Database Hierarchical

DBD (Database Definition) is used to define the physical structure of the database.

► One DBD for each database

► Created by a series of control statements - by a process called DBDGEN

► Used by DL/I whenever database is used

There are three control blocks (also known as DL/I control blocks)

► Database Description (DBD)

► Program Specification Block (PSB)

► Application Control Block (ACB)

Advantages:

► Speed of access is faster because of the predefined data paths.

Disadvantages:

► Complex implementation

► Predefined tree structure reduces flexibility.

► Difficult to manage

Atomicity:

Either all actions are carried out or none are. Users should not have to worry about the effect of incomplete transactions. DBMS ensures this by undoing the actions of incomplete transactions.

Aggregation:

A concept which is used to model a relationship between a collection of entities and relationships. It is used when we need to express a relationship among relationships.

In distributed deadlock detection, the delay in propagating local information might cause the deadlock detection algorithms to identify deadlocks that do not really exist. Such situations are called phantom deadlocks and they lead to unnecessary aborts.

► Follows inverted tree structure

► Data relationship is predefined by it's structure

► Program accesses the data through predefined paths.

IMS Software Environment consists of the below five main components:

► IMS DB

► DL/I

► DL/I Control Blocks

► IMS DC

► Application Programs

A Checkpoint is like a snapshot of the DBMS state. By taking checkpoints, the DBMS can reduce the amount of work to be done during restart in the event of subsequent crashes.

A hierarchical data base is a data model in which the data is organized into a tree-like structure. The structure allows repeating information using parent/child relationships: each parent can have many children but each child only has one parent

A hierarchical database is a database which formed with a root nodes and leaf nodes.An leaf node is the last node and each root node having at-least one node. this may not be the last node connected in a tree-like structure.

Program Specification Block defines the programs view of the database and application programs access authority to database. It defines the logical data structure. A PSB is made up of one or more PCBs (Program communication blocks).

DL/I is a command-level language to manipulate IMS databases. DL/I forms a bridge between application program and IMS database. It is used in batch and online programs to access data stored in IMS databases.

Every access to an IMS database is through DL/I. The DL/I interface must be called to update or read a IMS database. DL/I allows both sequential and random processing of a database.