<s>
Algorithms	O
said	O
to	O
employ	O
a	O
Ping-Pong	B-Algorithm
scheme	I-Algorithm
exist	O
in	O
different	O
fields	O
of	O
software	B-General_Concept
engineering	I-General_Concept
.	O
</s>
<s>
In	O
most	O
database	B-General_Concept
management	I-General_Concept
systems	I-General_Concept
durable	B-General_Concept
database	B-General_Concept
transactions	I-General_Concept
are	O
supported	O
through	O
a	O
log	B-General_Concept
file	I-General_Concept
.	O
</s>
<s>
Assuming	O
for	O
simplicity	O
that	O
the	O
log	B-General_Concept
file	I-General_Concept
is	O
organized	O
in	O
pages	O
whose	O
size	O
matches	O
the	O
block	B-General_Concept
size	I-General_Concept
of	O
its	O
underlying	O
medium	O
,	O
the	O
following	O
problem	O
can	O
occur	O
:	O
</s>
<s>
If	O
the	O
very	O
last	O
page	O
of	O
the	O
log	B-General_Concept
file	I-General_Concept
is	O
only	O
partially	O
filled	O
with	O
data	O
and	O
has	O
to	O
be	O
written	O
to	O
permanent	O
storage	O
in	O
this	O
state	O
,	O
the	O
very	O
same	O
page	O
will	O
have	O
to	O
be	O
overwritten	O
during	O
the	O
next	O
write	O
operation	O
.	O
</s>
<s>
If	O
a	O
crash	B-General_Concept
happens	O
during	O
that	O
later	O
write	O
operation	O
,	O
previously	O
stored	O
log	O
data	O
may	O
be	O
lost	O
.	O
</s>
<s>
The	O
Ping-Pong	B-Algorithm
scheme	I-Algorithm
described	O
in	O
Transaction	O
Processing	O
eliminates	O
this	O
problem	O
by	O
alternately	O
writing	O
the	O
contents	O
of	O
said	O
(	O
logical	O
)	O
last	O
page	O
to	O
two	O
different	O
physical	O
pages	O
inside	O
the	O
log	B-General_Concept
file	I-General_Concept
(	O
the	O
actual	O
last	O
page	O
i	O
and	O
its	O
empty	O
successor	O
i+1	O
)	O
.	O
</s>
<s>
it	O
is	O
completely	O
filled	O
with	O
log	O
data	O
)	O
,	O
it	O
is	O
written	O
one	O
last	O
time	O
to	O
the	O
regular	O
physical	O
position	O
(	O
i	O
)	O
inside	O
the	O
log	B-General_Concept
file	I-General_Concept
.	O
</s>
<s>
A	O
functionality	O
which	O
lets	O
a	O
computer	O
A	O
find	O
out	O
whether	O
a	O
computer	O
B	O
is	O
reachable	O
and	O
responding	O
is	O
built	O
into	O
the	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
.	O
</s>
<s>
Through	O
an	O
"	O
echo	B-Protocol
request	I-Protocol
"	O
Computer	O
A	O
asks	O
B	O
to	O
send	O
back	O
an	O
"	O
Echo	O
response	O
"	O
.	O
</s>
<s>
These	O
two	O
messages	O
are	O
also	O
sometimes	O
called	O
"	O
ping	B-Protocol
"	O
and	O
"	O
pong	O
"	O
.	O
</s>
<s>
this	O
in	O
Python	B-Language
:	O
</s>
