Understanding CDC Subscription

In this section:

Subscription is a logically and physically separated process defined for source-target CDC replication.

Each subscription consists of its own configuration files, Informix CDC Agent UNIX process, Shared Memory Buffer Structure, log file, trace files, output data files, and a separate instance of an iWay Service Manager (iSM) channel.

Any Informix database can have multiple subscriptions and any subscription can be defined on one database only. A database table cannot belong to more than one subscription.


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Subscription Control Files

Subscription control files are stored in the ctr subdirectory under CDC_HOMEDIR. Every subscription has two invisible control files:

The .seq control file stores IDs for the next output and trace file, for example:

TRACE_FILE_CNTRL_NO=11
TRANSACTION_FILE_CNTRL_NO=13

The next output or trace file will be created using this ID and will increment the value in the .seq control file.

The .lsn control file stores the value of the Last Sequence Number (LSN) that was written by the iWay Informix CDC Agent into the output file generated by the iWay Informix CDC Agent. This value is updated for every CDC record and has the following format:

89:0x16b4048$

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Subscription Environment Files

A subscription environment file is created for every subscription. The base name of this file must be identical to the subscription name and the environment variables in this file must only serve the specified subscription.

All directory names are derived from the iWay CDC environment file (iwaycdc.env) by default and can be configured. The variables include:


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Output CDC Data Files

Output files with CDC data are created in the subscription output directory and have the following format:

iwaycdc_<subscription>_<file ID>.txt

where:

<subscription>

Is the subscription name. The iwaycdc base name and .txt extension can be configured in the the iWay CDC environment file (iwaycdc.env).

<file_ID>

is a numeric incrementing number, starting from 1. When the output file reaches a maximum size, a new file with the next ID is created. When the CDC subscription process is restarted, a new file is created with the next ID.


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