Change Data Multicast (CDM) | Teradata Unity - 17.00 - Change Data Multicast (CDM) - Continuous Availability - Teradata Unity

Teradata® Unity™ User Guide

Product
Continuous Availability
Teradata Unity
Release Number
17.00
Published
September 2020
Language
English (United States)
Last Update
2020-09-15
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Change Data Multicast (CDM) is a method to replicate changes between multiple systems. When enabled, CDM replicates SQL changes that are non-deterministic and can produce different results if run in parallel on different systems. CDM sends a non-deterministic SQL to one of the managed Teradata systems and then captures all the resulting data changes. It then multicasts those data changes to the rest of the participating Teradata systems. This allows Unity to replicate changes that involve non-deterministic functions like RANDOM and SAMPLE and writes to tables that include Identity columns.

The replication of IDENTITY columns adversely affects performance. CDM requires that you redefine IDENTITY columns so they have different ranges on different systems.
Unity uses CDM across managed systems to keep non-deterministic objects and triggers synchronized. Non-deterministic objects and triggers include:
  • Identity Columns
  • Dynamic SQL in Stored Procedures
  • Non-deterministic ordering or sampling (i.e. Sample, TOP, ROWNUMBER)
  • Analytic functions

When non-deterministic objects or triggers are identified, Unity processes the action on the first Teradata system, then uses CDM to send and override the values on the other systems.