IDMS

Integrated Database Management System

Topics CoveredIntroduction to DBMSLogical and Physical Database StructureRecord CharacteristicsSet CharacteristicsIDD and DDDL Data Description Language (DDL)Data Manipulation Language (DML)Recovery & Restart of DatabaseLocking Facilities

Introduction to DBMS

TYPES of DBMSInverted List (e.g. Datacom DB)

Hierarchical (e.g. IMS)Data records are typically connected with embedded pointers to form a tree structure. Each node (except root) can have one and only one parent

Continue ..Network (e.g. IDMS)The database forms a mesh structureEntity-Relationship is implemented using Record type and Set.

Relational (e.g. Oracle, Sybase, etc)Entity-Relationship is implemented in the normalized form. Data represented in the form of rows and columns (two dimensional table)

Logical & Physical Database Structure

A database can be viewed as consisting of Entities. An entity is an object about which we are storing data. Every entity has attributes. e.g. Employee is an entity which has attributes like, emp-code, emp-name, emp-address, emp-sex, emp-dept etc. There exists relationship between different entities. We can represent the relationship between our entities using an entity-relationship (E-R) diagram as below.LOGICAL DATABASE STRUCTURE

SALARYBONUSTEAMPLAYERGAMEE-R Diagram

The example discussed here:Taken from a typical American baseball league.A team has many players but at a given time, a player is attached to only one team.A team plays many games. Games can be further identified by Game-Home and Game-Away.THE EXAMPLE

A player plays many games and same way a game is played by many players.A player has many salary records since his salary (contracts) may have undergone changes during a season. Similarly, a player might have many bonuses awarded to him.Team, Player, Game, Salary and Bonus represent Entities.Continue ..

Attributes of Team could be Team-Name, Team-City, Team-Address etc.The relationship between entities is shown by line diagram. We use cardinality symbols to represent type of relationship. The crow-foot end of the line diagram represents Many type and single line represents One type of the relationship.Continue ..

For example, consider relationship between Team and Player. Team end has a single line whereas Player end has a crow-foot. This represents a One-to-Many relationship. It means a team has many players. But a player is attached to only one team.Note that relationship between Player and Game is Many-to-Many. i.e a game has many players and a player plays many games.Continue ..

In IDMS, entities are implemented using Record types and relationships are implemented using Set types.IDMS does not allow direct implementation of many-to-many relationship.Continue ..

JUNCTION RECORDIt is a way of implementing many-to-many relationship in IDMS. It is a record type having attributes that are common (intersection) to both record types participating in many-to-many relation.Thus, many-to-many relation between player and game can be resolved as below.

NOTE:Here, referring to our earlier example of American baseball league, we had many-to-many relationship between entities player and game. PLAYERGAMEPOSITIONContinue ..

That is to say, a player has played in many games and conversely, a game has many players in it. To resolve this M:M relationship, we introduce a junction entity, Position record. Position is called Junction record because it represents junction or intersection between Player and Game record.Continue ..

A player has many positions assigned to him for different games. But for a game he has a fixed position. Main attribute of the record is player-position for a game. It thus resolves many-to-many relationship between player and game. In other words, we can easily move from a player record occurrence to a game record occurrence using the position record occurrence.Continue ..

BACHMAN DIAGRAMMost installations use a line with an arrow at one end to show each one-to-many relationship instead of using crows foot notation. Such a diagram is called Bachman Diagram. Here, the end of the relationship without an arrow always represents One side of the relationship and the end with arrow always represents Many side of the relationship. Our earlier E-R diagram is represented using Bachman diagram as:

SALARYTEAMPLAYERGAMEBONUSPOSITIONBACHMAN DIAGRAM

IDMS database is divided into one or more areas. An area is defined as the major named subdivision of addressable storage in database.An area is further subdivided into pages. Page is a smallest unit of data transfer between main memory and hard disk.PHYSICAL DATABASE STRUCTUREDatabase Area, Page and File

Each page stores control information and can store up to a maximum 255 database records.Database areas can be of different page sizes. e.g AREA-A can be of page size 2048 bytes where as AREA-B of same database can be of page size 4096 bytes. Continue ..

Page size for an area is decided based on the record types that will be residing within that area.ALL record occurances of a particular record type are stored within the SAME area.But, an area may store occurances of multiple record types. Hence this holds true for a database page as well.Continue ..

Record types are assigned to areas by designer.Areas are mapped to files.Many or all areas can be mapped into one file if all areas have the same page size.Each area can be mapped into a different file.One area can be mapped into several files.Continue ..

AREAS and FILES MAPPING

Page AreaLogical DatabasePhysical DatabaseFileBlock

Team Area921 922 930IDMS File 1Position Area1021 1022 1040Salary Area821 822 ... 830 IDMS File 2Area and File Mapping..

SALARYTEAMPLAYERGAMEBONUSPOSITIONADVANTAGES OF MULTIPLE AREASSALARY-AREAPOSITION-AREATEAM-AREA

Continue ..As shown in the previous slide, our example of baseball database is divided into three physical areas namely Team-area, Position-area and Salary-area.The advantages of dividing a database into separate physical areas are: Processing efficiencyRecords those are accessed together during most of the processing can be grouped together into same area for efficiency.

Continue .. SecurityOne can restrict access to certain record types. e.g. salary-area can be restricted to only finance department. Database recovery and backupDatabase can be initialized, reorganized and backed up on area-by-area basis. Backup of most updated areas can be made more frequent than other areas.

Continue .. Concurrent updatingA program can request exclusive use of an area and prevent other programs from accessing it concurrently.

CONCEPT of DB-KEYEach Record occurrence stored in the database is assigned an unique numeric identifier, called Database Key (db-key). A records db-key consists of a 32-bit field that contains a 23-bit page number and an 8-bit line number. The page number identifies the page in which the record is stored and the line number identifies location of the record occurrence within the page.The format of db-key is as below:

8-bits23-bits1-bit(Not Used)Line NumberDatabase Page NumberSign Bit

RECORD TYPE v/s RECORD OCCURRENCEIt is important to make distinction between Record Type and Record Occurrences.Record type is like a template. It describes the format of all occurrences of a given record type stored in database.Record occurrence represents the smallest directly addressable unit of data. It consists of fixed or variable number of characters that are subdivided into units called Data Elements.

Example of Record Type versus Record OccurrencesEmployee Record Type .Emp-codePIC 9(4)Emp-namePIC X(30)SexPIC XEmp-deptPIC X(10)Employee Record Occurrences .....1000ABCMTECH1500XYZFSALES

Database consists of collection of record occurrences.The physical records stored in a database consist of more than the data elements used by the application program.IDMS also maintains information about relationships that exists between records.Relationships are implemented by linking record occurrences together with pointers.STRUCTURE of DATABASE RECORDS

Pointers contain addresses of related record occurrences and are stored along with the data portion of the record occurrences.A record occurrence in database consists of two parts. Data portion and Prefix portion.Continue ..

