Some basic concepts explained

Learn about what is a resource definition atom, what is a Resource definition Atom Name block (RABN), what is a Resource Definition Update Block (RDUB), and what is syncpointing.

What is a resource definition atom

Certain resource definitions must be installed or deleted as a single set. These definitions are called a resource definition atom.

CICS® installs the members of a RDO group as individual resource definitions, which can fail without causing the other resources to fail except for these atoms, which bear the name of the logical set of definitions. For example:
A connection and its associated sessions
is named for the connection
A pool of terminals
is named for the pool of terminals

What is a Resource definition Atom Name block (RABN)

The RABN is only created for those atoms of resource recovery that are named. It holds the name of the atom, a chain of action elements for the atom, and the recovery outcome of the atom (whether it failed and was backed out, or succeeded and should be committed).

DFHTBSB uses the RABN to decide if a session definition should not be installed because the installation of the parent connection has already failed, for example. In our auto-install example, if the definition being installed is a parallel connection, there will be a RABN for it from which the action elements are chained.

What is a Resource Definition Update Block (RDUB)

The RDUB is a record of locks held by a UOW against names in three namespaces.

  1. Termids and Sysids
  2. Netnames
  3. Unique ids (Composed of the Netname of a terminal owning region followed by a period ‘.’ followed by the Termid or Sysid in that TOR)

During the installation, deletion, or replacement of a TCTTE definition the builders DFHBS* obtain locks by calling DFHZGTA. These locks guarantee exclusive or shared access to names in these namespaces. Exclusive access is used to prevent another task from installing another definition with the same name, netname or unique-id while this UOW is trying to install or delete (an action which may have to be reversed). Shared access is used to block another task from deleting an entry that a definition that this task is updating (for example, a system definition name may be locked by a remote terminal definition that refers to it).

RDUBs also exist on a global chain so that other UOWs can easily find out if a particular lock is held.

What is syncpointing

When DFHTBSBP has exhausted the list of builders, it returns to its caller. Similarly, DFHZCQIS returns to its caller, which could have been autoinstall. However, there is still an audit trail that is attached to the RRAB. It is only when the calling task terminates or issues DFHSP USER or EXEC CICS SYNCPOINT that the next two stages occur.

Syncpoint processing consists of two phases. The first phase (prepare phase) requires the resource manager to write a forward-recovery record to the log. Thus, if the second phase (commit phase) fails to write to the catalog, this recovery record can be used to forward-recover the action on an emergency restart.