<s>
Apache	B-Language
ZooKeeper	I-Language
is	O
an	O
open-source	B-Application
server	O
for	O
highly	O
reliable	O
distributed	O
coordination	O
of	O
cloud	O
applications	O
.	O
</s>
<s>
ZooKeeper	B-Language
is	O
essentially	O
a	O
service	O
for	O
distributed	B-Architecture
systems	I-Architecture
offering	O
a	O
hierarchical	B-General_Concept
key-value	B-Data_Structure
store	I-Data_Structure
,	O
which	O
is	O
used	O
to	O
provide	O
a	O
distributed	O
configuration	B-General_Concept
service	I-General_Concept
,	O
synchronization	O
service	O
,	O
and	O
naming	O
registry	O
for	O
large	O
distributed	B-Architecture
systems	I-Architecture
(	O
see	O
Use	O
cases	O
)	O
.	O
</s>
<s>
ZooKeeper	B-Language
was	O
a	O
sub-project	O
of	O
Hadoop	B-Application
but	O
is	O
now	O
a	O
top-level	O
Apache	O
project	O
in	O
its	O
own	O
right	O
.	O
</s>
<s>
ZooKeeper	B-Language
's	O
architecture	B-Architecture
supports	O
high	B-General_Concept
availability	I-General_Concept
through	O
redundant	O
services	O
.	O
</s>
<s>
The	O
clients	O
can	O
thus	O
ask	O
another	O
ZooKeeper	B-Language
leader	O
if	O
the	O
first	O
fails	O
to	O
answer	O
.	O
</s>
<s>
ZooKeeper	B-Language
nodes	O
store	O
their	O
data	O
in	O
a	O
hierarchical	B-General_Concept
name	O
space	O
,	O
much	O
like	O
a	O
file	O
system	O
or	O
a	O
tree	B-Application
data	I-Application
structure	I-Application
.	O
</s>
<s>
Clients	O
can	O
read	O
from	O
and	O
write	O
to	O
the	O
nodes	O
and	O
in	O
this	O
way	O
have	O
a	O
shared	O
configuration	B-General_Concept
service	I-General_Concept
.	O
</s>
<s>
ZooKeeper	B-Language
can	O
be	O
viewed	O
as	O
an	O
atomic	B-Operating_System
broadcast	I-Operating_System
system	O
,	O
through	O
which	O
updates	O
are	O
totally	O
ordered	O
.	O
</s>
<s>
The	O
ZooKeeper	B-Language
Atomic	B-Operating_System
Broadcast	I-Operating_System
(	O
ZAB	O
)	O
protocol	O
is	O
the	O
core	O
of	O
the	O
system	O
.	O
</s>
<s>
ZooKeeper	B-Language
is	O
used	O
by	O
companies	O
including	O
Yelp	B-Application
,	O
Rackspace	O
,	O
Yahoo	O
!,	O
Odnoklassniki	O
,	O
Reddit	B-Application
,	O
NetApp	O
SolidFire	O
,	O
Meta	O
,	O
Twitter	B-Application
and	O
eBay	B-Application
as	O
well	O
as	O
open	B-Application
source	I-Application
enterprise	B-Application
search	I-Application
systems	O
like	O
Solr	B-Language
.	O
</s>
<s>
ZooKeeper	B-Language
is	O
modeled	O
after	O
Google	O
's	O
Chubby	O
lock	O
service	O
and	O
was	O
originally	O
developed	O
at	O
Yahoo	B-Application
!	I-Application
</s>
<s>
for	O
streamlining	O
the	O
processes	O
running	O
on	O
big-data	O
clusters	O
by	O
storing	O
the	O
status	O
in	O
local	O
log	O
files	O
on	O
the	O
ZooKeeper	B-Language
servers	O
.	O
</s>
<s>
ZooKeeper	B-Language
was	O
developed	O
in	O
order	O
to	O
fix	O
the	O
bugs	O
that	O
occurred	O
while	O
deploying	O
distributed	O
big-data	O
applications	O
.	O
</s>
<s>
Some	O
of	O
the	O
prime	O
features	O
of	O
Apache	B-Language
ZooKeeper	I-Language
are	O
:	O
</s>
<s>
Simple	O
Architecture	B-Architecture
:	O
The	O
architecture	B-Architecture
of	O
ZooKeeper	B-Language
is	O
quite	O
simple	O
as	O
there	O
is	O
a	O
shared	O
hierarchical	B-General_Concept
namespace	O
which	O
helps	O
coordinating	O
the	O
processes	O
.	O
</s>
<s>
Fast	O
Processing	O
:	O
ZooKeeper	B-Language
is	O
especially	O
fast	O
in	O
"	O
read-dominant	O
"	O
workloads	O
(	O
i.e.	O
</s>
<s>
Scalable	O
:	O
The	O
performance	O
of	O
ZooKeeper	B-Language
can	O
be	O
improved	O
by	O
adding	O
nodes	O
.	O
</s>
<s>
Some	O
common	O
terminologies	O
regarding	O
the	O
ZooKeeper	B-Language
architecture	B-Architecture
:	O
</s>
<s>
The	O
services	O
in	O
the	O
cluster	O
are	O
replicated	O
and	O
stored	O
on	O
a	O
set	O
of	O
servers	O
(	O
called	O
an	O
"	O
ensemble	O
"	O
)	O
,	O
each	O
of	O
which	O
maintains	O
an	O
in-memory	O
database	O
containing	O
the	O
entire	O
data	O
tree	B-Application
of	O
state	O
as	O
well	O
as	O
a	O
transaction	O
log	O
and	O
snapshots	O
stored	O
persistently	O
.	O
</s>
<s>
Typical	O
use	O
cases	O
for	O
ZooKeeper	B-Language
are	O
:	O
</s>
<s>
In	O
addition	O
to	O
the	O
client	O
libraries	O
included	O
with	O
the	O
ZooKeeper	B-Language
distribution	O
,	O
a	O
number	O
of	O
third-party	O
libraries	O
such	O
as	O
Apache	O
Curator	O
and	O
Kazoo	O
are	O
available	O
that	O
make	O
using	O
ZooKeeper	B-Language
easier	O
,	O
add	O
additional	O
functionality	O
,	O
additional	O
programming	O
languages	O
,	O
etc	O
.	O
</s>