Data elements values are stored in data portion. Pointers to related record occurrences are stored in prefix portion.Application programs deal only with data portion of the record occurrence. Whereas, IDMS system maintains the pointers in the prefix portion.Continue ..PREFIX PortionDATA PortionPointer 1Pointer 2Pointer 3..Data element 1Data element 2...

Schema is the logical definition of a database. A schema is a complete database description (all records types & record elements, set types, files and areas).There is ONE and ONLY ONE schema for a given database.SCHEMA and SUBSCHEMA

A subschema defines a subset of the schema. One or more subschemas can be associated with a schema. A load module is created for each subschema. At runtime, a program can access only one subschema. A program can not access IDMS database by referring to the schema.A subschema is similar to a view in a relational database.Continue ..

Even if a program requires access to ALL record types, data elements and ALL set types defined in the schema, the designer must still define a subschema that includes all the entities defined in the schema.A subschema can restrict access to a program. For example, it may give only read-only access to an area, or it may not allow deletion of some record types.Continue ..

Record Characteristics

The characteristics that apply to record types are:Record NameRecord IdentifierStorage ModeRecord LengthLocation ModeDuplicates OptionArea Name

RECORD NAMEEach record type must be assigned a 1 to 16 character name that identifies the record type. The name must begin with an alphabetic character. The application program must reference the records name in DML (Data Manipulation Language).

It is a number that serves as in internal identifier for the record type. It is in the range 100 through 9999. Each record type must be assigned an unique record identifier within the installation. DBAs assign this number to each record type. Application programs do not refer to record type using this number.RECORD IDENTIFIER

STORAGE MODEIt indicates whether record occurances of the record type are fixed or variable length and whether they are stored in compressed format. Allowable codes are: F (fixed length) V (variable length) C (compressed)

NOTE:Compressed mode can be used along with fixed or variable length storage modes. For example, one can mention storage mode as FC or VC. DBMS takes care of compression and decompression of data.Continue ..

RECORD LENGTHIt is expressed in BYTES. It is the actual data length for fixed-length record or the maximum data length for variable-length record.

LOCATION MODEIt defines the way record occurances are stored in the database. Allowable location modes are:CALCVIADIRECT

In CALC mode, a particular data element within the record is declared as the CALC-key. At the time of storing the record in the database, IDMS system uses value of the data element to calculate the page number for storing a record. Records stored with a CALC mode can be retrieved from disk in a single access.CALC Mode

IDMS system uses randomizing routine to distribute records evenly over its area, minimizing overflow conditions and leaving space for adding new records.For retrieving the record (stored with CALC location mode), appropriate value for the data element is moved to the storage area in the application program and then DML retrieval function is executed.Continue ..

VIA ModeRecords stored with VIA location mode are stored near another database record. This mode is generally used for storing member records on the same page containing owner record or on a page near their owner record.This mode tends to reduce disk accesses needed to retrieve all the records of a set occurrence.

Continue ..CALC retrieval is not possible in case of records stored with VIA location mode. Generally the owner record is assigned CALC location mode for easy access of member records.

DIRECT ModeIn DIRECT location mode, application program explicitly specifies the page into which the record should be stored. To retrieve this record, programmer must specify its database address, i.e db-key of the record.This mode is less often used than CALC and VIA mode.

DUPLICATES OPTIONIt is specified only for record types that are stored using CALC location mode. It specifies whether records with duplicate CALC-key values are allowed and if so, how would they be stored in the database. Possible codes are as on next slides:

Continue .. DN (Duplicates Not Allowed)Record occurances with duplicate CALC-key value will not be accepted. IDMS will give an error if application program tries to store a record with duplicate CALC-key.

DF (Duplicates First)IDMS will store the record with duplicate CALC-key value Before any record in the database that has matching CALC-key value. When CALC retrieval is made using CALC-key value, newly stored record will be retrieved first. Continue ..

DL (Duplicates Last)IDMS will store the record with duplicate CALC-key value After any record in the database that has matching CALC-key value. When CALC retrieval is made using CALC-key value, newly stored record will be retrieved last.Continue ..

AREA NAMEIt is the name of the area into which all record occurances of the record type are to be stored. e.g. CUST-AREA, SALARY-AREA, SALES-AREA etc.

Data Structure Diagram (DSD) for Documenting Record Characteristics

Record name

Record-id

Storage mode

Record length

Location mode

Calc key or VIA set name

Duplicates option

Area name

Example : DSD for Employee Record Type

EMPLOYEE

1000

F

150

CALC

EMP-CODE

DN

HR-AREA

Set Characteristics

A Set Occurrence consists of one occurrence of owner record type and any number of member record occurrences.WHAT IS A SET ?A Set consists of an OWNER record type and one or more MEMBER record types.SET OCCURRENCE ?

A set occurrence that consists only of an owner record occurrence and no member record occurrences is called Empty Set occurrence.Walking a SetRecords in each set occurrence are physically linked together by pointers.Accessing members by following the pointers from one record occurrence to the next is called Walking the Set.Empty Set Occurrence

Any record type can be member in one or more sets.Any record type can be owner of one or more sets.Any record type can be member in one set and owner in another set.An owner record can own the same member record in more than one set.A record cannot be both owner and member in the same set.RULES & FEATURES FOR SET RELATIONSHIP

OfficeEmployeeDepartmentOffice-EmployeeDept-EmployeeAny record type can be member in one or more sets

Any record type can be owner of one or more setsOrderCust-OrderCustomerPartsCust-Parts

Any record type can be member in one set and owner in another setOrderCust-OrderCustomerItemsOrd-Items

An owner record can own the same member record in more than one setClassTeacherTeach-ClassExaminer-Class

DBA typically assigns characteristics to each set when defining the IDMS database in the schema DDL. The characteristics are:Set NameLinkage OptionsMembership OptionsOrder OptionDuplicates OptionSet Characteristics

SET NAMEA unique name must be given to each set type in the database. The name can be maximum of 16 characters. The set name must be referenced whenever an application program accesses records using that set relationship. Usually it will be owner record name followed by member record name e.g. CUST-ORDER.

LINKAGE OPTIONSIt indicates the types of pointers that are used to implement the set. Pointers provide flexibility in accessing records in a set occurrence. Pointers are stored in the prefix part of the database record occurrences. Available options are:

Continue ..N (NEXT pointer)Each record in the set contains a pointer to the next record occurrence. This option allows to access member records only in the forward direction. Next pointer is mandatory for all sets. All other pointers are optional.

Continue ..NP (NEXT and PRIOR pointer) With this, in addition to Next pointer, each record contains pointer to prior record occurrence in the set. This allows us to access member records in both forward and backward direction.

Continue ..NO (NEXT and OWNER pointer) With this, in addition to Next pointer, each record contains pointer to owner record occurrence in the set. This allows us to access the owner record directly from any member record occurrence.

NPO (NEXT, PRIOR and OWNER pointer) Each record in the set contains all three pointers. This option allows to access member records in both forward and backward direction and also allows to access the owner record directly from any member record occurrence.Continue ..

