1.3 ClearCase Data Storage

All ClearCase data is stored in VOBs and views. VOBs and views reside on one or more VOB or view server hosts in the ClearCase network. A networkwide registry of VOB-tags and view-tags allows users to access VOBs and views without having to know the name of the host where they reside. VOB and view maintenance is a critical aspect of ClearCase administration.

Versioned Object Bases (VOBs)

The ClearCase network's permanent data repository consists of one or (usually) more VOBs located on multiple hosts. Each VOB occupies a VOB storage directory, which holds file system objects and an embedded database.

VOB administration responsibilities include the following:

• Registration and tagging. Access information for all VOBs is kept in a networkwide registry. In a typical network, registry maintenance is minimal; ClearCase commands that create VOBs and VOB-tags update the registry automatically, though you may need to change a tag if you move a VOB. In addition, you may need to copy and modify tags to enable VOB access in mixed networks of UNIX and Windows computers.

• Backup. VOBs have special backup and recovery requirements, and must be backed up frequently and reliably. ClearCase does not include data-backup tools. You will have to select appropriate operating system backup and archive utilities or third-party backup tools for this critical task. VOB backup and restore procedures are described in Chapter 10, Backing Up and Restoring VOBs.

• Periodic maintenance. VOB administration requires that you balance the need to preserve important data with the need to conserve disk space. ClearCase includes tools for collecting data on disk space used and for occasional scrubbing of unneeded data. You can specify what data is unneeded on a per-VOB basis.

ClearCase includes a job scheduler that manages periodic execution of various administrative tasks, including disk-space data collection and VOB scrubbing. The job scheduler is installed with a predefined schedule of these maintenance operations, which you can change as needed. For more information on creating and managing scheduled jobs, see Chapter 28, Managing Scheduled Jobs.

• Access control. Each VOB has an owner, a primary group, a supplemental group list, and a protection mode. Together, they control access to VOB data. Understanding and managing VOB access controls is an important task for the ClearCase administrator. For more information, see Chapter 3, Understanding ClearCase Access Controls.

• Growth. As new projects begin or existing projects are placed under ClearCase control, you may need to create new VOBs and incorporate them into your data backup and periodic maintenance schedule.

• Reformatting. Occasionally, a major new ClearCase release may include a feature that requires reformatting of your existing VOBs. This process updates the schema of the embedded VOB database.

Views

ClearCase views provide

• Access to VOB data

• Short-Term storage for other data created during the development process

A view stores checked-out versions of file elements, view-private files that have no counterpart in the VOB (for example, text editor backup files), and newly built derived objects.

ClearCase supports two types of views:

• Snapshot views, which contain copies of versions of specified elements, along with view-private objects. The view never updates itself with new versions created from other views. Instead, the update command reevaluates the view's config spec and loads the newly selected versions into the view.

• Dynamic views, which provide transparent access to versions of elements in the VOB and to view-private objects. Each time you access an element through a dynamic view, the view's view_server process evaluates the view's config spec and selects a particular version of the element. Thus, such a view updates itself with new versions created in other views.

NOTE: Rational ClearCase LT does not support dynamic views.

Unlike VOBs, which are long-lived artifacts created by an administrator, views tend to be shorter lived and are usually created by individual developers. View administration is simpler than VOB administration, involving little more than occasional backups, periodic maintenance, and attention to access control issues.