<s>
Distributed	B-Operating_System
Data	I-Operating_System
Management	I-Operating_System
Architecture	I-Operating_System
(	O
DDM	O
)	O
is	O
IBM	O
's	O
open	O
,	O
published	O
software	B-Architecture
architecture	I-Architecture
for	O
creating	O
,	O
managing	O
and	O
accessing	O
data	O
on	O
a	O
remote	O
computer	O
.	O
</s>
<s>
DDM	O
was	O
initially	O
designed	O
to	O
support	O
record-oriented	B-Application
files	I-Application
;	O
it	O
was	O
extended	O
to	O
support	O
hierarchical	B-Application
directories	I-Application
,	O
stream-oriented	O
files	O
,	O
queues	O
,	O
and	O
system	O
command	O
processing	O
;	O
it	O
was	O
further	O
extended	O
to	O
be	O
the	O
base	O
of	O
IBM	O
's	O
Distributed	B-Language
Relational	I-Language
Database	I-Language
Architecture	I-Language
(	O
DRDA	B-Language
)	O
;	O
and	O
finally	O
,	O
it	O
was	O
extended	O
to	O
support	O
data	O
description	O
and	O
conversion	O
.	O
</s>
<s>
As	O
an	O
open	B-Architecture
architecture	I-Architecture
,	O
products	O
can	O
implement	O
subsets	O
of	O
DDM	B-Operating_System
architecture	I-Operating_System
and	O
products	O
can	O
extend	O
DDM	O
to	O
meet	O
additional	O
requirements	O
.	O
</s>
<s>
Taken	O
together	O
,	O
DDM	O
products	O
implement	O
a	O
distributed	B-Application
file	I-Application
system	I-Application
.	O
</s>
<s>
There	O
are	O
three	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
models	I-Operating_System
for	O
the	O
design	O
of	O
distributed	O
applications	O
:	O
</s>
<s>
File	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
FTP	O
)	O
copies	O
or	O
moves	O
whole	O
files	O
or	O
database	O
tables	O
to	O
each	O
client	O
so	O
they	O
can	O
be	O
operated	O
on	O
locally	O
.	O
</s>
<s>
Thin	B-Device
client	I-Device
applications	O
present	O
the	O
interface	O
of	O
an	O
application	O
to	O
users	O
while	O
the	O
computational	O
parts	O
of	O
the	O
application	O
are	O
centralized	O
with	O
the	O
affected	O
files	O
or	O
databases	O
.	O
</s>
<s>
Communication	O
then	O
consists	O
of	O
remote	B-Operating_System
procedure	I-Operating_System
calls	I-Operating_System
between	O
the	O
thin	B-Device
clients	I-Device
and	O
a	O
server	O
in	O
which	O
uniquely	O
designed	O
messages	O
specify	O
a	O
procedure	O
to	O
be	O
called	O
,	O
its	O
associated	O
parameters	O
,	O
and	O
any	O
returned	O
values	O
.	O
</s>
<s>
Fat	B-Protocol
client	I-Protocol
applications	O
perform	O
all	O
application	O
processing	O
tasks	O
on	O
client	O
systems	O
,	O
but	O
data	O
is	O
centralized	O
in	O
a	O
server	O
so	O
that	O
it	O
can	O
be	O
managed	O
,	O
so	O
that	O
it	O
can	O
be	O
accessed	O
by	O
any	O
authorized	O
client	O
application	O
,	O
so	O
that	O
all	O
client	O
applications	O
work	O
with	O
up-to-date	O
data	O
,	O
and	O
so	O
that	O
only	O
the	O
records	O
,	O
stream	O
sections	O
,	O
or	O
database	O
tables	O
affected	O
by	O
an	O
application	O
are	O
transmitted	O
.	O
</s>
<s>
The	O
DDM	B-Operating_System
architecture	I-Operating_System
was	O
initially	O
designed	O
to	O
support	O
the	O
fat	B-Protocol
client	I-Protocol
model	O
of	O
distributed	O
applications	O
;	O
it	O
also	O
supports	O
whole-file	O
transfers	O
.	O
</s>
<s>
The	O
DDM	B-Operating_System
architecture	I-Operating_System
provides	O
distributed	O
applications	O
with	O
the	O
following	O
benefits	O
:	O
</s>
<s>
DDM	B-Operating_System
architecture	I-Operating_System
is	O
a	O
set	O
of	O
specifications	O
for	O
messages	O
and	O
protocols	O
that	O
enable	O
data	O
distributed	O
throughout	O
a	O
network	O
of	O
computers	O
to	O
be	O
managed	O
and	O
accessed	O
.	O
</s>
<s>
IBM	O
's	O
Systems	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
(	O
SNA	O
)	O
was	O
initially	O
designed	O
to	O
enable	O
the	O
hierarchical	O
connection	O
of	O
workstations	O
to	O
IBM	O
mainframe	O
computers	O
.	O
</s>
<s>
When	O
IBM	O
's	O
SNA	O
Advanced	B-Protocol
Program	I-Protocol
to	I-Protocol
Program	I-Protocol
Communications	I-Protocol
(	O
APPC	O
)	O
architecture	O
was	O
defined	O
in	O
the	O
early	O
1980s	O
,	O
it	O
was	O
also	O
apparent	O
that	O
APPC	O
could	O
be	O
used	O
to	O
provide	O
operating	O
system	O
services	O
on	O
remote	O
computers	O
.	O
</s>
