<s>
The	O
Zope	B-Application
Object	I-Application
Database	I-Application
(	O
ZODB	B-Application
)	O
is	O
an	O
object-oriented	B-Application
database	I-Application
for	O
transparently	O
and	O
persistently	O
storing	O
Python	B-Language
objects	O
.	O
</s>
<s>
It	O
is	O
included	O
as	O
part	O
of	O
the	O
Zope	B-Language
web	B-Application
application	I-Application
server	I-Application
,	O
but	O
can	O
also	O
be	O
used	O
independently	O
of	O
Zope	B-Language
.	O
</s>
<s>
Features	O
of	O
the	O
ZODB	B-Application
include	O
:	O
transactions	O
,	O
history/undo	O
,	O
transparently	O
pluggable	O
storage	O
,	O
built-in	O
caching	O
,	O
multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
(	O
MVCC	B-Operating_System
)	O
,	O
and	O
scalability	O
across	O
a	O
network	O
(	O
using	O
)	O
.	O
</s>
<s>
Created	O
by	O
Jim	O
Fulton	O
of	O
Zope	B-Language
Corporation	O
in	O
the	O
late	O
90s	O
.	O
</s>
<s>
ZODB	B-Application
3	O
was	O
renamed	O
when	O
a	O
significant	O
architecture	O
change	O
was	O
landed	O
.	O
</s>
<s>
ZODB	B-Application
4	O
was	O
a	O
short	O
lived	O
project	O
to	O
re-implement	O
the	O
entire	O
ZODB	B-Application
3	O
package	O
using	O
100%	O
Python	B-Language
.	O
</s>
<s>
ZODB	B-Application
stores	O
Python	B-Language
objects	O
using	O
an	O
extended	O
version	O
of	O
Python	B-Language
's	O
built-in	O
object	B-Application
persistence	I-Application
(	O
pickle	O
)	O
.	O
</s>
<s>
A	O
ZODB	B-Application
database	O
has	O
a	O
single	O
root	O
object	O
(	O
normally	O
a	O
dictionary	O
)	O
,	O
which	O
is	O
the	O
only	O
object	O
directly	O
made	O
accessible	O
by	O
the	O
database	O
.	O
</s>
<s>
ZODB	B-Application
supports	O
concurrent	O
transactions	O
using	O
MVCC	B-Operating_System
and	O
tracks	O
changes	O
to	O
objects	O
on	O
a	O
per-object	O
basis	O
.	O
</s>
<s>
In	O
Python	B-Language
,	O
this	O
could	O
be	O
represented	O
the	O
following	O
way	O
:	O
</s>
<s>
Then	O
at	O
some	O
later	O
stage	O
,	O
this	O
object	O
can	O
be	O
altered	O
with	O
a	O
Python	B-Language
code	I-Language
like	O
:	O
</s>
<s>
There	O
is	O
no	O
declaration	O
of	O
the	O
data	O
structure	O
in	O
either	O
Python	B-Language
or	O
ZODB	B-Application
,	O
so	O
new	O
fields	O
can	O
be	O
freely	O
added	O
to	O
any	O
existing	O
object	O
.	O
</s>
<s>
For	O
persistence	O
to	O
take	O
place	O
,	O
the	O
Python	B-Language
Car	O
class	O
must	O
be	O
derived	O
from	O
the	O
persistence	O
.	O
</s>
<s>
The	O
ZODB	B-Application
machinery	O
does	O
n't	O
chase	O
modification	O
down	O
through	O
the	O
graph	O
of	O
pointers	O
.	O
</s>
<s>
In	O
the	O
example	O
above	O
,	O
carzz.wheel3	O
=	O
something	O
is	O
a	O
modification	O
automatically	O
tracked	O
down	O
by	O
the	O
ZODB	B-Application
machinery	O
,	O
because	O
carzz	O
is	O
of	O
(	O
Persistent	O
)	O
class	O
Car	O
.	O
</s>
<s>
The	O
ZODB	B-Application
machinery	O
does	O
this	O
by	O
marking	O
the	O
record	O
as	O
dirty	O
.	O
</s>
<s>
However	O
,	O
if	O
there	O
is	O
a	O
list	O
,	O
any	O
change	O
inside	O
the	O
list	O
is	O
n't	O
noticed	O
by	O
the	O
ZODB	B-Application
machinery	O
,	O
and	O
the	O
programmer	O
must	O
help	O
by	O
manually	O
adding	O
carzz._p_changed	O
=	O
1	O
,	O
notifying	O
ZODB	B-Application
that	O
the	O
record	O
actually	O
changed	O
.	O
</s>
<s>
The	O
storage	O
unit	O
(	O
that	O
is	O
,	O
an	O
object	O
whose	O
class	O
derives	O
from	O
Persistent	O
)	O
is	O
also	O
the	O
atomicity	B-General_Concept
unit	O
.	O
</s>
<s>
In	O
Python	B-Language
,	O
the	O
name	O
of	O
the	O
class	O
involves	O
the	O
hierarchy	O
of	O
directory	O
the	O
source	O
file	O
of	O
the	O
class	O
resides	O
in	O
.	O
</s>
<s>
If	O
it	O
is	O
,	O
the	O
ZODB	B-Application
machinery	O
is	O
unable	O
to	O
locate	O
the	O
class	O
of	O
an	O
object	O
when	O
retrieving	O
it	O
from	O
the	O
storage	O
,	O
resulting	O
into	O
a	O
broken	O
object	O
.	O
</s>
<s>
Zope	B-Language
Enterprise	O
Objects	O
(	O
ZEO	O
)	O
is	O
a	O
ZODB	B-Application
storage	O
implementation	O
that	O
allows	O
multiple	O
client	O
processes	O
to	O
persist	O
objects	O
to	O
a	O
single	O
ZEO	O
server	O
.	O
</s>
<s>
Network	O
Storage	O
(	O
aka	O
ZEO	O
)	O
-	O
Enables	O
multiple	O
python	B-Language
processes	O
load	O
and	O
store	O
persistent	O
instances	O
concurrently	O
.	O
</s>
<s>
File	O
Storage	O
-	O
Enables	O
a	O
single	O
python	B-Language
process	O
to	O
talk	O
to	O
a	O
file	O
on	O
disk	O
.	O
</s>
<s>
Similar	O
to	O
FSFS	O
in	O
Subversion	B-Application
.	O
</s>
<s>
BDBStorage	O
-	O
Which	O
uses	O
Berkeley	B-Language
DB	I-Language
back	O
end	O
.	O
</s>
<s>
Zope	B-Language
Replication	O
Services	O
(	O
ZRS	O
)	O
-	O
A	O
commercial	O
add-on	O
(	O
open-source	O
since	O
May	O
2013	O
)	O
that	O
removes	O
the	O
single	O
point	O
of	O
failure	O
,	O
providing	O
hot	O
backup	O
for	O
writes	O
and	O
load-balancing	O
for	O
reads	O
.	O
</s>
