<s>
Hybrid	B-General_Concept
Web	I-General_Concept
Cluster	I-General_Concept
is	O
a	O
software	O
product	O
developed	O
by	O
Hybrid	O
Logic	O
Ltd.	O
,	O
a	O
Bristol	O
,	O
United	O
Kingdom	O
based	O
company	O
.	O
</s>
<s>
The	O
software	O
is	O
designed	O
to	O
provide	O
commodity	O
web	O
hosting	O
in	O
a	O
distributed	O
and	O
fault-tolerant	O
manner	O
on	O
a	O
cluster	B-Architecture
of	O
either	O
real	O
physical	O
servers	O
or	O
virtual	O
cloud	B-Architecture
computing	I-Architecture
infrastructure	O
server	O
instances	O
.	O
</s>
<s>
Runs	O
standard	O
LAMP	B-Application
stack	I-Application
web	O
applications	O
.	O
</s>
<s>
Works	O
on	O
real	O
hardware	O
and	O
virtualized	B-General_Concept
cloud	B-Architecture
computing	I-Architecture
infrastructure	O
.	O
</s>
<s>
Enables	O
instant	O
and	O
automatic	O
scalability	B-Architecture
.	O
</s>
<s>
This	O
software	O
splits	O
each	O
website	O
and	O
database	O
into	O
individual	O
units	O
of	O
work	O
-	O
the	O
load	O
generated	O
by	O
each	O
is	O
monitored	O
closely	O
and	O
the	O
cluster	B-Architecture
automatically	O
and	O
transparently	O
transfers	O
websites	O
and	O
databases	O
between	O
cluster	B-Architecture
nodes	O
such	O
that	O
no	O
single	O
node	O
is	O
ever	O
overloaded	O
.	O
</s>
<s>
The	O
upper	O
bound	O
on	O
the	O
scalability	B-Architecture
of	O
this	O
system	O
is	O
that	O
no	O
single	O
website	O
can	O
ever	O
utilise	O
more	O
than	O
two	O
servers	O
at	O
once	O
-	O
under	O
this	O
condition	O
all	O
standard	O
LAMP	B-Application
applications	O
will	O
run	O
unmodified	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
possible	O
to	O
scale	O
beyond	O
this	O
limit	O
via	O
the	O
use	O
of	O
multi-master	B-Operating_System
database	I-Operating_System
replication	I-Operating_System
-	O
this	O
requires	O
that	O
applications	O
store	O
all	O
variable	O
state	O
information	O
within	O
a	O
database	O
-	O
the	O
database	O
can	O
then	O
be	O
live	O
on	O
many	O
servers	O
,	O
the	O
web	O
application	O
itself	O
must	O
remain	O
static	O
and	O
can	O
be	O
served	O
from	O
as	O
many	O
nodes	O
in	O
the	O
cluster	B-Architecture
as	O
required	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
specify	O
a	O
redundancy	O
invariant	O
-	O
this	O
sets	O
the	O
number	O
of	O
machines	O
in	O
the	O
cluster	B-Architecture
that	O
should	O
hold	O
a	O
copy	O
of	O
each	O
website	O
or	O
database	O
.	O
</s>
<s>
By	O
default	O
this	O
number	O
is	O
set	O
to	O
2	O
,	O
this	O
means	O
that	O
under	O
normal	O
operation	O
all	O
data	O
will	O
be	O
held	O
on	O
at	O
least	O
two	O
cluster	B-Architecture
nodes	O
at	O
any	O
one	O
time	O
.	O
</s>
<s>
If	O
a	O
cluster	B-Architecture
node	O
fails	O
,	O
the	O
system	O
acts	O
to	O
automatically	O
recover	O
its	O
data	O
and	O
resume	O
web	O
hosting	O
within	O
15	O
seconds	O
.	O
</s>
<s>
This	O
is	O
achieved	O
using	O
the	O
filesystem	O
snapshotting	O
capabilities	O
of	O
ZFS	B-Application
.	O
</s>
<s>
Hybrid	B-General_Concept
Web	I-General_Concept
Cluster	I-General_Concept
began	O
beta	O
testing	O
in	O
September	O
2010	O
.	O
</s>
