DMCLs

Defining, Generating, and Punching a DMCL › DMCLs

DMCLs

The DMCL is the runtime component that describes one or more physical databases. The DMCL:

Designates which physical databases are accessible at runtime
Describes the files used to journal database activities
Specifies buffers for database and journal files
Designates which areas are to be shared across members of a data sharing group
Specifies attributes that affect data sharing operations

What a DMCL Contains

A DMCL contains the following component definitions:

Component	Function
Database buffers	Hold database pages in memory while CA IDMS/DB accesses information on the pages.
Journal buffer	Maintains information to be written to journal files, which are used for recovery operations. One and only one journal buffer must be defined for a DMCL.
Journal files	Log database activity. You can define either disk and archive journal files or a tape journal file.
Segments	Contain the areas of the database and the files to which those areas map.

DMCL Area/File Overrides

A DMCL definition can also override area and file definitions in the segments added to the DMCL.

Designating Areas as Shared

The DMCL indicates which areas are eligible to be shared for update across members of a data sharing group. Sharability can be specified for an entire segment or for an individual area through an area override.

DMCL Identifies Database Name Table

A DMCL also identifies the database name table to be used at runtime. The database name table provides logical names for one or more segments associated with the DMCL.

Order of Component Definition

To define a DMCL and its components, issue the following statements in the listed order:

CREATE DMCL
CREATE BUFFER
CREATE JOURNAL BUFFER
Either:
- CREATE DISK JOURNAL
- CREATE ARCHIVE JOURNAL
  Or
- CREATE TAPE JOURNAL
ALTER DMCL, adding segments and optionally, any area and file overrides

DMCL Used Under the Central Version

All applications that execute under the central version use a single DMCL.

DCMLs Used in Local Mode

An application that uses local mode database services may use the same DMCL used under the central version or a DMCL tailored for local mode operations. You can define as many local DMCLs as you wish. However, generally a local mode DMCL should be created only for the following reasons:

To execute a local mode update application with journaling activated
To reduce core requirements in your local mode address space by reducing the number of segments in the DMCL
To use a different page reserve or buffer size for special processing such as load operations

Differences Between Central Version and Local Mode DMCLs

The table below highlights the main differences between a DMCL used under the central version and a DMCL used only in local mode:

Component	DMCL used under CV and in local mode	Local mode-only DMCL
Buffer size	Typically large for central version operations to accommodate concurrent processing and small for local mode operations to accommodate 1 application	Typically small, to accommodate 1 application
Journal files	2 or more disk journal files and 1 or more archive files	1 tape journal file

DMCLs Used for Data Sharing

In a data sharing environment, more than one central version may share the same DMCL. If all members of a data sharing group are identical with respect to the data that they access, then they should share the same DMCL. This type of group is referred to as a homogeneous group.

If members of a group share access to only a subset of data, they may use different DMCLs. This type of group is referred to as a heterogeneous group.

The choice of whether members of a data sharing group use the same DMCL is a matter of convenience and does not affect the operation of the group. However, if different DMCLs are used, they should all specify the same data sharing attributes.

Stored as a Load Module

Because the DMCL is a runtime component, its definition must be generated and stored as a load module, and then punched and link-edited to a load library.

Identifying the DMCL to the Runtime System

You must identify the DMCL to be used in the runtime system:

Under the central version, specify the name of the DMCL to be used as a startup parameter for the DC/UCF system. See the CA IDMS System Operations Guide for information about startup parameters.
In local mode, if the name of the DMCL is not IDMSDMCL, specify the name in the SYSIDMS parameter file.