<s>
A	O
primitive	O
form	O
of	O
distributed	O
file	O
services	O
,	O
called	O
Distributed	O
Data	O
File	O
Facility	O
(	O
DDFF	O
)	O
had	O
been	O
implemented	O
to	O
connect	O
the	O
IBM	B-Device
System/3	I-Device
,	O
IBM	B-Device
System/34	I-Device
,	O
and	O
IBM	B-Device
System/36	I-Device
minicomputers	O
.	O
</s>
<s>
Further	O
,	O
the	O
IBM	B-Device
System/36	I-Device
and	O
the	O
IBM	B-Device
System/38	I-Device
computers	O
were	O
being	O
sold	O
to	O
customers	O
in	O
multiples	O
and	O
there	O
was	O
a	O
clear	O
need	O
to	O
enable	O
,	O
for	O
example	O
,	O
the	O
headquarters	O
computers	O
of	O
a	O
company	O
to	O
interact	O
with	O
the	O
computers	O
in	O
its	O
various	O
warehouses	O
.	O
</s>
<s>
Further	O
,	O
he	O
narrowed	O
its	O
mission	O
to	O
include	O
only	O
support	O
for	O
Distributed	O
Data	O
Management	O
(	O
DDM	O
)	O
,	O
in	O
particular	O
,	O
support	O
for	O
record-oriented	B-Application
files	I-Application
.	O
</s>
<s>
He	O
then	O
convinced	O
an	O
experienced	O
software	O
architect	O
,	O
Richard	O
A	O
.	O
Demers	O
,	O
to	O
join	O
him	O
in	O
the	O
tasks	O
of	O
defining	O
DDM	B-Operating_System
architecture	I-Operating_System
and	O
selling	O
the	O
idea	O
of	O
DDM	O
to	O
the	O
IBM	O
system	O
houses	O
.	O
</s>
<s>
Further	O
,	O
as	O
Personal	B-Device
Computers	I-Device
began	O
to	O
be	O
used	O
as	O
terminals	O
attached	O
to	O
mainframe	O
computers	O
,	O
it	O
was	O
argued	O
that	O
simply	O
enhancing	O
the	O
3270	O
data	O
stream	O
would	O
enable	O
PCs	O
to	O
access	O
mainframe	O
data	O
.	O
</s>
<s>
Further	O
,	O
he	O
defined	O
a	O
generic	O
format	O
for	O
DDM	O
messages	O
based	O
on	O
the	O
principles	O
of	O
object-orientation	O
as	O
pioneered	O
by	O
the	O
Smalltalk	B-Application
programming	I-Application
language	I-Application
and	O
by	O
the	O
IBM	B-Device
System/38	I-Device
.	O
</s>
<s>
In	O
1982	O
,	O
the	O
System/36	B-Device
planners	O
became	O
convinced	O
there	O
was	O
a	O
sufficient	O
market	O
for	O
DDM	O
record-oriented	B-Application
file	I-Application
services	O
.	O
</s>
<s>
The	O
System/36	B-Device
file	O
system	O
had	O
been	O
defined	O
to	O
meet	O
the	O
record-oriented	O
needs	O
of	O
third	O
generation	O
programming	O
languages	O
(	O
3GLs	O
)	O
,	O
such	O
as	O
Fortran	B-Application
,	O
COBOL	B-Application
,	O
PL/I	B-Language
,	O
and	O
IBM	B-Language
RPG	I-Language
,	O
and	O
so	O
had	O
the	O
System/38	B-Application
file	O
system	O
and	O
the	O
Virtual	B-Application
Storage	I-Application
Access	I-Application
Method	I-Application
(	O
VSAM	O
)	O
file	O
system	O
of	O
the	O
IBM	O
mainframe	O
computers	O
.	O
</s>
<s>
And	O
yet	O
,	O
their	O
actual	O
facilities	O
and	O
interfaces	O
varied	O
considerably	O
,	O
so	O
what	O
facilities	O
and	O
interfaces	O
should	O
DDM	B-Operating_System
architecture	I-Operating_System
support	O
?	O
</s>
<s>
See	O
record-oriented	B-Application
files	I-Application
.	O
</s>
<s>
The	O
initial	O
work	O
on	O
DDM	O
by	O
the	O
Golden	O
Gate	O
project	O
had	O
followed	O
the	O
lead	O
of	O
the	O
File	B-Protocol
Transfer	I-Protocol
Access	I-Protocol
and	I-Protocol
Management	I-Protocol
(	O
FTAM	B-Protocol
)	O
international	O
standard	O
for	O
distributed	O
files	O
,	O
but	O
it	O
was	O
very	O
abstract	O
and	O
difficult	O
to	O
map	O
to	O
local	O
file	O
services	O
.	O
</s>
<s>
Kenneth	O
Lawrence	O
,	O
the	O
system	O
architect	O
responsible	O
for	O
System/36	B-Device
file	O
services	O
,	O
argued	O
that	O
it	O
would	O
be	O
better	O
to	O
define	O
messages	O
that	O
at	O
least	O
one	O
IBM	O
system	O
could	O
easily	O
implement	O
and	O
then	O
let	O
other	O
systems	O
request	O
whatever	O
changes	O
they	O
needed	O
.	O
</s>
<s>
Naturally	O
,	O
he	O
argued	O
for	O
support	O
of	O
System/36	B-Device
requirements	O
.	O
</s>
<s>
Richard	O
Sanders	O
joined	O
the	O
DDM	B-Operating_System
architecture	I-Operating_System
team	O
and	O
worked	O
with	O
Lawrence	O
and	O
Demers	O
to	O
define	O
the	O
specific	O
messages	O
needed	O
for	O
System/36	B-Device
DDM	O
.	O
</s>
<s>
Progress	O
in	O
the	O
definition	O
of	O
DDM	O
encouraged	O
System/38	B-Application
to	O
also	O
participate	O
.	O
</s>
<s>
This	O
broadened	O
