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What is the meaning of "Currency ????" ?- The current location in the database is known is Currency.
- IDMS/R maintains pointers to keep track of the location identification in the database.
- Pointers are maintained by IDMS/R to a number of different record occurrences as each run unit executes.
- IDMS/R maintains a Currency Table for each run unit.
- IDMS/R maintains four types of currency tables for every run unit.
Record Currency:
- Most recent record occurrence of each record type is identified by db-key. IDMS/R maintains unique record currency for each record.
Set Currency:
- This is the most recent record occurrence of every set that is accessed by the program. Separate currency key for each set is maintained by IDMS/R.
Area Currency:
- Area currency is the most recent record occurrence of every area that is accessed by the program. Separate currency for each area is maintained by IDMS/R.
Run Unit Currency:
- RUC is the database key of the most recent record occurrence that is accessed by the program.
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What is a database?- Def: Database is a collection of organized information.
- Database should have the property of controlled redundancy.
- Security and integrity should be managed by the database.
Types of database models.
- Database models are of four types:
1. Flat files:
- The whole data is persisted in a series of files without controls and redundancy.
2. Hierarchical Database:
- The database architecture resembles a tree structure.
3. Network Database:
- The network database architecture resembles that of hierarchical database with some variations.
- The main difference is that a relationship between two nodes at the same level, say F and G (siblings) can be established;
4. Relational Database:
- In relational database, the data is represented in the form of tables.
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What are storage attributes of records?- The following are the storage attributes of records:
Length : Records can either be stored as fixed length or variable length.
a. Fixed length : The whole record instance is persisted in a contiguous block of memory on the line
b. Variable length : The root part of the record is persisted on the current line. The remaining part of the record is persisted on some other part of the memory.
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Difference between Fixed length and Variable length records.- Fixed length records access is faster.
- Variable length records maintains better utilization of space.
2. Compressed : Records can be stored as compressed or uncompressed format.
a. Compressed : The record’s data can be compressed upto 50%.
b. Uncompressed: The records are represented as they are.
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What is DBKEY? Explain.- The DBKEY is used for identifying the record instance in the database in a unique address location.
- It is the combined usage of line and page numbers.
- A four byte address has 3 and 1, where 3 is the page nos and 1 is the line no.
- Pages are always unique across all the areas.
- DIRECT ACCESS: When the dbkey is specified, the record is persisted, if the location is free.
- SEQUENTIAL ACCESS: The instances of records are persisted one after the other, and line by line.
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What is the difference between local and central version operating modes?The following are the differences between local and central operating modes:
Local mode:
- A batch program, which is operates in local mode, has its own copy of IDMS DBMS loaded into its address space.
- It is suggested for Batch Retrieval programs.
- The operation of local mode is faster.
- When the records are updated, offline areas are taken. Other users have no access to the database even in retrieval mode.
- JCL can run in any of the regions.
Central Version:
- A single copy of IDMS DBMS will control various operations of a particular IDMS set application programs run unit.
- All CCOBOL/Online are to be used only Central Version.
- The operations of local mode is slow
- There is no need of taking offline area.
- Concurrent users can access the same area at a time.
- JCL mandatorily run in a particular region.
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Name and explain the three location modes?The three location modes are calc, via and direct:
1. calc:
- The values of any field of a record is converted into a combination of page no and line no.
- This process is implemented by means of a hashing algorithm.
- A record can be accessed directly .
2. via:
- The member is persisted near to the owner .
- The member can be accessed only by owner.
3. direct:
- The storage location is specified in the application program.
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What is an area sweep and when is it used?- IDMS/R scans through all the available records in an area in physical sequence, while performing an area.
- The order of records retrieval will have little bit of relationship to some logical sequence of records.
- Area sweeps are utilized when all the records need to be retrieved in an area irrespective of the order.
- Extraction jobs are commonly implemented by Area sweeps.What is OOK-Rec?- The full form of OOK is One Of a Kind.
- It is the meta schema record of IDD.
- The IDD meta schema is generation at the time of installation.
- For every database different name is given and there will be one OOK record that holds the database name and other information.
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What is a junction record?- A member record type.
- Allows many-to-many relationship between its 2 owner records.
- For example, for a company database, DEPARTMENT record is a junction record for the EMPLOYEE and PROJECT record types.What is a set? What pointers are required, what are possible? How may sets be ordered?- A set is known as a owner record.
- Members records can be included in owner record.
- There are three types of pointers are required:
1. next, prior and owner.
2. only next is required.
3. The possible order of arrangements of sets are five; they are:
i. first – inserts at the beginning.
ii. last – inserts at the end.
iii. next – inserts after the current record.
iv. prior – inserts prior to the current record.
v. sorted – inserts as per the sort value.