Xansa Mumbai DATAPNEMP-A DATA PNEMP-B DATAPNEMP-C DATAPNRecord Occurrences with Next, Prior & Owner PointersOfficeEmployeeOffice-EmployeeOOO

The Membership options specify how a member record may be connected to or disconnected from a set occurrence.The option is defined in two parts.MEMBERSHIP OPTIONSFirst part is a Disconnect Option, indicating the way a record is disconnected from a set. Second part is the Connect Option, indicating how a record is connected to a set.

It specifies whether a member record can be later disconnected from a set once its membership has been established. Possible values are: M (Mandatory)The record cannot be disconnected from a set unless that record is deleted (erased) from the database using the ERASE command.Disconnect Option

O (Optional)A record occurrence can be disconnected from a set. The record remains in the database. It can be connected to some other set occurrence. It is optional to use ERASE for such records.Continue ..

It specifies whether or not a member record is automatically connected to a set occurrence when it is added to the database.Possible values are: A (Automatic)Automatic means when a member record is inserted in database, IDMS will automatically connect it to all its owner records (provided currencies have been established for owners). Connect Option

M (Manual)Manual option specifies that after inserting a record, programmer must explicitly connect it to its owner record by issuing CONNECT statement. Continue ..

MA- Mandatory Automatic

MM- Mandatory Manual

OA- Optional Automatic

OM- Optional ManualCombinations for Membership Options

The order option specifies logical order in which member record occurrences are placed within a set occurrence. Options available are: FIRSTEach new member record occurrence is placed immediately after the owner record (in the next direction). This option achieves a member record in LIFO.ORDER OPTION

LASTEach new member record occurrence is placed immediately before the owner record (in the prior direction). This option achieves a member record in FIFO. Prior pointer is a must to specify this option. NEXTEach new member record occurrence is placed immediately after the member record occurrence that was last accessed (in the next direction).Continue ..

PRIOREach new member record occurrence is placed immediately before the member record occurrence that was last accessed within the set (in the prior direction). Prior pointer is a must to specify this option.Continue ..

SORTEDEach new member record occurrence is placed in ascending or descending sequence, based on the value of designated sort-control data element (sort-key) in each record occurrence. When the record is placed into a set, DBMS examines the sort-key value in each member to find the logical position of new member record in the set.Continue ..

This option is useful only in case where set is defined with order option as Sorted. It indicates the action to be taken when a duplicate sort-key value occurs. DN (Duplicates Not Allowed)Record occurances with duplicate sort-key value will not be stored in the set. IDMS returns an error code if program tries to store such a record. DUPLICATES OPTION

DF (Duplicates First)Record with duplicate sort-key value is stored Before any existing record in the set that has matching sort-key value. Most recently stored duplicate record will be retrieved first. Continue ..

DL (Duplicates Last)The record occurrence with duplicate sort-key value is stored After existing record in the set that has matching sort-key value. Most recently stored duplicate record will be retrieved last.Continue ..

IDMS allows retrieval of records using an index.In conventional sets, member records are chained together by pointers. In an indexed set, DB-key values of member record occurrences are stored in a specified order in one or more index records.Adds flexibility to data retrieval and retrieval is made faster in some cases.INDEXED SETS

Indexed sets are useful in cases where:Records need to be accessed using alternate-key (apart from CALC key).Walking very long sets and when only key values of a member record is required.Member records need to be retrieved randomly using partial key value.Continue ..

Indexed sets can be implemented using two database record types or a single record type and a system defined record type.Continue ..

PlayerPositionPlayer-Position-Index(Position-Name)Continue .....User-defined Owner Record

EmployeeSystem-defined owner record(Employee-Name)Continue .....SR7

Index between Player and Position records allows to access position records using Position-name values.System owned index on employee implements alternate index on employee-name.Continue ..

SALARYPLAYERBONUSTEAMPOSITIONGAMESALARY-AREASALARY-AREATEAM-AREATEAM-AREATEAM-AREAPOSITION-AREADNDNDNVIAVIAVIACALCCALCCALC407025508070100011001200200025003000FFFFFFPLAYER-BONUSGAME-POSITIONPLAYER-NAMEPLAYER-SALARYGAME-PLAY-DATETEAM-NAMEPLAYER-SALARYNPOOANEXTTEAM-PLAYERNPOOMLASTPLAYER-BONUSNPOOANEXTPLAYER-POSITIONNPOOANEXTGAME-POSITIONNPOMANEXTTEAM-GAMENPOMMNEXT

IDD & Data Dictionary Definition Language (DDDL)

IDD stores meta-data about all the data items in the database (The data values are not stored in IDD).

Stores information about users, application programs, files, record, data element, module (date routine, error handling routine) and application systems.

IDD is integrated with every software component provided in IDMS like ADSO, OLQ and Report Generator. Each software component accesses IDD to get information about data items and programs.INTEGRATED DATA DICTIONARY (IDD)

The data dictionary definition language (DDDL) is used to create, update or delete entity occurrences in IDD.

DDDL provides five verbs to manipulate entities:ADD to add a new entity.

MODIFY to change entity description.DATA DICTIONARY DEFINITION LANGUAGE (DDDL)

DELETE to remove an entity.

DISPLAY to display an entity description.

PUNCH to copy entity information from IDD to a file.Continue ..

ADD ELEMENT CITY-NAME ELEMENT DESCRIPTION TEAM CITY PICTURE X(20) USAGE DISPLAY.

DISPLAY PROGRAM STATSEDIT.

DELETE PROGRAM NAME IS STATSEDIT.

Examples of DDDL

MODIFY ELEMENT CITY-NAME PICTURE X(25). MODIFY PROGRAM NAME IS STATSEDITVERSION IS 1LANGUAGE IS COBOLESTIMATED LINES 5000.

SIGNOFF. ( to exit from IDD )Continue ..

Data Description Language (DDL)

Data Description Language (DDL) is used to define:

SCHEMASDBA describes the logical structure of the database by coding set of schema DDL statements. There is a single schema for a given database.

DEVICE-MEDIA CONTROL LANGUAGE (DMCL) MODULESMany aspects of the databases physical structure, such as disk device assignments and page sizes are described in DMCL module definition.

SUBSCHEMASThe views that individual applications programs have of the database are defined in subschemas and described with set of subschema DDL statements. Any number of subschemas can be defined for a given database.

The logical structure of a database is defined in a set of schema DDL statements.Schema DDL statements define the following:Schema nameDatabase file namesDatabase areasData elements and recordsSetsSCHEMA DDL STATEMENTS

The schema DDL is processed by the schema compiler. The schema compiler reads the schema DDL statements and stores a description of the schema in the data dictionary.The data dictionary is the central repository of information that IDMS maintains about the databases under its control.Process

SchemaDDL StatementsSchema CompilerData DictionaryThe schema compiler can also copy from the data dictionary descriptions of records that have been previously defined and stored in the data dictionary.Continue ..