the	O
scope	O
of	O
DDM	O
record-file	O
support	O
to	O
meet	O
many	O
of	O
the	O
requirements	O
of	O
the	O
System/38	B-Application
'	O
s	O
advanced	O
file	O
system	O
.	O
</s>
<s>
In	O
level	O
1	O
of	O
DDM	O
,	O
access	O
to	O
remote	O
file	B-Application
directories	I-Application
was	O
not	O
supported	O
beyond	O
the	O
transmission	O
of	O
the	O
fully	O
qualified	O
name	O
of	O
the	O
file	O
to	O
be	O
used	O
.	O
</s>
<s>
TCP/IP	B-Protocol
.	O
</s>
<s>
With	O
the	O
completion	O
of	O
the	O
System/36	B-Device
DDM	O
product	O
,	O
Lawrence	O
worked	O
with	O
programmers	O
from	O
the	O
IBM	O
Hursley	O
Park	O
,	O
UK	O
laboratory	O
to	O
adapt	O
much	O
of	O
the	O
System/36	B-Device
DDM	O
server	O
programming	O
for	O
use	O
in	O
the	O
IBM	O
Customer	B-Application
Information	I-Application
Control	I-Application
System	I-Application
(	O
CICS	B-Application
)	O
transaction	O
processing	O
environment	O
,	O
thereby	O
making	O
CICS	B-Application
a	O
DDM	O
server	O
for	O
both	O
the	O
MVS	B-Application
and	O
VSE	B-Application
mainframe	O
operating	O
systems	O
.	O
</s>
<s>
Lawrence	O
also	O
worked	O
with	O
programmers	O
from	O
the	O
IBM	O
Cary	O
,	O
North	O
Carolina	O
laboratory	O
to	O
implement	O
a	O
DDM	O
record-oriented	O
client	O
for	O
IBM	B-Operating_System
PC	I-Operating_System
DOS	I-Operating_System
.	O
</s>
<s>
Level	O
1	O
of	O
DDM	B-Operating_System
Architecture	I-Operating_System
was	O
formally	O
published	O
in	O
1986	O
.	O
</s>
<s>
In	O
this	O
article	O
,	O
System/38	B-Application
will	O
be	O
henceforth	O
used	O
to	O
refer	O
to	O
the	O
System/38	B-Application
and	O
its	O
successors	O
:	O
the	O
IBM	O
AS/400	O
(	O
which	O
merged	O
the	O
functionality	O
of	O
the	O
System/36	B-Device
and	O
the	O
System/38	B-Application
)	O
,	O
the	O
IBM	O
iSeries	O
,	O
and	O
the	O
IBM	O
Power	O
Series	O
(	O
which	O
merged	O
the	O
iSeries	O
with	O
the	O
IBM	B-Device
RS/6000	I-Device
,	O
IBM	O
's	O
RISC/UNIX	O
-based	O
server	O
and	O
workstation	O
product	O
line	O
)	O
.	O
</s>
<s>
With	O
the	O
increasing	O
importance	O
of	O
the	O
IBM	B-Device
PC	I-Device
and	O
the	O
Unix	B-Application
operating	I-Application
system	I-Application
in	O
network	O
environments	O
,	O
DDM	O
support	O
was	O
also	O
needed	O
for	O
the	O
hierarchical	B-Application
directories	I-Application
and	O
stream-oriented	O
files	O
of	O
the	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
running	O
IBM	B-Operating_System
PC	I-Operating_System
DOS	I-Operating_System
and	O
the	O
IBM	B-Device
RS/6000	I-Device
running	O
IBM	B-Application
AIX	I-Application
(	O
IBM	O
's	O
version	O
of	O
Unix	B-Application
)	O
.	O
</s>
<s>
DDM	B-Operating_System
Architecture	I-Operating_System
Level	O
2	O
was	O
published	O
in	O
1988	O
.	O
</s>
<s>
Jan	O
Fisher	O
and	O
Sunil	O
Gaitonde	O
did	O
most	O
of	O
the	O
architecture	O
work	O
on	O
DDM	O
support	O
for	O
directories	B-Application
and	O
stream	O
files	O
.	O
</s>
<s>
In	O
1986	O
,	O
IBM	O
marketed	O
four	O
different	O
relational	B-Application
database	I-Application
(	O
RDB	O
)	O
products	O
,	O
each	O
built	O
for	O
a	O
specific	O
IBM	O
operating	O
system	O
.	O
</s>
<s>
Roger	O
Reinsch	O
from	O
the	O
IBM	O
Santa	O
Theresa	O
Programming	O
Center	O
lead	O
a	O
cross-product	O
team	O
to	O
define	O
a	O
Distributed	B-Language
Relational	I-Language
Database	I-Language
Architecture	I-Language
(	O
DRDA	B-Language
)	O
.	O
</s>
<s>
Richard	O
Sanders	O
and	O
Richard	O
Demers	O
from	O
the	O
DDM	B-Operating_System
architecture	I-Operating_System
team	O
to	O
define	O
appropriate	O
models	O
,	O
messages	O
and	O
protocols	O
.	O
</s>
<s>
In	O
1990	O
,	O
DDM	B-Operating_System
Architecture	I-Operating_System
Level	O
3	O
and	O
DRDA	B-Language
were	O
published	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Both	O
DDM	O
and	O
DRDA	B-Language
were	O
designated	O
as	O
strategic	O
components	O
of	O
IBM	O
's	O
Systems	B-Device
Application	I-Device
Architecture	I-Device
(	O
SAA	O
)	O
.	O
</s>
<s>
DRDA	B-Language
was	O
implemented	O
by	O
all	O
four	O
of	O
the	O
IBM	O
RDB	O
products	O
and	O
by	O
other	O
vendors	O
.	O
</s>
<s>
Awards	O
were	O
given	O
to	O
key	O
participants	O
