Tag Archive for 'Continuously Available'

Continuous Availability versus High Availability

Wikipedia’s page on Continuous Availability is available here:


A quick perusal tells us that High Availability can be ‘accomplished by providing redundancy or quickly restarting failed components’. This is very different from ‘Continuously Available’ systems that enable continuous operation through planned and unplanned outages of one or more components.

As large global organizations move from using Hadoop for batch storage and retrieval to mission critical real-time applications where the cost of even one minute of downtime is unacceptable, mere high availability will not be enough.

Solutions such as HDFS NameNode High Availability (NameNode HA) that come with Apache Hadoop 2.0 and Hadoop distributions based on it are subject to downtimes of 5 to 15 minutes.  In addition, NameNode HA, is limited to a single data center, and only one NameNode can be active at a time, creating a performance as well as an availability bottleneck. Deployments that incorporate WANdisco Non-Stop Hadoop are not subject to any downtime, regardless of whether a single NameNode server or an entire data center goes offline. There is no need for maintenance windows with Non-Stop Hadoop, since you can simply bring down the NameNode servers one at a time, and perform your maintenance operations.  The remaining active NameNodes continue to support real-time client applications as well as batch jobs.

The business advantages of a Continuously Available, multi-data center aware systems are well known to IT decision makers. Here are some examples that illustrate how both real-time and batch applications can benefit and new use cases can be supported:

  • A Batch Big Data DAG is a chain of applications wherein the output of a preceding job is used as the input to a subsequent job. At companies such as Yahoo, these DAGs take six to eight hours to run, and they are run every day. Fifteen minutes of NameNode downtime may cause one of these jobs to fail. As a result of this single failure, the entire DAG may not run to completion, creating delays that can last many hours.
  • Global clickstream analysis applications that enable businesses to see and respond to customer behavior or detect potentially fraudulent activity in real-time.
  • A web site or service built to use HBase as a backing store will be down if the HDFS underlying HBase goes down when the NameNode fails. This is likely to result in lost revenue and erode customer goodwill.  Non-Stop Hadoop eliminates this risk.
  • Continuous Availability systems such as  WANdisco Non-Stop Hadoop are administered with  fewer staff. This is because failure of one out of five NameNodes is not an emergency event. It can be dealt with by staff during regular business hours. Significant cost savings in staffing can be achieved since Continuously Available systems do not require 24×7 sysadmin staff .  In addition, in a distributed multi-data center environment, Non-Stop Hadoop can be managed from one location.
  • There are no passive or standby servers or data centers that essentially sit idle until disaster strikes.  All servers are active and provide full read and write access to the same data at every location.

See a demo of Non-Stop Hadoop for Cloudera and Non-Stop Hadoop for Hortonworks in action and read what leading industry analysts like Gartner’s Merv Adrian have to say about the need for continuous Hadoop availability.



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