be cached to improve performance
- Locking - for maintaining consistency to update the central registry sequentially

The big takeaway from Pepper’s design is that Zookeeper can be used as much more than a coordination service among distributed systems. Rather than relying on a database or a complicated state machine, the Zookeeper here plays a pivotal role in simplifying cloud provisioning.