in	O
the	O
design	O
of	O
DRDA	B-Language
.	O
</s>
<s>
The	O
Distributed	O
File	O
Management	O
(	O
DFM	O
)	O
project	O
was	O
initiated	O
to	O
add	O
DDM	O
services	O
to	O
IBM	O
's	O
MVS	B-Application
operating	I-Application
system	I-Application
to	O
enable	O
programs	O
on	O
remote	O
computers	O
to	O
create	O
,	O
manage	O
,	O
and	O
access	O
VSAM	O
files	O
.	O
</s>
<s>
John	O
Hufferd	O
,	O
the	O
manager	O
of	O
the	O
DFM	O
project	O
looked	O
to	O
the	O
DDM	B-Operating_System
Architecture	I-Operating_System
team	O
for	O
a	O
means	O
of	O
converting	O
the	O
data	O
fields	O
in	O
records	O
as	O
they	O
flowed	O
between	O
systems	O
.	O
</s>
<s>
The	O
following	O
additional	O
services	O
were	O
defined	O
by	O
Richard	O
Sanders	O
,	O
Jan	O
Fisher	O
and	O
Sunil	O
Gaitonde	O
in	O
DDM	B-Operating_System
architecture	I-Operating_System
at	O
Level	O
4	O
:	O
</s>
<s>
For	O
DRDA	B-Language
,	O
two-phase	O
commitment	O
control	O
protocols	O
for	O
application	O
directed	O
distributed	O
units	O
of	O
work	O
.	O
</s>
<s>
Queue	B-Operating_System
entries	O
are	O
application	O
defined	O
records	O
that	O
are	O
added	O
to	O
or	O
received	O
from	O
a	O
queue	B-Operating_System
.	O
</s>
<s>
DDM	B-Operating_System
architecture	I-Operating_System
level	O
4	O
was	O
published	O
in	O
1992	O
.	O
</s>
<s>
mainframe	O
Partitioned	B-Application
Data	I-Application
Sets	I-Application
,	O
which	O
are	O
files	O
consisting	O
of	O
an	O
internal	O
directory	B-Application
and	O
multiple	O
members	O
;	O
in	O
effect	O
,	O
they	O
are	O
libraries	O
of	O
similar	O
files	O
.	O
</s>
<s>
further	O
enhancements	O
to	O
DRDA	B-Language
.	O
</s>
<s>
Shortly	O
thereafter	O
,	O
the	O
IBM	B-Operating_System
DDM	I-Operating_System
architecture	O
group	O
was	O
disbanded	O
.	O
</s>
<s>
DDM	B-Operating_System
architecture	I-Operating_System
is	O
a	O
formally	O
defined	O
and	O
highly	O
structured	O
set	O
of	O
specifications	O
.	O
</s>
<s>
DDM	B-Operating_System
architecture	I-Operating_System
defines	O
a	O
client/server	B-Operating_System
protocol	I-Operating_System
;	O
that	O
is	O
,	O
a	O
client	O
requests	O
services	O
from	O
a	O
server	O
which	O
interacts	O
with	O
its	O
local	O
resources	O
to	O
perform	O
the	O
requested	O
service	O
,	O
the	O
results	O
of	O
which	O
,	O
data	O
and	O
status	O
indicators	O
,	O
are	O
returned	O
to	O
the	O
client	O
.	O
</s>
<s>
(	O
The	O
common	O
terminology	O
of	O
clients	O
and	O
servers	O
is	O
used	O
here	O
,	O
but	O
in	O
DDM	B-Operating_System
architecture	I-Operating_System
,	O
a	O
client	O
is	O
called	O
a	O
Source	O
Server	O
and	O
a	O
Server	O
is	O
called	O
a	O
Target	O
Server	O
.	O
)	O
</s>
<s>
DDM	B-Operating_System
architecture	I-Operating_System
does	O
not	O
specify	O
how	O
this	O
redirection	O
is	O
to	O
occur	O
as	O
it	O
does	O
not	O
support	O
a	O
directory	B-Application
of	O
remote	O
resources	O
.	O
</s>
<s>
One	O
method	O
of	O
redirection	O
used	O
by	O
several	O
DDM	O
file-oriented	O
products	O
is	O
to	O
have	O
the	O
application	O
open	O
a	O
special	O
local	O
file	O
,	O
called	O
a	O
DDM	O
File	O
by	O
the	O
System/38	B-Application
,	O
that	O
provides	O
location	O
and	O
access	O
information	O
about	O
the	O
remote	O
file	O
.	O
</s>
<s>
DDM	B-Operating_System
Architecture	I-Operating_System
defines	O
Manager	O
level	O
entities	O
for	O
files	O
,	O
relational	B-Application
databases	I-Application
,	O
access	B-Application
methods	I-Application
,	O
etc	O
.	O
</s>
<s>
Various	O
telecommunication	O
protocols	O
can	O
be	O
used	O
,	O
including	O
IBM	O
's	O
SNA	O
APPC	O
and	O
the	O
Internet	O
's	O
TCP/IP	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
DDM	B-Operating_System
architecture	I-Operating_System
is	O
object-oriented	B-Language
.	O
</s>
<s>
Structurally	O
,	O
DDM	B-Operating_System
architecture	I-Operating_System
consists	O
of	O
hierarchical	O
levels	O
of	O
objects	O
,	O
each	O
level	O
manifesting	O
emergent	O
properties	O
at	O
increasingly	O
higher	O
levels	O
.	O
</s>
<s>
The	O
encoding	O
methods	O
used	O
for	O
the	O
values	O
of	O
these	O
scalar	O
objects	O
are	O
fully	O
defined	O
by	O
DDM	B-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
