<s>
ISAM	B-Application
(	O
an	O
acronym	O
for	O
indexed	B-Application
sequential	I-Application
access	I-Application
method	I-Application
)	O
is	O
a	O
method	O
for	O
creating	O
,	O
maintaining	O
,	O
and	O
manipulating	O
computer	B-Operating_System
files	I-Operating_System
of	O
data	O
so	O
that	O
records	O
can	O
be	O
retrieved	O
sequentially	O
or	O
randomly	O
by	O
one	O
or	O
more	O
keys	B-Application
.	O
</s>
<s>
Indexes	O
of	O
key	B-Application
fields	I-Application
are	O
maintained	O
to	O
achieve	O
fast	O
retrieval	O
of	O
required	O
file	O
records	O
in	O
Indexed	B-Language
files	I-Language
.	O
</s>
<s>
IBM	O
originally	O
developed	O
ISAM	B-Application
for	O
mainframe	B-Architecture
computers	I-Architecture
,	O
but	O
implementations	O
are	O
available	O
for	O
most	O
computer	O
systems	O
.	O
</s>
<s>
The	O
term	O
ISAM	B-Application
is	O
used	O
for	O
several	O
related	O
concepts	O
:	O
</s>
<s>
The	O
IBM	O
ISAM	B-Application
product	O
and	O
the	O
algorithm	O
it	O
employs	O
.	O
</s>
<s>
A	O
database	B-Application
system	I-Application
where	O
an	O
application	O
developer	O
directly	O
uses	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
to	O
search	O
indexes	O
in	O
order	O
to	O
locate	O
records	O
in	O
data	O
files	O
.	O
</s>
<s>
In	O
contrast	O
,	O
a	O
relational	B-Application
database	I-Application
uses	O
a	O
query	B-Language
optimizer	I-Language
which	O
automatically	O
selects	O
indexes	O
.	O
</s>
<s>
Most	O
databases	B-Application
use	O
some	O
variation	O
of	O
the	O
B-tree	B-Architecture
for	O
this	O
purpose	O
,	O
although	O
the	O
original	O
IBM	O
ISAM	B-Application
and	O
VSAM	O
implementations	O
did	O
not	O
do	O
so	O
.	O
</s>
<s>
Indexes	O
are	O
used	O
by	O
almost	O
all	O
databases	B-Application
.	O
</s>
<s>
In	O
an	O
ISAM	B-Application
system	O
,	O
data	O
is	O
organized	O
into	O
records	O
which	O
are	O
composed	O
of	O
fixed	O
length	O
fields	O
,	O
originally	O
stored	O
sequentially	O
in	O
key	O
sequence	O
.	O
</s>
<s>
This	O
differs	O
from	O
the	O
contemporaneous	O
navigational	B-General_Concept
databases	I-General_Concept
,	O
in	O
which	O
the	O
pointers	O
to	O
other	O
records	O
were	O
stored	O
inside	O
the	O
records	O
themselves	O
.	O
</s>
<s>
The	O
key	O
improvement	O
in	O
ISAM	B-Application
is	O
that	O
the	O
indexes	O
are	O
small	O
and	O
can	O
be	O
searched	O
quickly	O
,	O
possibly	O
entirely	O
in	O
memory	O
,	O
thereby	O
allowing	O
the	O
database	O
to	O
access	O
only	O
the	O
records	O
it	O
needs	O
.	O
</s>
<s>
When	O
an	O
ISAM	B-Application
file	O
is	O
created	O
,	O
index	O
nodes	O
are	O
fixed	O
,	O
and	O
their	O
pointers	O
do	O
not	O
change	O
during	O
inserts	O
and	O
deletes	O
that	O
occur	O
later	O
(	O
only	O
content	O
of	O
leaf	O
nodes	O
change	O
afterwards	O
)	O
.	O
</s>
<s>
Relational	B-Application
databases	I-Application
can	O
easily	O
be	O
built	O
on	O
an	O
ISAM	B-Application
framework	O
with	O
the	O
addition	O
of	O
logic	O
to	O
maintain	O
the	O
validity	O
of	O
the	O
links	O
between	O
the	O
tables	O
.	O
</s>
<s>
Typically	O
the	O
field	O
being	O
used	O
as	O
the	O
link	O
,	O
the	O
foreign	B-Application
key	I-Application
,	O
will	O
be	O
indexed	O
for	O
quick	O
lookup	O
.	O
</s>
<s>
ISAM	B-Application
is	O
simple	O
to	O
understand	O
and	O
implement	O
,	O
as	O
it	O
primarily	O
consists	O
of	O
direct	O
access	O
to	O
a	O
database	O
file	O
.	O
</s>
<s>
To	O
prevent	O
this	O
,	O
some	O
ISAM	B-Application
implementations	O
provide	O
whole-file	O
or	O
individual	O
record	O
locking	B-Application
functionality	O
.	O
</s>
<s>
Locking	B-Application
multiple	O
records	O
runs	O
the	O
risk	O
of	O
deadlock	B-Operating_System
unless	O
a	O
deadlock	B-Operating_System
prevention	I-Operating_System
scheme	O
is	O
strictly	O
followed	O
.	O
</s>
<s>
The	O
problems	O
of	O
locking	B-Application
,	O
and	O
deadlock	B-Operating_System