ADD SCHEMA NAME IS DDSCHEMA VERSION 1

ADD FILE NAME IS CUST-FILE ASSIGN TO CUST.ADD FILE NAME IS ORDER-FILE ASSIGN TO ORDER. ADD AREA NAME IS CUST-AREA PAGE RANGE IS 505000 FOR 100 WITHIN FILE CUST-FILE.ADD AREA NAME IS ORDER-AREA PAGE RANGE IS 809000 FOR 100 WITHIN FILE ORDER-FILE.Example Of Schema DDL

ADD RECORD NAME IS CUST-REC RECORD ID IS 3000 LOCATION MODE IS CALC USING (CUST-NO) DUPLICATES ARE NOT ALLOWEDWITHIN AREA CUST-AREA.02 CUST-NO PICTURE IS 9(6) USAGE IS DISPLAY.02 CUST-NAME PICTURE IS X(40) USAGE IS DISPLAY.

ADD RECORD NAME IS ORDER-REC RECORD ID IS 3100 LOCATION MODE IS CALC USING (ORDER-NO) DUPLICATES ARE NOT ALLOWEDWITHIN AREA ORDER-AREA.02 ORDER-NO PICTURE IS 9(7) USAGE IS DISPLAY.02 ORDER-STATUS PICTURE IS X(1) USAGE IS DISPLAY.02 ORDER-REQ-DT PICTURE IS 9(8) USAGE IS DISPLAY.Continue ..

ADD SET NAME IS CUST-ORDERORDER IS NEXTMODE IS CHAIN LINKED TO PRIOROWNER IS CUST-RECMEMBER IS ORDER-REC LINKED TO OWNERMANDATORY AUTOMATIC.Continue ..

It completes the mapping of logical areas into physical files.It also identifies and describes the journal files that are used by IDMS for recording changes made to the database. Journal files are used to implement IDMS backup and recovery facilities.DMCL DDL STATEMENTS

DMCL DDL statements define the following:

DMCL module nameSchema name associated with this DMCL moduleSizes of the I/O buffersInformation about database areasInformation about journal files used for backup and recoveryContinue ..

The DMCL definition is processed by the DMCL compiler. The DMCL compiler reads the DMCL source statements and stores a description of the DMCL module in the data dictionary.As the DMCL compiler processes DMCL source, it must have access to the corresponding schema definition in the data dictionary. For this reason, schema must be compiled before its associated DMCL module.Process

The DMCL compiler also generates a set of assembler language source statements. These are in turn processed by assembler and the linkage editor to form an executable DMCL load module. The linkage editor places the load module into a load module library. The DMCL load module is used by IDMS at run-time when an application program accesses the database files described by the DMCL module.Continue ..

DMCL CompilerAssemblerLinkage EditorDMCL Source StatementsData DictionaryLoad LibraryContinue ..

DMCL DDL statements contain following four sections: Device-Media Description sectionThis sections gives name to the DMCL statements. It further provides other documentary details about the DMCL module. Buffer sectionIt assigns page size and defines buffers that IDMS will use to handle database accesses.Continue ..

Area sectionInformation about each area is copied from the schema into this section of the DMCL. The section assigns each area to buffers that were defined in the buffer section. Journal sectionThe section describes disk files to be used for journaling. Journal files are used to keep track of database activity.Continue ..

DEVICE-MEDIA DESCRIPTION.DEVICE-MEDIA NAME IS DMCLBASE OF SCHEMA NAME SCHBASE VERSION 1.DATE.DD/MM/YY.INSTALLATION.XANSA MUMBAIINDIA.BUFFER SECTION.BUFFER NAME IS XXBUFFPAGE CONTAINS 4096 CHARACTERSBUFFER CONTAINS 5 PAGES.JOURNAL BUFFER NAME IS JJBUFFPAGE CONTAINS 3188 CHARACTERSBUFFER CONTAINS 3 PAGES.Example Of DMCL DDL

AREA SECTION.COPY SALARY-AREA AREABUFFER IS XXBUFFJOURNAL SECTION.JOURNAL BUFFER IS JJBUFF.FILE CONTAINS 3000 BLOCKS.FILE NAME IS XANJRNL1ASSIGN TO SYSJRNL1.FILE NAME IS XANJRNL2ASSIGN TO SYSJRNL2.Continue ..

Subschema DDL statements specify the following:Subschema, Schema and DMCL module nameAreas accessible in this subschemaRecords and data elements accessible andSets included in the subschemaSUBSCHEMA DDL STATEMENTS

The subschema compiler reads subschema source statements and stores a description of the subschema in the data dictionary.The compiler also stores an executable load module in an area of the data dictionary, known as Load Area.IDMS uses this load module whenever an application program using this subschema is under execution. Process

Like DMCL compiler, subschema compiler must have access to the corresponding schema definition in the data dictionary. Thus, schema must have been compiled before any of its associated subschemas can be compiled.After compiling subschema, data dictionary has all the required information about the logical and physical structures of the database.Continue ..

Data DictionarySubschemaDDLSubschemaCompilerLoad AreaContinue ..

ADD SUBSCHEMA NAME SUBSCHA OF SCHEMA NAME SCHDBA VERSION 1 DMCL NAME SUBADMCL OF SCHEMA NAME SCHDBA VERSION 1 COMMENTS LIMITED VIEW OF SCHDBA

ADD AREA NAME IS CUST-AREA.ADD AREA NAME IS ORDER-AREA.ADD RECORD NAME CUST-RECELEMENTS ARE CUST-ID CUST-NAME.ADD RECORD NAME ORDER-RECELEMENTS ARE ALL.ADD SET NAME ORDER-ACTION.ADD SET NAME CUST-ORDER.GENERATE.Example Of Subschema DDL

Data Manipulation Language (DML)

In an IDMS application program, there are no file description statements or Open and Close statements found in a typical batch Cobol file processing application system.To access data stored in the database, DML verbs are used.Two new terms Run Unit and Currency are important to understand.

The BIND RUN-UNIT statement is used to specify the start of a run unit.A run unit ends when program completes all database processing and issues a FINISH statement.CONCEPT of RUN UNITThe run unit is that portion of programs processing during which it has access to one or more database areas and services.

The program may perform some processing before it begins a run unit. The program may perform some processing even after it finishes a run unit.A program may begin and end more than one run unit.Continue ..

IDMS internally stores pointers to the most recently accessed records. It uses these pointers to access subsequent records. These pointers are referred to as CURRENCY. At the beginning of a program, all currencies are null.CONCEPT of CURRENCY

Currency is established by DML verbs FIND, OBTAIN, STORE, CONNECT, DISCONNECT and ERASE.There are four currencies maintained for a database:current of run unitcurrent of record typecurrent of setcurrent of areaContinue ..

Current of Run Unit is a record which was most recently accessed.Current of Record type is the most recently retrieved or inserted record of that record type.Current of Set is the most recently retrieved or stored record of that set. Record may be a member or owner of that set. Continue ..

Current of Area is the most recently retrieved or stored record of that area.

When a record is erased, currency of its record type and its sets is set to null. Run unit and area currencies are unchanged.Continue ..