A	O
mapped	O
object	O
consists	O
of	O
one	O
or	O
more	O
fields	O
,	O
such	O
as	O
the	O
fields	O
of	O
an	O
application	O
defined	O
record	B-Application
.	O
</s>
<s>
The	O
encoding	O
methods	O
and	O
the	O
alignment	O
of	O
these	O
fields	O
is	O
not	O
defined	O
by	O
DDM	B-Operating_System
architecture	I-Operating_System
;	O
instead	O
,	O
it	O
is	O
defined	O
by	O
application	O
program	O
declaration	O
statements	O
and	O
the	O
encoding	O
and	O
alignment	O
methods	O
of	O
its	O
programming	O
language	O
.	O
</s>
<s>
Manager	O
entities	O
at	O
this	O
level	O
were	O
inspired	O
by	O
the	O
System	O
Objects	O
of	O
the	O
System/38	B-Application
operating	O
system	O
.	O
</s>
<s>
The	O
Managers	O
defined	O
by	O
DDM	O
include	O
:	O
Dictionary	O
,	O
Supervisor	O
,	O
Agent	O
,	O
Directory	B-Application
,	O
File(s )	O
,	O
Access	O
Method(s )	O
,	O
Relational	B-Application
Database	I-Application
,	O
SQL	B-Language
Application	O
Manager	O
,	O
Queue	B-Operating_System
,	O
Lock	O
Manager	O
,	O
Security	O
Manager	O
,	O
Recovery	O
Manager	O
,	O
System	O
Command	O
Processor	O
,	O
Communication	O
Manager(s )	O
.	O
</s>
<s>
Examples	O
are	O
clients	O
and	O
servers	O
specialized	O
for	O
distributed	O
file	O
or	O
distributed	O
relational	B-Application
database	I-Application
management	O
.	O
</s>
<s>
While	O
DDM	B-Operating_System
architecture	I-Operating_System
is	O
object-oriented	B-Language
,	O
the	O
DDM	O
products	O
were	O
implemented	O
using	O
the	O
languages	O
and	O
methods	O
typical	O
of	O
their	O
host	O
systems	O
.	O
</s>
<s>
A	O
Smalltalk	B-Application
version	O
of	O
DDM	O
was	O
developed	O
for	O
the	O
IBM	B-Device
PC	I-Device
by	O
Object	B-Language
Technology	I-Language
International	O
,	O
with	O
appropriate	O
Smalltalk	B-Application
classes	O
automatically	O
created	O
from	O
the	O
DDM	O
Reference	O
Manual	O
.	O
</s>
<s>
DDM	O
is	O
an	O
open	B-Architecture
architecture	I-Architecture
.	O
</s>
<s>
DDM	O
products	O
can	O
implement	O
subsets	O
of	O
DDM	B-Operating_System
architecture	I-Operating_System
;	O
they	O
can	O
also	O
create	O
their	O
own	O
extensions	O
.	O
</s>
<s>
It	O
identifies	O
the	O
client	O
and	O
specifies	O
the	O
managers	O
the	O
client	O
requires	O
and	O
the	O
level	O
of	O
DDM	B-Operating_System
architecture	I-Operating_System
at	O
which	O
support	O
is	O
required	O
.	O
</s>
<s>
For	O
example	O
,	O
DDM/PC	O
is	O
only	O
a	O
client	O
,	O
CICS/DDM	O
is	O
only	O
a	O
server	O
,	O
and	O
System/38	B-Application
DDM	O
is	O
both	O
a	O
client	O
and	O
a	O
server	O
.	O
</s>
<s>
to	O
support	O
specific	O
managers	O
,	O
such	O
as	O
record-oriented	B-Application
files	I-Application
,	O
stream-oriented	O
files	O
,	O
relational	B-Application
databases	I-Application
(	O
as	O
part	O
of	O
DRDA	B-Language
)	O
,	O
or	O
any	O
combination	O
thereof	O
.	O
</s>
<s>
For	O
example	O
,	O
MVS	B-Application
Database	B-Application
2	I-Application
provides	O
client	O
and	O
server	O
support	O
for	O
only	O
the	O
subset	O
of	O
DDM	O
required	O
by	O
DRDA	B-Language
.	O
</s>
<s>
to	O
support	O
selected	O
parameters	O
of	O
a	O
command	O
,	O
such	O
as	O
the	O
'	O
Return	O
Inactive	O
Records	O
 '	O
parameter	O
of	O
the	O
'	O
Get	O
Record	B-Application
 '	O
command	O
.	O
</s>
<s>
Such	O
extensions	O
can	O
be	O
defined	O
within	O
DDM	O
's	O
object-oriented	B-Language
framework	O
so	O
that	O
existing	O
DDM	O
message	O
handling	O
facilities	O
can	O
be	O
used	O
.	O
</s>
<s>
In	O
a	O
purely	O
object-oriented	B-Language
implementation	O
of	O
DDM	O
,	O
clients	O
and	O
servers	O
and	O
all	O
of	O
their	O
contained	O
managers	O
and	O
objects	O
exist	O
in	O
a	O
memory	O
heap	O
,	O
with	O
pointers	O
(	O
memory	O
addresses	O
)	O
used	O
to	O
interconnect	O
them	O
.	O
</s>
<s>
DDM	B-Operating_System
architecture	I-Operating_System
assigns	O
unique	O
code	O
points	O
to	O
each	O
class	O
of	O
object	O
.	O
</s>
<s>
In	O
DDM	B-Operating_System
architecture	I-Operating_System
,	O
these	O
envelopes	O
are	O
called	O
Data	O
Stream	O
Structures	O
(	O
DSS	O
)	O
.	O
</s>
<s>
Classes	O
are	O
defined	O
by	O
an	O
inheritance	O
hierarchy	O
;	O
for	O
example	O
,	O
