Familiarize yourself with terms that have special meanings
in IMS™.
- Batch message processing programs (BMPs)
- Programs that run as batch jobs but access databases that they
share with online transactions. BMPs can access message queues like
MPPs for batch processing and can also access operating system files.
You start BMPs using JCL. There are two types of BMPs:
- Transaction-oriented BMP
- A BMP that accesses the message queue for input. It can also access
databases and operating system files. Output can be sent to databases,
operating system files, or a message queue. Only one input file can
be associated with the message queue.
- Batch-oriented BMP
- A BMP that does not access a message queue for input. Databases
and operating system files are available for input and output processing,
and message queues are available for output.
- Batch programs
- Programs that can access private databases and operating system
files directly. Batch jobs do not access message queues or databases
shared with an online system. In IMS documentation,
these are referred to as DL/I batch jobs.
- BTS
- Batch Terminal Simulator. This is an IBM® product
that enables you to run IMS database
and data communication programs in a z/OS® or
batch environment. BTS provides a comprehensive way to check program
logic, IMS program interfaces,
teleprocessing activity, 3270 format control blocks, and database
activity.
- DBD
- The database definition of DL/I, fast path, or GSAM databases.
You specify segment content, hierarchy, and physical characteristics
such as organization and access method, in the DBD.
- Fast path
- A type of IMS processing
that involves expedited handling for certain transactions and supports
special databases designed for large volumes of data with a high availability
rate. There are two types of fast path databases:
- DEDB
- Data Entry Database. This is a type of IMS fast path database that contains large volumes
of data with a high rate of availability. Subset pointers help manage
long chains of segment occurrences. One segment type is stored near
the root segment and the occurrences are in chronological order.
- MSDB
- Main Storage Database. A type of IMS fast
path database that uses fixed-length root segments that reside in
virtual storage for quick access. The segments can be related to a
specific terminal or be defined so all terminals can access the data.
- GSAM
- Generalized sequential access method- GSAM enables MVS™ batch programs and BMPs to access a sequential
OS/VS data set as a database. The database is a root-only database,
and the entire root segment represents a record. Unlike sequential
OS/VS data sets, you can checkpoint and restart GSAM files just like
DL/I databases.
- Message processing programs (MPPs)
- IMS programs that process
requests from terminals and from other programs. The requests or messages
are stored in message queues accessible to the MPP. The MPP can perform
required database access and write new messages to output queues for
further terminal and program processing. MPPs cannot access GSAM files.
There are two types of MPPs:
- Conversational MPP
- A type of MPP that saves data in a scratchpad area (SPA) during
user think time, even though locks on the database are lost. IMS conversational mode is similar
to CICS® pseudoconversational
mode. IMS has no capability that
is similar to CICS conversational
mode.
- Nonconversational MPP
- A type of MPP that can process a single input message with, at
most, a single response. No data can be saved during user think time
(except for data saved on the screen or in a database).
- Message Queue
- A location in IMS where information
being sent to an alternate terminal or to another transaction can
be stored so IMS can handle the
I/O to the terminal or schedule the other transaction to start processing.
It is accessed through DL/I calls.
- MFS
- Message Format Services. Use this editing facility with IMS so that programs can access message
data from a terminal. MFS enables you to customize the presentation
of the data, but shields the program from panel formats and device
dependencies by providing access to only the data required from the
terminal.
- PCB
- Program Communication Block. In IMS,
this collection of information is related to an IMS resource that a program can use. IMS uses this control block to determine the
resource being used and to return the results of an I/O operation
against the resource to the program. IMS uses
the following types of PCBs:
- I/O
- Represents an IMS logical
terminal or a message from another program. A program uses DL/I calls
for this PCB to read input from a terminal or program and write output
messages back to that same terminal.
- Alternate
- Represents the message queue for an IMS logical
terminal or an alternate transaction. It differs from an I/O PCB because
the alternate PCB can represent logical terminals other than the terminal
from where the input message came. An alternate PCB is also known
as a teleprocessing (TP) PCB.
- Express®
- An alternate PCB that sends a completed message immediately to
its destination. A non-express alternate PCB does not send a completed
message to its destination until a commit point. When this process
is not successful, complete messages sent to an express PCB cannot
be backed out. A message is complete when a PURG call is issued. You
can force a PURG call by using the EGL close statement.
An express PCB is also known as a teleprocessing (TP) PCB.
- DB
- Represents a DL/I database that a program can access. In addition,
the DB PCB specifies the data that the program can access, the segment
or field level, and the type of processing valid with that database.
- GSAM
- Represents a GSAM file that a program can access and contains
the processing option available for the program.
- PSB
- A Program Specification Block. In IMS,
a PSB is a set of statements that define the PCBs a program can use.
The database PCBs identify the required databases, segments to be
accessed, and database options for a given program. Alternate PCBs
define message queues for terminals or other programs.
- Program switch
- A way of transferring control from one program to another. There
are two types of program switches:
- Deferred program switch
- Occurs when Program A responds to the terminal and informs IMS to start another transaction
that is associated with Program B on the next input from the terminal.
- For a conversational MPP, the program modifies the SPA to specify
the new transaction name before sending that name back to IMS through the I/O PCB.
- For a nonconversational MPP, the program includes the next transaction
name on the form so it is the first 8 bytes of the input message.
- Immediate program switch
- Occurs when Program A passes control directly to another transaction
that is associated with Program B without responding to the originating
terminal first.
- For a conversational MPP, the program inserts the SPA to an alternate
PCB that has its destination set to the new transaction name.
- For a nonconversational MPP, the program inserts a message to
an alternate PCB that has its destination set to the new transaction
name.
- SPA
- The scratchpad area for IMS.
The SPA is used for conversational processing to save data while the
map is displayed during user think time.
- Work database
- A database that saves information about a running program during
user think time. Except for defining a PCB for the work database or
including it in your DB2® plan,
you do not need to do any special processing for a work database. Rational® COBOL Runtime manages
and uses the work database for you. The work database can be a DL/I
or a DB2 database.