DML ProcessorCompilerLinkage EditorSource StatementsData DictionaryLoad LibraryIDMS APPLICATION PROGRAM PREPARATION

The previous slide shows how an IDMS application program written in Cobol host language is compiled and made ready for execution.Application program with embedded DML statements is first read by DML processor. It validates DML statements and creates translated version of the source program where in DML statements are replaced by appropriate calls to IDMS.

DML processors are available for a number of commonly used programming languages. DML processor may also generate a listing of errors. DML processor interacts with data dictionary for copying descriptions of various database elements like schema, subschema, record types etc.Output from the DML processor acts as input to the host programming language compiler like a Cobol compiler. Continue ..

After the translated source program is compiled by host language compiler, the resulting object module is processed by Linkage editor to create an executable load module. The Linkage editor also includes copy of a routine called IDMS Interface Module in final load module. IDMS service requests from the program are converted in to calls to IDMS Interface Module.Continue ..

Following are typical changes one notices in a n IDMS Cobol application program.IDENTIFICATION DIVISION remains exactly the same as before.ENVIRONMENT DIVISION has IDMS-CONTROL section.Continue ..

DATA DIVISION begins with SCHEMA SECTION which gives the subschema used by the program.PROCEDURE DIVISION has DML verbs for data retrieval and data updation. Continue ..

..ENVIRONMENT DIVISION.IDMS-CONTROL SECTION.PROTOCOL.MODE IS BATCH DEBUG.IDMS-RECORDS WITHIN WORKING-STORAGE SECTION.CONFIGURATION SECTION... INPUT-OUTPUT SECTION.FILE CONTROL...IDMS-CONTROL SECTION

The PROTOCOL statement is a compiler-directive statement that specifies the manner in which DML processor will generate CALL statements. Application program must specify one of the following modes to access database:Continue ..

BATCH (a batch program with database processing)IDMS-DC (a non-batch program i.e. an online program called by a dialog process)DC-BATCH (a batch program with access to DC queues)Continue ..

DEBUG instructs DML processor to produce additional code in the resulting source code which is helpful during program debugging.The IDMS-RECORDS clause specifies that the data definitions for records and data elements will be copied automatically from the data dictionary and placed at the end of working storage section.Continue ..

..DATA DIVISION.SCHEMA SECTION.DB SUBEMP WITHIN SCHEMP.FILE SECTION. .. WORKING-STORAGE SECTION...DATA DIVISION

As explained earlier, DATA DIVISION begins with SCHEMA SECTION which gives the subschema used by the program.As shown in previous slide, SUBEMP is the name of the subschema which will be used by the program. Further, the SCHEMP is the schema from which SUBEMP subschema is derived.Continue ..

BIND RUN UNIT.BIND EMPLOYEE.BIND MANAGER.BIND PROJECT.BIND SALARY.BIND STATEMENTAbove example displays the typical code that might be found in the Procedure Division of an IDMS Cobol program.

Continue ..The first statement BIND RUN-UNIT establishes addressability for the IDMS communications block and causes IDMS to load the subschema identified in the Schema section. Next four BIND statements establish addressability for the working-storage areas that will be used to contain record occurrences.

Continue ..Generally, following one statement is found in the source code.COPY IDMS SUBSCHEMA-BINDS.When the source is pre-compiled, the above single statement is replaced by previous set of five Bind statements.BIND statement identifies the record types that the program will access.

COPY IDMS SUBSCHEMA-BINDS. COPY IDMS IDMS-STATUS.COPY STATEMENTCopy statement is a compiler directive statement.In first example above, the copy statement generates appropriate Bind statements at the time of pre-compile.

Continue ..As in second example, the copy statement copies module called IDMS-STATUS from data dictionary into the source code when it is pre-compiled. This particular module (routine) checks result of DML statement.This helps in reducing coding effort.

READY EMPLOYEE-AREA USAGE IS RETRIEVAL. READY SALARY-AREA USAGE EXCLUSIVE UPDATE. READY USAGE IS RETRIEVAL.READY STATEMENTReady statement can be thought as an equivalent to Open statement for a conventional file. It gives the program access to all or named areas defined in the subschema and begins a run-unit.

Continue ..The usage-mode clause in the Ready statement indicates whether the program will only retrieve data or update it as well.If no areas are mentioned in the Ready statement, then all areas mentioned in the subschema will be made available to the program by default in the mode specified in the Ready statement.

FINISH.FINISH STATEMENTFinish statement is used to end a run unit by releasing database resources and terminating database processing.Non-database processing of the program may continue after the Finish statement, but database can not be accessed (unless a fresh run unit is started).

MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.FIND CALC EMPLOYEE.FIND STATEMENTFind statement is used to locate a record occurrence in the database. The record occurrence is placed in the system buffers but the data element values are still not available in the variable storage of the program.

Continue ..There are situations when we do not need actual values of a record occurrence like:We may need to verify that a particular record occurrence existsWe need to establish currencies for some subsequent record retrievalIn the example, the required employee information is moved to the CALC-key data element of Employee.

Continue ..Next, Find statement will locate the record occurrence with matching employee-code.If the Find is successful, currencies are set appropriately but the individual data element values for the located record are not yet made available to the program. In case, data element values are required then Find can be followed by Get statement.

MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.FIND CALC EMPLOYEE.GET.GET STATEMENTGet statement is used to make available to the program a record occurrence that was previously located by the Find statement. Get statement can qualify the record type or it can be unqualified as in this case.

Continue ..To continue with our earlier Find example, we have followed it with unqualified Get. This will transfer record occurrence values to the variable storage.If Get is qualified then IDMS will verify whether the previously located record is Employee record or not. If it does not match then IDMS will return error code.

The Obtain statement is equivalent of a Find followed by a Get.It is most widely used statement for data retrieval in IDMS.There are many syntax variations available for Obtain statement, based on location mode of records and mode of retrieval.OBTAIN STATEMENT

CALC-key RetrievalTo perform Calc-key retrieval, the record must have location mode specified as CALC and we must know the Calc-key value for the record to be retrieved.Continue ..MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.OBTAIN CALC EMPLOYEE.

As shown in previous example, we move the Calc-key value for the record to be retrieved to the data element of Employee record type that is defined as Calc-key (Employee-Code) and issue Obtain statement to get the record contents.IDMS uses the Calc-key value supplied to calculate the target page for the data retrieval.Continue ..

Duplicate CALC-key RetrievalSometimes the record type is defined with duplicate allowed option. We can try and find out if duplicate record exists for a given Calc-key value.Continue ..MOVE INPUT-EMP-NAME TO EMPLOYEE-NAME.OBTAIN CALC EMPLOYEE.OBTAIN DUPLICATE EMPLOYEE.

As shown in previous example, we obtain the first record with matching Calc-key value in normal way.The second Obtain statement tries to obtain duplicate record with same Calc-key value. If it exists then IDMS fetches the record, else an error code is returned.Continue ..