Record	B-Application
File	O
is	O
a	O
subclass	O
of	O
File	O
which	O
is	O
a	O
subclass	O
of	O
Manager	O
and	O
inherits	O
their	O
data	O
and	O
commands	O
.	O
</s>
<s>
the	O
status	O
of	O
the	O
class	O
relative	O
to	O
ongoing	O
work	O
on	O
DDM	B-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
Three	O
general	O
file	O
models	O
are	O
defined	O
by	O
DDM	B-Operating_System
architecture	I-Operating_System
:	O
record-oriented	B-Application
files	I-Application
,	O
stream-oriented	O
files	O
and	O
hierarchical	B-Application
directories	I-Application
.	O
</s>
<s>
The	O
following	O
services	O
are	O
provided	O
by	O
DDM	B-Operating_System
architecture	I-Operating_System
for	O
managing	O
remote	O
files	O
:	O
</s>
<s>
Record-oriented	B-Application
files	I-Application
were	O
designed	O
to	O
meet	O
the	O
data	O
input	O
,	O
output	O
,	O
and	O
storage	O
requirements	O
of	O
third	O
generation	O
(	O
3GL	O
)	O
programming	O
languages	O
,	O
such	O
as	O
Fortran	B-Application
,	O
Cobol	B-Application
,	O
PL/I	B-Language
,	O
and	O
RPG	B-Language
.	O
</s>
<s>
A	O
record	B-Application
is	O
a	O
series	O
of	O
related	O
data	O
fields	O
,	O
such	O
as	O
the	O
name	O
,	O
address	O
,	O
identification	O
number	O
and	O
salary	O
of	O
a	O
single	O
employee	O
,	O
in	O
which	O
each	O
field	O
is	O
encoded	O
and	O
mapped	O
to	O
a	O
contiguous	O
string	O
of	O
bytes	O
.	O
</s>
<s>
Application	O
records	O
,	O
such	O
as	O
employee	O
data	O
records	O
,	O
were	O
sequentially	O
read	O
or	O
written	O
a	O
record	B-Application
at	O
a	O
time	O
and	O
processed	O
in	O
batches	O
.	O
</s>
<s>
When	O
direct	O
access	O
storage	O
devices	O
became	O
available	O
,	O
programming	O
languages	O
added	O
ways	O
for	O
programs	O
to	O
randomly	O
access	O
records	O
one	O
at	O
a	O
time	O
,	O
such	O
as	O
access	O
by	O
the	O
values	O
of	O
key	O
fields	O
or	O
by	O
the	O
position	O
of	O
a	O
record	B-Application
in	O
a	O
file	O
.	O
</s>
<s>
The	O
DDM	O
record-oriented	B-Application
file	I-Application
models	O
consist	O
of	O
file	O
attributes	O
,	O
such	O
as	O
its	O
creation	O
date	O
,	O
the	O
date	O
of	O
last	O
update	O
,	O
the	O
size	O
of	O
its	O
records	O
,	O
and	O
slots	O
in	O
which	O
records	O
can	O
be	O
stored	O
.	O
</s>
<s>
DDM	O
defines	O
four	O
kinds	O
of	O
record-oriented	B-Application
files	I-Application
:	O
</s>
<s>
DDM	B-Operating_System
architecture	I-Operating_System
also	O
defines	O
a	O
variety	O
of	O
access	B-Application
methods	I-Application
for	O
working	O
with	O
record-oriented	B-Application
files	I-Application
in	O
various	O
ways	O
.	O
</s>
<s>
An	O
access	B-Application
method	I-Application
is	O
an	O
instance	O
of	O
the	O
use	O
of	O
a	O
file	O
created	O
by	O
means	O
of	O
an	O
OPEN	O
command	O
that	O
connects	O
itself	O
to	O
the	O
file	O
after	O
determining	O
if	O
the	O
client	O
is	O
authorized	O
to	O
use	O
it	O
.	O
</s>
<s>
The	O
access	B-Application
method	I-Application
is	O
disconnected	O
from	O
a	O
file	O
by	O
means	O
of	O
a	O
CLOSE	O
command	O
.	O
</s>
<s>
An	O
access	B-Application
method	I-Application
keeps	O
track	O
of	O
the	O
record	B-Application
being	O
currently	O
processed	O
by	O
means	O
of	O
a	O
cursor	O
.	O
</s>
<s>
Using	O
various	O
SET	O
commands	O
,	O
the	O
cursor	O
can	O
be	O
made	O
to	O
point	O
to	O
the	O
beginning	O
or	O
end	O
of	O
the	O
file	O
,	O
to	O
the	O
next	O
or	O
previous	O
sequential	O
record	B-Application
of	O
the	O
file	O
,	O
to	O
the	O
record	B-Application
with	O
a	O
specific	O
key	O
value	O
,	O
or	O
to	O
the	O
next	O
or	O
previous	O
record	B-Application
as	O
ordered	O
by	O
their	O
keys	O
.	O
</s>
<s>
Multiple	O
instances	O
of	O
access	B-Application
methods	I-Application
can	O
be	O
opened	O
on	O
a	O
file	O
at	O
the	O
same	O
time	O
,	O
each	O
serving	O
a	O
single	O
client	O
.	O
</s>
<s>
If	O
a	O
file	O
is	O
opened	O
for	O
update	O
access	O
,	O
conflicts	O
can	O
occur	O
when	O
the	O
same	O
record	B-Application
is	O
being	O
accessed	O
by	O
multiple	O
clients	O
.	O
</s>
<s>
Also	O
,	O
if	O
a	O
file	O
is	O
opened	O
for	O
update	O
a	O
lock	O
is	O
obtained	O
on	O
a	O
record	B-Application
by	O
the	O
first	O
client	O
to	O
read	O
it	O
