Fall Back,Journals and NODES

FALLBACK

Fallback is a Teradata database feature that protects data in the case of an AMPvproc failure. Fallback guarantees the maximum availability of data. We can specify Fallback protection at the table or database level. It ensures high availability of the applications.

Fallback protects our data by storing a second copy of each row of a table on a different AMP in the same cluster(cluster has more than one AMPs).Fallback provides AMP fault tolerance at the table level. With Fallback tables, if one AMP fails, all data is still available and we can continue to use Fallback tables without any loss of access to data.

During table creation or after a table is created, we may specify whether or not the system should keep a Fallback copy of the table.

Below are the examples of create table syntax with Fallback and  without Fallback :-

1.Create multiset table employee, fallback,

datablocksize=32 kbytes,freespace= 10 percent,checksum=none

(empno integer,

salary integer); 

2.Create set table deptartment,No fallback,

       No Before Journal,

       No After  Journal,

       Checksum =default

(

employee_number integer,

deptno integer

);

Benefitsof Fallback:-

1. A level of protection beyond RAID disk array protection.
2. Can be specified on a table-by-table basis to protect data requiring the highest availability.
3. Permits access to data while an AMP is off-line.
4. Automatically restores data that was changed during the AMP off-line period.
5. When an AMP is down with a table that is defined as Fallback, Teradata will access the Fallback copies of the rows which is available in same cluster.

Drawback of Fallback:

Fallback-protected tables requires 100% additional storage

The highest level of data protection is Fallback and RAID1 (Redundant Array of Inexpensive Disks which provides data protection at the disk drive level)

Journals

There are  3 different types of journals available in Teradata. They are

1. Transient Journal  - This maintains current transaction history. Once the query is successful it deletes entries from its table .  If the current query transaction fails, It rolls back data from its table.

2. Permanent Journal  - This is defined when a table is created.  It can store BEFORE or AFTER image of tables. DUAL copies can also be stored. Permanent Journal maintains Complete history of table.

3.Down AMP recovery Journal (DARJ)  - This journal activates when the AMP which was supposed to process goes down.  This journal will store all entries for AMP which went down.  Once that AMP is up, DARJ copies all entries to that AMP and makes that AMP is sync with current environment.

NODE

A Node is a term used in Teradata for a processing unit under the control of a single operating system.

The Node is where the processing occurs for the Teradata Database.

In other words we can say that a Node is the basic building block of a Teradata Database system which contains a large number of hardware and software components.

Hardware components of Node are –

CPU
System disk
Memory
System bus
Ethernet
LAN Cables

Software components of Node are –

AMP
PE
Application
Gateway
Channel Driver
PDE
OS

There are two types of Teradata Database systems mentioned below:

SMP(symmetric multiprocessing) - An SMP Teradata Database has a single node that contains multiple CPUs sharing a memory pool.

MPP(massively parallel processing) - Multiple SMP nodes working together comprise a larger, MPP implementation of a Teradata Database.In MPP nodes are connected with BYNET, which allows multiple virtual processors on multiple nodes to communicate with each other.