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Distinguish among erase, erase permanent, erase selective and erase all?- Erase : Cancels the record’s membership in a specified set of occurrences.
Named records only will be removed.
- Erase Permanent :
Deletes the specific record.
Deletes owned mandatory occurrences.
Disconnects all the optional members.
- Erase Selective :
Deletes the record.
All mandatory members are deleted.
All optional members not connected to the remaining sets will be removed.
- Erase All : Deletes a specified records, mandatory records and optional records.
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Explain about few error status codes of IDMS.- All error codes are of 4 characters. - First two are called as: 1. Major 2. Minor MAJOR CODES:| Code | Database Function | | 00 | All the DML Statements | | 01 | FINISH | | 02 | JERASE | | 03 | FIND/OBTAIN | | 05 | GET | | 06 | KEEP | | 07 | CONNECT | | 08 | MODIFY |
MINOR CODES:| Code | Database Function Status | | 00 | Successful completion of the DML operation, if combined with major code 00. Incomplete DML operation , if combined with major non zero. | | 01 | When combined with major code 16, the IF operation yields a false value. | | 02 | The db-key is used either FIND/OBTAIN or direct db-key is suggested for a STORE. | | 05 | Violation of the specified DML function is caused a duplicates-not-allowed option for a CALC, sorted or index set. | | 06 | Indicates the non establishment of the currency for the named record / set / area. |
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Discuss some of the terminology of IDMS.- The following are some of the IDMS jargon:
a. DBKEY: It is a 4 byte key, typically 3 byte page number and 1 byte line index, used to uniquely identify each physical stored IDMS record.
b. AREA: Represents a range of IDMS pages used to persist IDMS data. The critical factors are starting-page, number of pages, and page-size.
c. FILE: IBM data set is represented as a FILE. Various areas are mapped to different files on one-to-one, one-to-many or many-to-one basis.
d. DMCL: Journals, buffers and files for an IDMS system are tied together. One global DMCL for a CV is always available. Local jobs have custom tailored. The DMCLs are managed by the DBAs
e. CV vs Local: The batch jobs are run under Central Version or in Local mode
Central Version provides lock protection facility A backup is needed before running a local-update job.
Various report jobs will run in local mode
f. Schema/Sub Schema: A schema has logical database definition
A sub schema is a sub set of schema. One batch program should refer for one schema/ sub schemaEvery batch program must refer to one sub schema / schema.
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What is Integrated Data Dictionary?- IDD’s prime requirement is to maintain the database definitions.
- IDD was interfaced by DBAs and other designated users using a database language known as Data Dictionary Definition Language(DDDL).
- IDD is used even to store definitions and code for various other products of IDMS such as ADS/Online and IDMS-DC.
- The power of IDD is the extensibility that could be utilized for creation of definitions of anything. Certain enterprises uses IDD for the development of in-house documentation.
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Explain about logical data model of IDMS.- The logical data model is the data model offered to CODASYL(Conference On Data System Languages).
- The prime structure of this model is records and sets.
- All the records essentially follows COBOL pattern, consisting of fields of different types, which allows internal structure, like repeating items and groups.
- The most distinctive structure is the 'set'.
- The ‘set’ represents a one-to-many relationship among records – one owner and many members.
- A record can be a member in different sets, which is the key factor for distinguishing network model from hierarchical models.
- Each set belongs to a set identified by a name. Every set is an ordered sequence of records.
- A record is an owner and member of any number of sets.
- The database key is directly related to the physical address of the record on a disk.
- Records could be accessed directly by using database key, followed by set of relationships or by direct access using the key values.
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Explain Storage architecture of IDMS.- The organization of databases is in terms of files.
- The files are mapped to areas , which are pre-formatted.
- The areas are subdivided into pages.
- Each page corresponds to physical blocks on the disk.
- A fixed number of pages is allotted in a file for each area by the DBA, and defines the records to be stored in each area and how they are to be stored.
- The free space is only tracked for all the pages in order to reduce I/O requirements.
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What methods are available for persisting records in an IDMS database? Explain.- There are four methods are available for persisting records in an IDMS database.
1. Direct:
- The target is specified by the user and persists as close as possible to that page.
2. Sequential:
- The records are simply places each record at the end of the area.
3. CALC:
- A hashing algorithm is utilized for deciding the place of the record.
- The hash key provides an efficient retrieval of the records.
- The CALC records are linked to the page of CALC owner record by utilizing a single link-list.
4. VIA:
- VIA attempts to store a record near its owner in a particular set.
- The records are clustered on the physical page as the owner
- The records are persisted in a different IDMS areas, yet remain clustered together for efficiency.
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