and	O
released	O
when	O
that	O
client	O
updates	O
it	O
.	O
</s>
<s>
Stream	O
files	O
are	O
the	O
primary	O
file	O
model	O
supported	O
by	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
and	O
by	O
Windows	B-Application
.	O
</s>
<s>
DDM	O
defines	O
a	O
single	O
stream	O
file	O
model	O
and	O
a	O
single	O
stream	O
access	B-Application
method	I-Application
.	O
</s>
<s>
The	O
stream	O
can	O
be	O
accessed	O
by	O
means	O
of	O
the	O
Stream	O
Access	B-Application
Method	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
the	O
data	O
stream	O
of	O
document	O
files	O
is	O
defined	O
by	O
a	O
text	O
processing	O
program	O
such	O
as	O
Microsoft	B-Application
Word	I-Application
and	O
that	O
of	O
a	O
spreadsheet	O
file	O
by	O
a	O
program	O
such	O
as	O
Microsoft	B-Application
Excel	I-Application
.	O
</s>
<s>
A	O
Stream	O
access	B-Application
method	I-Application
is	O
an	O
instance	O
of	O
use	O
of	O
a	O
stream	O
file	O
by	O
a	O
single	O
client	O
.	O
</s>
<s>
Multiple	O
instances	O
of	O
the	O
Stream	O
access	B-Application
method	I-Application
can	O
be	O
opened	O
on	O
a	O
file	O
at	O
the	O
same	O
time	O
,	O
each	O
serving	O
a	O
single	O
client	O
.	O
</s>
<s>
Hierarchical	B-Application
directories	I-Application
are	O
files	O
whose	O
records	O
each	O
associate	O
a	O
name	O
with	O
a	O
location	O
.	O
</s>
<s>
A	O
hierarchy	O
occurs	O
when	O
a	O
directory	B-Application
record	B-Application
identifies	O
the	O
name	O
and	O
location	O
of	O
another	O
directory	B-Application
.	O
</s>
<s>
Using	O
DDM	O
client	O
and	O
server	O
products	O
,	O
a	O
program	O
can	O
create	O
,	O
delete	O
and	O
rename	O
directories	B-Application
in	O
a	O
remote	O
computer	O
.	O
</s>
<s>
They	O
can	O
also	O
list	O
and	O
change	O
the	O
file	O
attributes	O
of	O
remote	O
directories	B-Application
.	O
</s>
<s>
The	O
records	O
in	O
a	O
directory	B-Application
can	O
be	O
sequentially	O
read	O
by	O
using	O
the	O
DDM	O
Directory	B-Application
Access	B-Application
Method	I-Application
.	O
</s>
<s>
The	O
files	O
identified	O
by	O
directory	B-Application
records	O
can	O
be	O
renamed	O
,	O
copied	O
,	O
and	O
moved	O
to	O
a	O
different	O
directory	B-Application
.	O
</s>
<s>
A	O
DDM	O
queue	B-Operating_System
resides	O
in	O
a	O
single	O
system	O
,	O
but	O
it	O
can	O
be	O
accessed	O
by	O
programs	O
on	O
multiple	O
systems	O
.	O
</s>
<s>
The	O
DDM	O
queue	B-Operating_System
model	O
consists	O
of	O
queue	B-Operating_System
attributes	O
,	O
such	O
as	O
its	O
creation	O
date	O
,	O
the	O
number	O
of	O
records	O
the	O
queue	B-Operating_System
can	O
contain	O
,	O
and	O
the	O
length	O
of	O
the	O
records	O
.	O
</s>
<s>
The	O
records	O
in	O
a	O
queue	B-Operating_System
can	O
be	O
either	O
fixed	O
or	O
varying	O
length	O
.	O
</s>
<s>
Unlike	O
the	O
DDM	O
file	O
models	O
,	O
it	O
is	O
not	O
necessary	O
to	O
open	O
an	O
access	B-Application
method	I-Application
on	O
a	O
queue	B-Operating_System
.	O
</s>
<s>
Programs	O
can	O
add	O
records	O
to	O
a	O
queue	B-Operating_System
and	O
receive	O
records	O
from	O
a	O
queue	B-Operating_System
as	O
determined	O
by	O
the	O
class	O
of	O
the	O
queue	B-Operating_System
.	O
</s>
<s>
Programs	O
can	O
also	O
clear	O
records	O
from	O
a	O
queue	B-Operating_System
,	O
stop	O
operations	O
on	O
a	O
queue	B-Operating_System
,	O
list	O
the	O
attributes	O
of	O
a	O
queue	B-Operating_System
,	O
and	O
change	O
the	O
attributes	O
of	O
a	O
queue	B-Operating_System
.	O
</s>
<s>
Programs	O
can	O
also	O
lock	O
a	O
queue	B-Operating_System
or	O
individual	O
records	O
in	O
a	O
queue	B-Operating_System
to	O
inhibit	O
contention	O
from	O
other	O
programs	O
.	O
</s>
<s>
A	O
relational	B-Application
database	I-Application
(	O
RDB	O
)	O
is	O
an	O
implementation	O
of	O
the	O
Structured	B-Language
Query	I-Language
Language	I-Language
(	O
SQL	B-Language
)	O
that	O
supports	O
the	O
creation	O
,	O
management	O
,	O
querying	O
,	O
updating	O
,	O
indexing	O
and	O
interrelationships	O
of	O
tables	O
of	O
data	O
.	O
</s>
<s>
An	O
interactive	O
user	O
or	O
program	O
can	O
issue	O
SQL	B-Language
statements	O
to	O
a	O
RDB	O
and	O
receive	O
tables	O
of	O
data	O
and	O
status	O
indicators	O
