Features Supported by Hadoop Release Series

Comparison between Big Data and RDBMS

	RDBMS	Big Data
Data size	Gigabytes	Petabytes
Access	Interactive and batch	Batch
Updates	Read and write many times	Write once, read many times
Structure	Static schema	Dynamic schema
Integrity	High	Low
Scaling	Nonlinear	Linear

HBase Backup

HBase Backup:

We need to have backup of HBase table offline in some point of time, in spite of the fact that Hadoop and HBase provide replication and redundancy. For this we have some backup option in HBase. These are categorized in two ways:

Offline backup and

Online backup

Online Backup:

Again this is categorized in three ways

Replication: In this method you need to have a 2^nd cluster where you will keep your replication for the data from the 1^st cluster.

Hadoop/HBase Export command: which runs a map reduce job to copy table from one cluster to the same cluster or to other Hadoop cluster. This does not require any kind of downtime for backing/ exporting data.

In this method we need to export the data to the cluster and if we need to restore we need to restore it by Importing.

CopyTable: this is also online backup method which copies table from one cluster to another cluster or to the same cluster.

Offline Backup:

Distcp : this is a kind of file system backup, this copies a directory from HDFS to same cluster or to other cluster.

copyToLocal : this is less reliable way of copying directories from HDFS to local backup drive. If large amount of data is there then you need lot of Hadoop tune-up to copy successfully.

Offline Backup methods are full shutdown backup method, suppose you need to copy HBase you need to stop your HBase cluster, for a successful backup, as the files are being continuously moved, modified and changes while cluster is online, and copying in this scenario may fail.

Linux: Crontab - Brief

I always get confuse whenever I want to set a new cron job. The confuse is with regard to the options too be set!
For those who new to 'cron', its nothing but, an event scheduler in Linux. That means, you can schedule any script to run at any time you wanted to. Its just the system/server should be up and running!

cron job is specific to every user in Linux/Unix. So, one can't see other's cron unless the necessary privileges or sudo root access given.

Whatever, here is the options in cron:


To check cron jobs:

[root@localhost kiran]# crontab -l
no crontab for root

To set cron jobs:

[root@localhost kiran]# crontab -e

After adding, here is how it looks:

[root@localhost kiran]# crontab -l

##Script to test

00 */2 1-31 * 0,2,3   sh /home/kiran/test.sh >> /dev/null

Every Cron job should be given with 5 options:

• minute -> 0-59
• hour -> 0-23
• day of month -> 1-31
• month -> 1-12 
• day of week -> 0-7 (0 is Sunday )

In the above example:

00 -- 0th Minute

*/2 -- Every 2 hours

1-31 -- Every day (1 to 31)

* -- Every Month

0,2,3 -- Sunday,Tuesday,Wednesday

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