Member Record RetrievalMany times, we need to retrieve owner record for a set occurrence and then proceed to retrieve one or more of its member records.Following are some of the examples of such typical member retrieval criteria.Continue ..

Continue ..OBTAIN FIRST EMPLOYEE WITHIN DEPT-EMP.

OBTAIN LAST EMPLOYEE WITHIN DEPT-EMP.

OBTAIN NEXT EMPLOYEE WITHIN DEPT-EMP.

OBTAIN PRIOR EMPLOYEE WITHIN DEPT-EMP.

OBTAIN OWNER WITH DEPT-EMP.

OBTAIN 5 WITHIN DEPT-EMP.

By first retrieving the owner record, we establish the currency for the subsequent member retrievals. Owner becomes current of run unit, record type and set type. Now to access members within the set becomes relatively simple task.Obtain First gets the first member record within the set.Continue ..

Obtain Last gets the last member record within the set. For this, the set must have been defined with prior linkage option.Next option is known as Walking the Set. Obtain Next gets each member in sequence. This statement is executed in a loop generally till end-of-set is reached.Continue ..

Obtain Prior gets the prior member record within the set. For this, the set must have been defined with prior linkage option. This works in same manner as Next option, except that it is in the reverse direction.Continue ..

We can obtain owner of the set by Obtain Owner statement. If set is defined with Owner linkage pointer then owner is obtained directly. In case such pointer is absent, IDMS walks through the entire set and obtains the set owner.Continue ..

We can obtain specific member record within the set by its relative position with respect to owner in the next direction.Obtain 5 will fetch the fifth member record in the set. Continue ..

Current Record RetrievalMany times, we need to retrieve records based directly on various currency.Continue ..OBTAIN CURRENT EMPLOYEE.OBTAIN CURRENT WITHIN DEPT-EMP.OBTAIN CURRENT WITHIN EMPLOYEE-AREA.

In first case, IDMS obtains the record that is current of the Employee record type.In second case, current of the Dept-Emp set type is fetched. Depending on currency established, either Dept record or an employee record is obtained.Continue ..

In last case, current of employee-area is obtained. e.g. if employee-area contains employee, project, salary record types then depending on currency, the statement will fetch record occurrence of any one of these record types.Continue ..

Area Sweep RetrievalSometimes, we need to retrieve records by doing an area sweep. IDMS scans through the area in physical sequence. Area sweep is used when we need to access all records within an area, irrespective of the record types and/or their sequence. Continue ..

Continue ..OBTAIN FIRST WITHIN EMPLOYEE-AREA.OBTAIN NEXT WITHIN EMPLOYEE-AREA.OBTAIN LAST WITHIN EMPLOYEE-AREA.In first case, the record having lowest db-key value within the employee-area will be fetched. In second case, record having next highest db-key value will be obtained. The sweep in reverse direction as in last case can also be made.

Continue ..OBTAIN FIRST EMPLOYEE WITHIN EMPLOYEE-AREA.OBTAIN LAST EMPLOYEE WITHIN EMPLOYEE-AREA.As shown above, the record type can also be qualified during the area sweep. Only record type mentioned in the statement (employee) are obtained during the sweep. Occurrences of other record types are ignored.

Sort-Key RetrievalWe can retrieve records based on sort-key values, provided the set is defined as Sorted order option.In our earlier example of American baseball league database, suppose that set Player-Position is defined as Sorted.Continue ..

The sort-key data element is Position-name of Position record.Then we can retrieve Position record directly based on sort-key value as below.Continue ...OBTAIN CALC PLAYER-REC.MOVE INPUT-POSITION-NAME TO POSITION-NAME.OBTAIN POSITION-REC WITHIN PLAYER-POSITION USING POSITION-NAME.

In the example, a position-name value is moved to sort-key data element and then Obtain statement is issued using reference to the sort-key element.The retrieval appears similar to CALC, but in this case the search is made only within the current set occurrence and not within entire record type.Continue ..

DB-Key Value RetrievalIf we know the dB-key value of a record then we can directly obtain the record. This is useful in cases where we know that a particular record would be processed again later on during the execution of the program.Continue ..

When a record is successfully retrieved or located, its dB-key value is available in the IDMS Communication Block. We can store this value in a variable storage area for later use. Later on, we can use the stored dB-key value to retrieve the record and process it.Continue ..

..MOVE INPUT-PLAYER-NAME TO PLAYER-NAME.OBTAIN CALC PLAYER-REC...MOVE DBKEY TO SAVE-DBKEY...OBTAIN DB-KEY IS SAVE-DBKEY.Continue ..

Continue ..Note that the dB-key of a record may undergo change whenever a database is reorganized. Hence it is not always advisable to save dB-key values for a long time with an intention of using them later on.

Index Set RetrievalUsing index sets, following types of retrieval is made possible which is otherwise very difficult. Generic key retrieval. Random and sequential retrieval based on a key other than Calc-key.Continue ..

Suppose in our baseball example, Player record has a system indexed set IX-PLAYER-NAME on Player-Name element, then we can retrieve a player record by supplying a partial key to IDMS. Such retrieval is not possible unless we have index set. In Calc, we have to supply complete key value.Continue ..

01WS-PLAYER-NAME.05 FIRST-INITIALPIC X.05 FILLERPIC X(29)...MOVE LOW-VALUES TO WS-PLAYER-NAME.MOVE WS-INITIAL-LETTER TO FIRST-INITIAL.MOVE WS-PLAYER-NAME TO PLAYER-NAME.OBTAIN PLAYER-REC WITHIN IX-PLAYER-NAME USING PLAYER-NAME...OBTAIN NEXT PLAYER-REC WITHIN IX-PLAYER-NAME.Continue ..

Say we want to retrieve all players starting with C.We move C to ws-initial-letter and rest is filled with low values. This is called Partial key value.Then the program will obtain first player starting with name C.Continue ..

First obtain statement is random retrieval using index set. Whereas the second obtain statement is the example of the sequential retrieval using index set.Program has to check name field after every retrieval to determine if it has reached the last record with specific name.Continue ..

The Modify statement is used to change data element values in an existing record occurrence.

Important considerations for a record to be successfully modified are:Ready the concerned area(s) with one of the update options.MODIFY STATEMENT

Making the record to be modified as current of run unit. This is done by retrieving the record before modifying it, by issuing Obtain.Continue ..

MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.OBTAIN CALC EMPLOYEE.IF DB-STATUS-OKMOVE NEW-EMP-ADDRESSTO EMPLOYEE-ADDRESSMODIFY EMPLOYEEPERFORM IDMS-STATUS.Continue ..Above example shows how an existing Employee record is modified.

The Store statement is used to add new record occurrence to the database.Adding a new record involves:Constructing new record occurrence in the records area in variable storage.STORE STATEMENT

Establish currency with correct set occurrences in which the new record participates as automatic member. FIND is used to set currencies.Issue Store statement specifying the name of the record type we are adding.Continue ..

Syntax for Store is :

STORE EMPLOYEE.

While adding a new record, following information is analyzed.Continue ..