in	O
reply	O
.	O
</s>
<s>
However	O
,	O
SQL	B-Language
statements	O
can	O
also	O
be	O
compiled	O
and	O
stored	O
in	O
the	O
RDB	O
as	O
packages	O
and	O
then	O
invoked	O
by	O
package	O
name	O
.	O
</s>
<s>
The	O
Distributed	B-Language
Relational	I-Language
Database	I-Language
Architecture	I-Language
(	O
DRDA	B-Language
)	O
fits	O
nicely	O
into	O
the	O
overall	O
DDM	O
framework	O
,	O
as	O
discussed	O
in	O
Object-Orientation	O
.	O
</s>
<s>
(	O
However	O
,	O
DDM	O
can	O
also	O
be	O
viewed	O
as	O
a	O
component	O
architecture	O
of	O
DRDA	B-Language
since	O
other	O
specifications	O
are	O
also	O
required	O
)	O
.	O
</s>
<s>
The	O
DDM	O
manager-level	O
objects	O
supporting	O
DRDA	B-Language
are	O
named	O
RDB	O
(	O
for	O
relational	B-Application
database	I-Application
)	O
and	O
SQLAM	O
(	O
for	O
SQL	B-Language
Application	O
Manager	O
)	O
.	O
</s>
<s>
Transparency	O
is	O
a	O
key	O
objective	O
of	O
DDM	B-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
For	O
files	O
,	O
this	O
was	O
largely	O
accomplished	O
by	O
DDM	O
clients	O
at	O
the	O
interface/functional	O
level	O
,	O
but	O
what	O
about	O
the	O
data	O
fields	O
in	O
a	O
record	B-Application
?	O
</s>
<s>
Complete	O
transparency	O
requires	O
that	O
client	O
application	O
programs	O
be	O
able	O
to	O
write	O
and	O
read	O
fields	O
as	O
encoded	O
by	O
their	O
local	O
data	O
management	O
system	O
,	O
regardless	O
of	O
how	O
any	O
remote	O
server	O
encodes	O
them	O
,	O
and	O
that	O
implies	O
automatic	O
data	B-General_Concept
conversions	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
IBM	O
mainframe	O
computers	O
encode	O
floating	O
point	O
numbers	O
in	O
hexadecimal	B-Device
format	O
and	O
character	O
data	O
in	O
EBCDIC	B-Language
,	O
while	O
IBM	B-Device
Personal	I-Device
computers	I-Device
encode	O
them	O
in	O
IEEE	O
format	O
and	O
ASCII	B-Protocol
.	O
</s>
<s>
Further	O
complexity	O
arose	O
because	O
of	O
the	O
ways	O
in	O
which	O
various	O
programming	O
language	O
compilers	O
map	O
record	B-Application
fields	O
onto	O
strings	O
of	O
bits	O
,	O
bytes	O
,	O
and	O
words	O
in	O
memory	O
.	O
</s>
<s>
Transparent	O
conversion	O
of	O
a	O
record	B-Application
requires	O
detailed	O
descriptions	O
of	O
both	O
the	O
client	O
view	O
and	O
the	O
server	O
view	O
of	O
a	O
record	B-Application
.	O
</s>
<s>
The	O
key	O
issue	O
is	O
obtaining	O
sufficiently	O
detailed	O
record	B-Application
descriptions	O
,	O
but	O
record	B-Application
descriptions	O
are	O
generally	O
specified	O
abstractly	O
in	O
application	O
programs	O
by	O
declaration	O
statements	O
defined	O
by	O
the	O
programming	O
language	O
,	O
with	O
the	O
language	O
compiler	O
handling	O
encoding	O
and	O
mapping	O
details	O
.	O
</s>
<s>
In	O
a	O
distributed	O
processing	O
environment	O
,	O
what	O
is	O
needed	O
is	O
a	O
single	O
,	O
standardized	O
way	O
of	O
describing	O
records	O
that	O
is	O
independent	O
of	O
all	O
programming	O
languages	O
,	O
one	O
that	O
can	O
describe	O
the	O
wide	O
variety	O
of	O
fixed	O
and	O
varying	O
length	O
record	B-Application
formats	O
found	O
in	O
existing	O
files	O
.	O
</s>
<s>
The	O
following	O
IBM	O
products	O
implemented	O
various	O
subsets	O
of	O
DDM	B-Operating_System
architecture	I-Operating_System
:	O
</s>
<s>
CICS	B-Application
-	O
record	B-Application
file	O
server	O
within	O
the	O
CICS	B-Application
transaction	O
processing	O
environment	O
.	O
</s>
<s>
Discontinued	O
in	O
CICS	B-Application
for	O
z/OS	B-Application
V5.2	O
and	O
later	O
.	O
</s>
<s>
CICS	B-Application
-	O
Record	B-Application
file	O
server	O
within	O
the	O
CICS	B-Application
transaction	O
processing	O
environment	O
.	O
</s>
<s>
Discontinued	O
in	O
CICS	B-Application
for	O
z/VSE	B-Application
V2.1	O
and	O
later	O
.	O
</s>
<s>
DDM/PC	O
-	O
Directory	B-Application
and	O
stream	O
file	O
client	O
.	O
</s>
<s>
PC	O
Support/36	O
-	O
Directory	B-Application
and	O
stream	O
file	O
client	O
.	O
</s>
<s>
PC	O
Support/400	O
-	O
Directory	B-Application
and	O
stream	O
file	O
client	O
.	O
</s>
<s>
For	O
a	O
complete	O
list	O
of	O
the	O
products	O
that	O
have	O
implemented	O
DRDA	B-Language
,	O
see	O
the	O
.	O
</s>