are	O
typically	O
solved	O
with	O
the	O
addition	O
of	O
a	O
client-server	B-Operating_System
framework	O
which	O
marshals	O
client	O
requests	O
and	O
maintains	O
ordering	O
.	O
</s>
<s>
Full	O
ACID	O
transaction	O
management	O
systems	O
are	O
provided	O
by	O
some	O
ISAM	B-Application
client-server	B-Operating_System
implementations	O
.	O
</s>
<s>
These	O
are	O
the	O
basic	O
concepts	O
behind	O
a	O
database	B-Application
management	I-Application
system	I-Application
(	O
DBMS	B-General_Concept
)	O
,	O
which	O
is	O
a	O
client	O
layer	O
over	O
the	O
underlying	O
data	O
store	O
.	O
</s>
<s>
ISAM	B-Application
was	O
replaced	O
at	O
IBM	O
with	O
a	O
methodology	O
called	O
VSAM	O
(	O
virtual	B-Application
storage	I-Application
access	I-Application
method	I-Application
)	O
.	O
</s>
<s>
Still	O
later	O
,	O
IBM	O
developed	O
SQL/DS	B-Application
and	O
then	O
Db2	B-Application
which	O
IBM	O
promotes	O
as	O
their	O
primary	O
database	B-Application
management	I-Application
system	I-Application
.	O
</s>
<s>
VSAM	O
is	O
the	O
physical	O
access	O
method	O
used	O
in	O
Db2	B-Application
.	O
</s>
<s>
The	O
OpenVMS	B-Operating_System
operating	O
system	O
uses	O
the	O
Files-11	B-Application
file	O
system	O
in	O
conjunction	O
with	O
RMS	B-Application
(	O
Record	B-Application
Management	I-Application
Services	I-Application
)	O
.	O
</s>
<s>
RMS	B-Application
provides	O
an	O
additional	O
layer	O
between	O
the	O
application	O
and	O
the	O
files	O
on	O
disk	O
that	O
provides	O
a	O
consistent	O
method	O
of	O
data	O
organization	O
and	O
access	O
across	O
multiple	O
3GL	O
and	O
4GL	O
languages	O
.	O
</s>
<s>
RMS	B-Application
provides	O
four	O
different	O
methods	O
of	O
accessing	O
data	O
;	O
sequential	O
,	O
relative	O
record	O
number	O
access	O
,	O
record	O
file	O
address	O
access	O
,	O
and	O
indexed	O
access	O
.	O
</s>
<s>
The	O
indexed	O
access	O
method	O
of	O
reading	O
or	O
writing	O
data	O
only	O
provides	O
the	O
desired	O
outcome	O
if	O
in	O
fact	O
the	O
file	O
is	O
organized	O
as	O
an	O
ISAM	B-Application
file	O
with	O
the	O
appropriate	O
,	O
previously	O
defined	O
keys	B-Application
.	O
</s>
<s>
Multiple	O
keys	B-Application
,	O
overlapping	O
keys	B-Application
and	O
key	O
compression	O
within	O
the	O
hash	O
tables	O
are	O
supported	O
.	O
</s>
<s>
A	O
utility	O
to	O
define/redefine	O
keys	B-Application
in	O
existing	O
files	O
is	O
provided	O
.	O
</s>
<s>
IBM	O
engineers	O
designed	O
the	O
ISAM	B-Application
system	O
to	O
use	O
a	O
minimum	O
amount	O
of	O
computer	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
The	O
tradeoff	O
was	O
that	O
the	O
Input/Output	B-Device
channel	I-Device
,	O
control	O
unit	O
,	O
and	O
disk	O
were	O
kept	O
busier	O
.	O
</s>
<s>
An	O
ISAM	B-Application
file	O
consists	O
of	O
a	O
collection	O
of	O
data	O
records	O
and	O
two	O
or	O
three	O
levels	O
of	O
index	O
.	O
</s>
<s>
The	O
track	O
index	O
contains	O
the	O
highest	O
key	O
for	O
each	O
disk	B-Device
track	I-Device
on	O
the	O
cylinder	O
it	O
indexes	O
.	O
</s>
<s>
The	O
cylinder	O
index	O
stores	O
the	O
highest	O
key	O
on	O
a	O
cylinder	O
,	O
and	O
the	O
disk	B-Device
address	I-Device
of	O
the	O
corresponding	O
track	O
index	O
.	O
</s>
<s>
An	O
optional	O
master	O
index	O
,	O
usually	O
used	O
only	O
for	O
large	O
files	O
,	O
contains	O
the	O
highest	O
key	O
on	O
a	O
cylinder	O
index	O
track	O
and	O
the	O
disk	B-Device
address	I-Device
of	O
that	O
cylinder	O
index	O
.	O
</s>
<s>
ISAM	B-Application
's	O
use	O
of	O
self-modifying	O
channel	O
programs	O
later	O
caused	O
difficulties	O
for	O
CP-67	B-Application
support	O
of	O
OS/360	B-Application
,	O
since	O
CP-67	B-Application
copied	O
an	O
entire	O
channel	O
program	O
into	O
fixed	O
memory	O
when	O
the	O
I/O	O
operation	O
was	O
started	O
and	O
translated	O
virtual	O
addresses	O
to	O
real	O
addresses	O
.	O
</s>