Location where new record is added.If location mode is CALC, then based on Calc-key value, storage page is calculated. For VIA, optimum location is found based on the location of owner record. If its DIRECT, then program specifies the page number through db-key. Continue ..

Where in the set occurrences the new record should be inserted.Before issuing Store, set order option is also examined. Depending on order option like first, last, next etc, the new record is inserted in appropriate position within the set.Continue ..

The Erase statement is used to delete a record occurrence from the database. Erase statement often affects not only the record that is being deleted but, in some cases, also members of sets owned by that record.ERASE STATEMENT

There are four options available for the Erase statement. ERASE ERASE ALL MEMBERS ERASE PERMANENT MEMBERS ERASE SELECTIVE MEMBERSContinue ..

ERASEWe first begin by retrieving the record we want to delete and thus the record is made current of run unit. Then Erase is issued naming the record type.The record is first disconnected from all sets in which it participates as a member. Continue ..

Then the record occurrence is removed (erased) from the database.If the record is the owner of any sets, then the record will be erased ONLY IF those sets are empty. If we try to erase an owner record of a non-empty set, IDMS returns an error status code.Continue ..

MOVE INPUT-EMP-CODE TO EMPLOYEE-CODE.OBTAIN CALC EMPLOYEE.IF DB-STATUS-OKERASE EMPLOYEEPERFORM IDMS-STATUS.Continue ..Above example shows how an existing Employee record is erased from the database with unqualified Erase statement.

ERASE ALL MEMBERSFirst the record occurrence is removed from the database.Then all its members, both mandatory and optional, are also removed from the database.Continue ..

ERASE PERMANENT MEMBERSFirst the record occurrence is erased from the database.Then all mandatory members in all sets owned by the record are also removed. Optional members are NOT removed but they are disconnected from their respective set occurrences. Continue ..

Lastly, if the erased member is the owner of any sets then it is treated as if an Erase Permanent is issued for it. Its mandatory members are removed and optional members are disconnected. This process continues until all members have been processed. Continue ..

ERASE SELECTIVE MEMBERSFirst the record occurrence is erased from the database.Then all mandatory members in all sets owned by the record are also removed. Optional members are removed, provided they do not participate in any other sets.Continue ..

Lastly, if the erased member is the owner of any sets then it is treated as if an Erase Selective is issued for it. Its mandatory members are removed and optional members are removed if they do not participate in any other sets.Continue ..

Connect statement is used to link a record occurrence into a set.Here, we name both the record type and the set type.The statement causes the current of record type to be connected to the set occurrence that is current of set type.CONNECT STATEMENTCONNECT EMPLOYEE TO DEPT-EMP.

In case of set defined with Automatic member option, the member record gets connected automatically at the time of addition to the database.With set defined with Manual member option, Connect statement is used to manually set up link with a set occurrence.Continue ..

Disconnect statement cancels the membership of a record in a set occurrence in which it currently participates as a member.The record that is being disconnected, must be defined as an Optional member of that set. If not then IDMS returns an error code.DISCONNECT STATEMENTDISCONNECT EMPLOYEE FROM DEPT-EMP.

QUEUE RECORDSWe write temporary data to a queue record in data dictionary and make this data available to other IDMS COBOL programs (DC-BATCH Mode).Queue records are relatively permanent records in that they are not deleted when the system is shutdown or when the system crashes. They are deleted either explicitly by programmer or after the retention period specified for the queue is over.

Continue ..As the data remains permanent, it is global in a sense. Thus, queues are used to pass data information from one application to another. Specifically this feature is very widely used in online applications using ADSO.

QUEUE MANAGEMENT COMMANDSCommands to write a record to a queue, retrieve a queue record and delete a queue record are:

PUT QUEUE ID queue-id GET QUEUE ID queue-id DELETE QUEUE ID queue-id

Continue ..PUT QUEUE ID queue-idThis command is used to store a queue record in the data dictionary in the queue identified by the queue-id. An added parameter of Last or First can be mentioned to instruct whether the new queue record will be placed at the beginning or end of the queue. Default is Last.

Continue ..GET QUEUE ID queue-idThis command is used to transfer the contents of a queue record to a specified location in the variable storage. Added parameter is Delete or Keep. Delete will remove the record from the queue after retrieval. Keep will retain the record in the queue.Default is Delete.

Continue ..DELETE QUEUE ID queue-idThis command is used to delete one or all queue records in a specified queue. Added parameter is Current or All. Current will delete only the current record of the queue. All will delete all queue records, including the header record. Default is Current.

MAJOR CODES OF ERROR-STATUS01:FINISH02:ERASE03:FIND/OBTAIN05:GET06:KEEP07:CONNECT08:MODIFY09:READY

Continue ..11:DISCONNECT12:STORE14:BIND18:COMMIT19:ROLLBACK

Recovery and Restart of Database

IDMS provides facilities for recovering from problem situations and for restarting after failures occur.Journal files are used for the recovery and restart facilities.These facilities are dependant on the two operating environments in which an IDMS run unit executes.

IDMS application program can be run in two operating environments: Local Mode and Central Version.All online applications that use IDMS/DC or some other telecommunication facilities must run under central version.An application program that operates in batch mode can be run either under the control of central version or in local mode.IDMS OPERATING ENVIRONMENTS

In central version, a single copy of the IDMS DBMS controls the operation of a particular set of IDMS application program run units.No two central version are allowed to update the same database area. All run units that execute under the control of a particular central version are chosen such that they all access the same group of database areas.Central Version

Multiple copies of IDMS central version can execute in same region with each copy accessing different set of database areas.Central version implements facilities for recovering from system failures. It is the preferred method of running an IDMS application.Continue ..

A batch program that operates in local mode has its own copy of IDMS DBMS loaded into its partition, region or address space.IDMS provides no automatic restart and recovery facilities for applications running in local mode.Batch retrieval programs are good candidates for running in local mode.An application runs faster in local mode.Local Mode

IDMS maintains set of journal files that are used to recover from failures and to restart aborted run units.IDMS provides automatic restart and recovery facilities for applications running under central version AND for run units those use journal files. Journal files can be put on disk or tapes.JOURNAL FILES

CHECKPOINTINGWhen a run unit begins by issuing BIND run-unit statement, IDMS system writes Begin checkpoint record on the journal file. This contains information about current status of the run-unit.As run-unit executes, it may update records in database. As each update request is processed, IDMS writes Before Image and After Image of the updated record to the journal file.

Continue ..When applications ends the run-unit by issuing FINISH statement, IDMS writes End checkpoint record on to the journal file.Thus, at the end of execution of a run-unit the journal files contains:Begin checkpointBefore and After images of all updated records andEnd checkpoint

RECOVERY and RESTARTFollowing are typical scenarios when run unit is terminated abnormally.Program error may cause the run unit to terminate abnormally.IDMS system may encounter situations that require it to terminate one or more run units abnormally. e.g. Wait time limit has been exceeded to avoid deadlock.

Continue ..In situations like these, when a run unit is terminated abnormally after it has updated one or more database records; IDMS works backwards in the journal file until it reaches the Begin checkpoint record and uses the information in the Before images to reverse effect of database updates already made by the aborted run unit. The run unit can then be restarted from the beginning.

RECOVERY UNITAll run units that terminate normally, create at least two checkpoint records on the journal file: BEGIN CHECKPOINT when Ready/Bind statement is executed and END CHECKPOINT when FINISH statement is executed. Before and After images that are recorded between these two checkpoints constitute a record of all updating the run unit performed.

Continue ..The processing that a run unit performs between checkpoints is called Recovery Unit.In the simplest form, the entire run unit constitutes a single recovery unit.

INTERMEDIATE CHECKPOINTSIn real life scenario (for online applications), a separate run unit is started for each incoming transaction. Very few database updates are made during each run unit. Each run unit is a single recovery unit. At times, it is required to divide the run unit into many smaller run units to reduce the time for recovery in case of run unit failure.

Continue ..This leads us to the concept of Intermediate checkpoints.To create intermediate checkpoints, run unit periodically issues a COMMIT statement to make permanent all database updates that has already been made.

THE COMMIT VERBCommit verb is used to write Intermediate checkpoints on the journal file.There are two forms of Commit verb:COMMIT andCOMMIT ALL

Continue ..In COMMIT, IDMS releases all locks except those placed on current records.In COMMIT ALL, IDMS releases all locks including those on current records.In case of abnormal termination of run unit, IDMS works backwards in the journal file and reads till the most recent COMMIT checkpoint is encountered.

Continue ..Now the journal file is not read till Begin checkpoint. The run unit can now be restarted from the point at which it wrote the most recent Commit checkpoint.A simple thumb rule of 500 or 1000 database updates is used to issue a Commit statement.

THE ROLLBACK VERBSometimes an application program may itself decide to back out all the updates it made to the database since it wrote the last intermediate (commit) checkpoint.To back out the changes, we can issue a ROLLBACK statement.It has two forms:ROLLBACK andROLLBACK CONTINUE

ROLLBACKIt writes a checkpoint record to the journal file, nullifies all currencies, automatically restore the database using the before images from the journal and terminate the run unit.After issuing the unqualified Rollback, to access the database again, we must reissue the Bind.

ROLLBACK CONTINUEThis form causes IDMS to restore the database but does not terminate the run unit. One can access the database immediately without reissuing the Bind.

Locking Facilities

Locking facilities are provided for controlling access to areas and to individual records when two or more run units require access to the same database areas.IDMS provides three levels of locking facilities.Area LocksArea Usage ModesRecord Locks

AREA LOCKSArea locks are used to prevent areas from being updated by more than one IDMS central version or by more than one application running in local mode.If an area is locked that is required by a local mode application, that local mode application ABENDS.

Continue ..If required area is NOT locked, area lock is set by the local mode application and no other local mode application is given access to that area.An area locked by a local mode application is released when that application executes its FINISH statement (end of run-unit).

Continue ..If required area is locked that is required by an IDMS central version, execution of that central version continues, but no run-unit executing under the control of that central version will be given access to the already locked area. The area is said to be Varied Offline to that central version. Run-units that attempt to access an area that is varied offline are abnormally terminated.

Continue ..If required areas are NOT locked, then those areas are locked by the central version. Run-units executing under the control of that central version can access those areas. No other central version will be given access to locked areas.An area that is locked by a central version is released only when the central version is shut down.

AREA USAGE MODESArea usage modes control how database areas under the control of a central version can be accessed.The six possible usage modes for a database area are as follows:

Continue ..(Shared) RetrievalProtected RetrievalExclusive Retrieval(Shared) UpdateProtected UpdateExclusive Update

The Shared word is optional.

Retrieval or Shared RetrievalWe are requesting only retrieval access to the area. All other run units under the same central version will be allowed to RETRIEVE or UPDATE records in the same area, provided they do no specify usage mode Exclusive.

Protected RetrievalWe are requesting retrieval access to the area and it also specifies that no other run unit should update records in the same area for the duration of our run unit. All other run units under the same central version will be allowed to retrieve records in the same area provided they did not specify usage mode Exclusive.If another run unit is already updating records in the area, our run unit will wait till that run unit finishes.

Exclusive RetrievalWe are requesting retrieval access to the area and it also specifies that no other run unit should retrieve or update records in the same area till our run unit finishes execution. If another run unit is already accessing the area, our run unit will wait till that run unit finishes.This mode is not often used.

Update or Shared UpdateWe are requesting updating access to the area. All other run units under the same central version will be allowed to retrieve or update records in the same area as long as they do not specify Protected or Exclusive usage mode options in Ready statements.

Protected UpdateWe are requesting update access to the area and specifying that other run units should not be able to update records in the same area till our run unit finishes. All other run units will be allowed to retrieve records in the same area as long as they do not specify Protected or Exclusive as usage modes.If another run unit is already accessing the area, our run unit will wait till that run unit finishes.

Exclusive UpdateWe are requesting update access to the area and further specifying that no other run unit should either retrieve or update records in the same area while our run unit is executing. This option gives our run unit exclusive use of the area.

Sheet1

Run Unit 2

Shared UpdateShared RetrievalProtected UpdateProtected RetrievalExclusive UpdateExclusive Retrieval

Run Unit 1Shared UpdateYYNNNN

Shared RetrievalYYYYNN

Protected UpdateNYNNNN

Protected RetrievalNYNYNN

Exclusive UpdateNNNNNN

Exclusive RetrievalNNNNNN

&CArea Usage ModesOptions for Concurrent Run Units

Sheet2

Sheet3

IMPLICIT RECORD LOCKSIt is maintained only for run-units operating under Central Version. When we use usage mode options that allow more than one run-unit to access the same area while updating is taking place, IDMS sets Implicit record locks to prevent same record from being updated simultaneously by two or more run-units.

Continue ..Record locks are generally maintained for shared and protected update mode and shared retrieval mode. Implicit locks can be either Shared or Exclusive.Implicit Shared lock guarantees that only one run-unit is allowed to update a record while others can retrieve the same record.Implicit Exclusive lock ensures that no other run-unit can either update or retrieve the record.

Implicit shared locks is placed on a record when it is retrieved and Implicit Exclusive lock is placed on a record when it is accessed through a DML update verb.Shared Lock remains in effect till currency changes.Exclusive Lock remains in effect till run-unit ends (Finish) or until Commit is issued.Continue ..

Explicit locking is the most efficient way of placing record locks. It can be set by coding the KEEP statement or by the KEEP clause of the FIND/OBTAIN statement.Explicit lock remain is effect until Finish is executed or Commit is issued.e.g.Obtain Keep Calc Employee.Obtain Keep Exclusive Calc Employee.First is explicit shared lock and second is explicit exclusive lock.EXPLICIT RECORD LOCKS

Another way of setting explicit locks is to code separate Keep statement after a record is retrieved.e.g.Keep exclusive current.Keep current within Dept-Employee.Keep exclusive current within employee-area.Continue ..